Could the new spring at full compression be too big to allow the catch notch to reach the catch plate?

That would actually be a pretty effective panic blaster. Can you wear that and still hold another blaster? You might want to compare the weight of a vortex disc to the pure foam ones before doing anything too crazy. I'm thinking the vortex discs might be a bit heavier - affecting flight potential and range.

That's pretty smooth looking. Hope you don't mind if i have some questions:

Did you keep any of the bladder to level out the air pressure, or does the compressor have enoug volume to maintain the needed pressure?

Did you keep the valve trigger, or has it been replaced with an electronic switch to the compressor.?

Does it run on the same pressure as it did stock?

What kind of batteries and voltages are used?

How many clips can you fire from a full charge?

Sorry i'm at work and cant get to youtube through the firewall. I'd like to see some pics of the internals to see how it all fits together if it's at all possible.

May i suggest that you scale down the pictures a bit? Pictures needing a horizontal scroll bar are often met with mockery.

When I do test the rifled barrels vs smooth bore, I think I'll throw in a ported barrel as well - though I don't know what the "optimum" barrel length is for my PAS. (Using 10" of cpvc. Dart fit is certainly snug, but not "twist fit".) If I got lucky and nailed the perfect barrel length solely on guess work, porting won't display any effect. Maybe I'll have to test two additional short barrels (5" or so) ported, and unported to see if there is any difference. If porting can be proven to make a difference in a mid power blaster like my stock PAS, it should have an even greater effect on the high end ones like snaps.

Ooh! Porting a barrel may be an easy, effective way check your barrel length! Coat the ports very lightly with dish soap or other bubble blowing solution, and fire a dart. No bubbles, or bubble solution inside the tip barrel = too long of a barrel, you're drawing a vacuum before the dart leaves the barrel. If you get bubbles, that means there is excess pressure that could of been used to accelerate the dart. If it just sputters violently, it's too short. That's purely theoretical of course. I don't know if ports on a barrel can even blow bubbles. Perhaps a similar test can be done with a weak tape (painters tape), or glitter/chalk line dust. The chalk line dust may work better with only a single small port drilled.

Only problem with drilling ports in a barrel is deburring the inside.

I'm no gunsmith, but ports should be placed close to the tip of the barrel. I'd guess towards the last 1/2" to 1/4". Should be easy to clean out with a small knife, or hobby file. You are right though, there is definitely a right way and a wrong way to deburr drilled ports holes. My first instinct would be to grab a .5" drill bit. But if the drill bit expands the barrel ID enough for air to pass around the dart - you've defeated the purpose of the ports.

Firstly, Boot - you rock! Is there any chance of getting some of that footage posted, or possibly even re-shooting and comparing non ported and ported barrels? That kind of testing can really only be done effectively with a high speed camera.

Secondly, the initial (and objective) testing I've done with rifling (comparing a gentle 18:1 twist, a tight 9:1 twist and a smooth bore have been so obviously against rifling that I've had a tough time forcing my self to set up the "precision shooting through paper test" I promised everyone and gather hard data. The darts weighted with a #6 washer swam like a fishy when shot out of the rifled barrel. I shot a heavier silicone head dart, and that looked like it flew perfectly straight, but it almost torqued the head off after the second shot. It's possible I'm not using a heavy enough dart. I think I'm going to have to wait till my 3 rd generation silicone head darts are done before doing more testing. They'll have more surface area to bond with an may be able to handle the stresses better. They'll also be heavier than the #6 washer. I also added a ls spring to my pas, which allows me to add another 2 inches to the barrel (confirmed by comparative testing 10" to 12") So the fishtailing with the 10" rifled barrels may be caused by the increase in power and not the rifling. I could add ports to the tip, but I only want to change one variable at a time while testing.

Seeing this reminded me of a really old school technique for a grenade that frags mid-air. I couldn't find the post, and I cant remember who created it - or even which nerfy site it was featured on, but the basic concept involves the type of cost effective home brewery that you might like to explore. 

 

Materials: 

Darts, 

Hair elastic

1" wide Strip of fabric (I cant remeber what type of fabric they used. I think it might of been felt but the material might also be irrelevant)

 

Group the darts into a bundle about the size of a fist. Wrap the fabric strip around the front, stretch the hair tie over the tails.  The fabric strip acts as your fuse. The longer it takes to unwrap, the longer it will fly undetonated. You may need to unwrap a little of the fabric strip so it drags in the air and helps the strip to unwind as it flies.That'll shorten the fuse. Once it unwraps the heads are free and they'll spread apart rolling the elastic off of the tail - allowing the grenade to explode and the compressed to darts separate. If you place the elastic slightly further up the tail it takes a little shock to get it to release and it becomes an impact grenade that explodes once it impacts a wall or ceiling - useful for indoor wars with entrenched enemies. If you're outside in the open, mid air - high altitude explosions  produced pretty impressive spread. 

 

I can't remember the exact specifics of how well it works ( fabric strip length, dart spread, sensitivity, fuse timing...) but I remember being impressed. It was the only functional grenade nerf I'd ever read about, and it was stupid cheap and easy to make. Just be careful not to crush the tails of your darts so much that they no longer stay in a blaster. You'll need to tune the size of the dart bundle to the hair tie.

I was browsing on Nrev, and found this. I'm pretty sure its what your looking for.

ah yes exactly, i was thinking of using some kind of caulk.

thanks

Thanks for taking interest. If you try them, make special note to read, and re-read the part that discusses what silicone caulks work, and which do not. (If it reeks like vinegar and has a bunch of warning labels - that's the stuff. GE II brand silicone will not work.)

They're still in the developmental phase. I need to expand the mold to do only blunt heads (heavier and easier to extract) and make up another batch. If you're interested, pm me your address, I'll send you some to experiment with- if you let me know how they work for you (comparative ranges, flight chararistics...) I think they fit exactly what you're looking for - no metal weights, and soft enough to actually bounce when dropped on a hard surface from a few inches (the silicone has more give than rubber, and far more give than hot glue.) and is easily mass producible. (making a mold is as easy as drilling a polyethylene cutting board with a counter sink at carefully repeated depths) Filling the mold is as simple as mixing caulk and corn starch and mooshing it into the holes.

It's about 3-4 hours to full cure, (depending on how much corn starch you add) and the ONLY glue I could find that bonds to silicone is "liquid nails small projects silicone adhesive" (blue and gold toothpaste type tube pictured in the post - not the liquid nails caulk tube; though I haven't tried the caulk tube...) which has a 6 hr set time, 16 hr full cure time. So you won't be able to fab up a bunch of darts the morning of a war and expect them to be set in time. If you find anything better - please for the love of Nerf let me know!!

Once I feel confidant enough in their performance to defend the concept, I'll post it here. Things have just been to crazy to do even the slightest bit of testing with them.

Sounds like fun. Wish SLC was less than 5 hours out. That would be a bit far to drive for an 11 person war. Good to know you're out there though - should I ever find myself in ID.

I occasionally host as well, though we would be lucky to get that many at one of ours.

Droid31 was kind enough to toss some samples in with a broken set of x-bow arms.

The sample that really got my attention was a small stub of dense blue foam - similar to the "craft block" or building block foam. I haven't been able to play with that yet, but I'd imagine that foam to be darn near indestructible. Was that also something that you extruded? I'm sure you get a lot less foam with that high of a density, but there really isn't anything out there that even comes close to a foam that dense. I could see there being a much larger market for the High Density foam than the large pore pool noodle-esque foam.

Wow. There is a lot of potential for game types there! You could do everything from a mobile nerfy suicide bomber to blow up an opposing teams base, to a CTF'esque game type where you're either trying to arm the opposing teams bomb, or disarm your own.

What game types have you come up with so far?

I really like how easy it would be to reload the unused mag while nerfing away. It looks much more manageable than dump pouches and tac vests.

Writing over it with a dry erase marker also works. It returns the ink to a liquid state, mix with the dry erase ink and wipes off. Quick and simple, but like you said it works the best when removing ink from smooth glossy surfaces.

Or the white elite streak?

I don't think any repair will ever get a torn bladder holding pressure again. No adhesive has the inherent stretch necessary. That bladder is toast.

While we're talking about it, specialty hose stores (not the common hardware stores) often sell latex tubing in various sizes. They are usable for bladder replacements. Might want to shop around in your country before you start shipping from the states.

With the exception of an indoor war, I favor darts over discs.

Ranges aside, it's easier to see a disc approaching and dodge it than a dart. It has a much bigger surface area from the targets perspective.

Discs are also heavily affected by wind, and can easily be blown way off course.

Discs cost about twice as much as a dart, which could hinder your load out. Spare vortex clips may also be hard to find.

Vortex blasters are also much more complex mechanically. I was tearing apart a praxis yesterday looking for the single failure that was keeping it from firing. What a headache!

If it's an indoor war, it might be worth considering vortex blasters - especially if ricochet hits count. Great fun bouncing a disc down a tight hallway and around a corner to nail someone while they're reloading. No wind, closer quarters (shorter ranges/harder to pay dart ninja - I'd give the advantage to vortex blasters. Discs impact with a bit more of a "thump" too, which is never a bad thing.

Goo. Double post. Apologies all.

Tobias, You'd be suprised how uncommon "common sense" can be. There is an age appropriateness to blasters that even I've managed to overlook when shopping for a younger nerfer. For example, I bought my 4 year old a rebel messenger for christmas. Guess what? She can barely prime it and when she does she flexes the priming bar upwords a full inch because she lacks the wrist strength needed to hold the pistol handle steady. My 3 year old had to learn to not squeeze the trigger when priming his doublestrike blaster so the hammer would catch. Both blasters probably could of been a little better chosen and I picked them out myself, and I've been nerfing since the Bn'A was on the shelves.

I've found panthers and jolt esque' blasters to be a fantastic fit for smaller younger hands. The mechanical advantage gained in air powered blasters is a huge assistance for smaller arms. Give a small kid a titan to play with and you'll be his hero for the day. Since they can be pumped on the ground like a floorstanding bike pump any kid who can lift the thing will feel like the biggest bad*** ever - and you don't have to worry about them loosing an eye if they decide to shoot themselves in the face. (which will happen!!!)

Ducnha, you're absolutely right about avoiding clip fed blasters and slide to prime blasters. Simple front loaders are the way to go for young nerfers.

The L.A. region has one of the more significant Nerfing Communities in the country. S.C.U.N. has some extremely well attended wars - including "Armageddon" - the second biggest annual war in the nation. You're very fortunate to live there. I'm in utah and have on multiple years driven 20 hours (round trip) just to war with the fine folks of S.C.U.N.

If memory serves - they even have their own forum... or at least did at one point in time. I'm not sure if that's where they still hang out, but you will see some SoCal nerfers here from time to time. Baghead, or Apollo will probably fill you in a little more if you message them.

Oh, wow. That is very interesting. I just e-mailed them asking about their pricing.
I've played "foam distributor" before and enjoyed it. With some of the recent inquiries for foam, I'm thinking it might be time for another order sooner than I thought. With the next batch I order I hope to not only sell foam - but also sell completed gumdrop 3.0 darts. Glory1610 has recently made a pretty big breakthrough and we're nearing the end of the development phase. Before I will place the next order though, I need finish my write up for the "auto blanker", as well as build another automated machine to drill holes quickly (I've got it conceptualized but not built). I also need to see if I can cut square straight off of the roll. Pre-stretching sucked, especially when I decided to run it through the dryer anyway.

Thanks for the heads up! If you're only looking for smaller quantities, maybe you and I can work out a group buy? One thing that makes me hesitate is the risk of getting stuck with a ton of foam without any buyers. I thought I might of dried up the Beige market with my last order.

I've found 2 100' bags of pre-cut 1.38", straightened beige Hot rod XL blanks from orders that had fallen through if you're interested. I'm selling them on a first post - first serve basis. I'm also waiting on payment for a third larger order of 2" cut blanks. Once they're all sold, I should be ready to buy another 2500' order of foam.

Pre-cut hot rod blanks

Also, minimizing the ammount of tubing between the hornet tanks, blast button, and check valve will result in a more explosive discharge from the hornet tanks. The less air there is to vent, the quicker the internal piston moves. The quicker it moves, the better. It makes the difference between a hissing discharge, and an explosive "pop".

Each individual tank is about 1.125" in diameter, and about 2" long, not including the port on the back, or the barrel.

That's interesting, I was going to say I don't know that anyone has ever posted exact dimensions before, but here is a really useful thread that has good pics and provides decent relative dimensions (by showing the tanks inserted into standard pvc fittings):

http://nerfhaven.com... tank expansion

It is also looks like a really cool thing to try with extra hornet tanks.

Just ignore the second part (output expansion) I eventually got it to seal, but enlarging the output hole sends all the contained air back through the piston stem instead of shifting the piston and venting out the barrel. But that whole mess is covered in later posts.

and 3- Yes, I am in fact interested in making an all-custom tank of roughly the same size or maybe larger. Though the dimensions of a hornet tank specifically are not as important to this goal. The dimensions give me a rough approximation of the volume of such a tank though, which is helpful.

Building your own backfire tank sounds like a really challenging project. Let me share some lessons learned.

From my failures in working with hornet tanks I've learned that you need to:
Make sure your barrel side internal peg is a smaller diameter with less contact surface area than the inlet peg (the peg the piston sits on) or it will just vent back out of the piston stem instead of shifting the piston and venting out the barrel.

From my successes in working with hornet tanks I've learned that you need to:
Keep your blast valve as close to the tank with the shortest amount of tubing possible. The less air there is to vent the quicker your piston will move out of the way, and the more force your tank will fire. (And yes force. I can actually feel and see it kick my thumb when held above the tank nozzle.)

And from my recent observations I've decided that Cheyner is a mean-spirited cur who would probably flame his own grandma if he found the opportunity. Apologies to Phillypretzel, i should of said "later replies" instead of "later posts". That was unintentionally misleading.

If you want to buy a bigger volume back fire tank, try one from a big salvo. I haven't played with them, but they come highly recommended. (or expand a hornet tank) [self serving plug].

Firstly, disclamer: the filters at my work knock out all non-hosted pics so I don't know if any of these links have pics. Secondly, while not familiar with "FVJ's", RSCB's will generally use anything with a tip that isn't wider than the foam - including full size 3" long darts (the dart needs to be able to slide down the feed tube without getting stuck. Full size 3" long darts rarely hopper well due to the amount of bending required to get past the wye. 

 

I'm a big fan of RSCB's they're a great piece of tech that shoots every time without fail, and works well on a small air supply. Just remember to point the barrel down once before each shot to seat the next dart. I'm not sure if i've seen a write up either, but the basic concept is that you have a loose fitting tube (feed tube) capped on the back end teeing into a tighter fitting barrel and an air supply coming into the middle leg of the tee.

 

You can either sand down the internal stop of the tee so the feed tube slides past the middle leg and drill a hole through the feed tube at the middle leg or (my personal preference) harvest a clear feed tube from a basic marshmallow blaster, chop one end off to the desired length and wrap e-tape (to create a tee friendly seal) around the chopped side such that the feed tube hangs past the middle leg of the tee once it's inserted into the tee. The "magic PVC" refers to the 5' pre cut lengths at most hardware stores. For some reason, they're slightly thicker walled so you can pound a cpvc barrel into them to adapt the cpvc to pvc fittings.I forgot to lable the bottom black elbow, but I've found a cpvc elbow nests well into 1/2" pvc fittings which reduced dead space a little. 

 

These may be helpful...

 

http://nerfhaven.com...stions/?hl=rscb

 

and... have a crappy MS paint drawing...

The bbq skewer cylinder your using looks a little sketchy. Go to your hardware store light bulb aisle. They sell 3' and 6' clear petg fluorescent light bulb storage tubes for a few bucks. They're the perfect fit for vortex discs. I use one to simply store mine in, you could easily make a large magazine out of one as well.

Seriously awesome first post! I think it's safe to say that that we're all looking forward to seeing where you go with this next. What psi were you running at for that test? 80' full - auto? That will be a serious gun to contend with if you aren't spending your entire time pumping.

A write up explaining magstrike piston theory was done by "Gas Mask Guy" and can be found here:

Magstrike Firing Mechanism Autopsy

If you're willing to consider using a back pressure style of tank (panther, xbz, big salvo, hornet...) you should be able to modify a shcrader valve stem from a bicycle tire inner-tube. simply pushing in the pin will vent the fill line to the tank and fire the tank. The threads on the valve stem might be useful for mounting the valve and you could also cut out some of the thread wall to guide a trigger bar over the schrader valve. You can also stretch some vinyl tubing over the back of the valve if you warm up the tubing first with a hair dryer (maybe), heat gun or carefully with a flame and double wrap a zip tie to seal it.

 

You might also want to consider the slightly less expensive MAVO-3. the cfm's are a little less but i've used them successfully on many of my air tank builds so they've proven sufficient for stock nerfy tanks (just keep the air line between the pump, trigger valve and air tank to a minimum. The shorter that line is, the faster it can vent and the sharper the tank's air release will be.

 

I don't know if the mjvo-3 will shut off the inlet before back venting the outlet but the mavo-3's will - which means they can be used to make a true semi auto. (they won't vent off your reserve tank with each trigger pull.)

 

https://www.amazon.c...keywords=mavo-3

If you're willing to move away from the homemade pre-requisite, and you're using an air gun, a ball valve teed off of the pump, leading to a "pop-safety valve" (aka relief valve) would be worth looking into. McMaster has some "soft seal valves" starting at $6.25. you could open the ball valve to limit pressure durring HVZ, and close it when more power is appropriate. If they can be disassembled without being distroyed, you could slip in /remove springs to adjust when it triggers.

And to quote the McMaster:
"Soft-seal valves have an airtight silicone seal, a brass disc, a music wire spring (unless noted), and a pull ring for testing. Temp. range is -15° to +250° F. Connections: NPT male bottom inlet and vented side outlet. Please specify set pressure. All are available from 25 to 150 psi in 5 psi increments, and from 150 to 300 psi in 25 psi increments, unless noted." They are only open while pressure exceeds the set limit.

And they meet ASME VIII (consctuction code for pressure vessels) for air, which really isn't relivant to nerf, but still a fun note for the other poor people who are forced to deal with such a beastly code.

Oh dear, where to start...

I've spent a lot of time chasing the perfect semi-auto platform. To start with the origional question asked:
I once went to my local EVCO house of hose and bought some varied diameter latex tubing. After having a foot or two of odd sizes rung up I would call it cost prohibitive. The nerf bladders use foam latex. I think it has a higher ductility that straight latex (surgical tubing) and therefore more fills. In discussing the materials with Doom (he's spent a lot of time with them in water blasters) I also learned that when an air filled latex bladder ruptures, it 'splodes as loud as a shotgun and might draw unwanted attention. I would honestly search out a rf-20/magstrike to gut.

Another thought you might want to think about: using a pull pin type tank will vent your reserve bladder so long as the trigger is pulled. You'll inevitably be wasting precious air even with good trigger control. You might want to consider switching to a back pressure style tank (big salvo, hornet, panther) and a "clippard mavo-3" valve (from ebay (clippard.com charges you $10 for shipping + $10 for handling on a $7 valve)). That valve triggers the tank - without venting your reserve. It's the route I went when I made my semi auto dart tag gun. I also sleeved a 1" inner-tube over the rf 20 bladder to increase the pressure to a functional level. Fair warning though - it's a lot of work.

What isn't a lot of work?
Pick up one of these for less than $10:
http://www.amazon.co...r/dp/B002YOXQRY
Marshmallow Mforcers are semi auto/rear loading by design. Once re-turreted to pteg barrels they'll give you ~80' ranges with half the work, and a 15 round turret to boot. Less work, more fire power. One of my favorite go-to primaries. They do look kinda silly - as far as blasters go, but that all changes when you've dropped 2 people during a rush and you still have half a turret full of darts.

Mforcer write up

Ball pumps often have a bigger diameter and the 6 to 10" pumps are still small enough to integrate onto most shells. For higher volume lower pressure the key thing to watch for is pump diameter. The bigger the diameter the more volume it will move - at the cost of pressure.

Simple and extremely effective, deffinately a Boot creation! Great work as always.

Based on the plunger diameter and such, It almost looks like an unshelled xbow!

Much of this topic actually hits on a failed prototype I made many many years ago. I think I still have it in a box somewhere. I'll have to dig it out. Basically, 2 ribbons from 2 tape measures are supposed to neatly guide a line of darts through a mag and wrap them around the center of a spool with the ribbons forming the walls while the darts transition from out the bottom of the mag to the center of the spool. Because the entire spool is rotating along with the wound up line of darts, you only get friction on the few darts that are spinning towards or away from the spool. The spool would also perform better as more darts were loaded in - increasing the leverage the tape measures could exert on retracting the ribbon.

 

The functioning theory was that as darts are placed inside the mag (between the two ribbons) the ribbons would travel with the darts, guiding them to the cog in the center of the spool and as the spool is wound the ribbons & darts, the ribbons would feed in and out from the tape measures keeping potentially hundreds of in a neatly wound spiral. 

 

Theory collapsed into reality when I tried loading the thing. The ribbons failed to keep the darts neatly aligned during the mag to spool transition and as soon as they exited bottom of the mag they broke rank and jumbled into an un-extractable mess. I made a crappy ms paint drawing of expectations vs reality of that prototype but my work filters photobucket and I'm out of mobile data, so you'll all just have to wait till later. It could possibly work if there was a swinging guide running from the bottom of the mag to the cam, (keeping the darts aligned as it transitions) but after my initial failure I kinda stopped caring and threw it in a box. Also, I think I'd need something more sturdy than the 1/2" wide ribbon I was using. The ribbons were prone to bunching and twisting and falling off of the tape measure spool.

 

Another down side is that I had to chop off the fingers on top of the mag (the ones that keep darts from jumping completely out of the magazine). Without those fingers, a loaded drum has to be in a blaster or it will unload itself in quick glorious fashion. Knowing what i know now I probably could of kept the fingers and had just cut a slit for the ribbon to pass through instead. Better yet, instead of folding the ribbon over the top of the mag, simply pull the ribbon through slits at the bottom of the mag and have a longer follower (I used 1/2" lengths of dowel to make dummy darts for a follower) to make up the difference. Poof. no sharp fold on the ribbon and I'd get to keep the dart fingers at the top of the mag... wow, i'm actually starting to care about that project again!

My experience with RSCB's tells me that moist or damp darts do not like to slide - even through a straight tube. (my group recently had a nerf war during a rain storm). The thread is moving away from lung power to transition darts, and I think a fail has been avoided there. The  humidity that blowing in the tube would of transferred would probably of impacted dart movement after a few hours of nerfing and advancing darts down the tube.

 

As far as pumps go, you can easily source a battery powered air blower by looking near the air mattresses. I've got one, it takes 4 "D" batteries so it's stupid heavy but it puts out more air than a computer fan. A little modding to a lipo would cut down on the weight and seriously increase the output.

So, Hoping that worked. I can't actually see the pics due to work filters. Here's some pics of my failed prototype. The entire spool rotates inside the cookie tin so only the darts segueing between the mag and cam are moving independently of the spool. With a little more guidance between the stock mag and the cam, and a lot more rethinking for spring placement and materials (like having the ribbon feeding out through slots much lower in the mag and having a longer follower - instead of wrapping up and over the top of the mag, I think it could be viable.

I don't know if it can handle that spring load but harvesting a spring from a standard clickable pen might be a good supplement or replacement even to the original catch spring.

What you have to understand, is that there is glue on top of the weight, thus there is no way the weight can punch through the felt. In your experience, did you put hot glue on top go the weights? If you didn't your results make sense, if you did, I have no Idea what is going on.

Granted, they were my very first attempts at dartmaking. I burned a hole, added glue, put in a bb, added more glue and capped it with foam. Not sure why the bb's punched through, but they definitely did. I've had little trust for bb's since.

There are a few things to consider when capping a dart with more fbr. 1) fbr compresses quickly, especially when it's sitting between your weight and the target. Your darts won't live very long and you'll have to replace them more quickly. 2)the more foam you have in front of your weight the further back your center of gravity will be. Its the reason airplanes don't put the flaps on the front of their planes. A little shift on the front makes a big difference in the flight path. The tradtional felt pads provide a long term, durable cushion without adding much material in front of the weight.

I'm going to have to disagree with birch. Theirs no way I would allow someone to use ball bearings as weights in their darts at a war I hosted. Even if the bearings weighed the same, theirs the very real possibility that the bearing will punch through a dart head and hurt the target. In my early dart making days I tried dropping a few pellets in a dart head and encased them in hot glue and capped it with a bit of FBR. The first test shot against a cabinet door, and the pellets punched through the hot glue and fbr tip. The pellets still had enough momentum to dent the oak cabinet. The fbr quickly and simply became a sabbot to accelerate the pellets. Washers are used due to their flat surface area. So long as they're well made, and they don't have any metal edges exposed, theirs very little chance of drawing blood. since they spread their momentum across the entire face of the washer. I would also worry about small ball bearings weighing too much. The last thing you want flying around a field is a dart heavy enough to take out a tooth.

I have seen people use a craft foam punched disc to cap their steffans. That worked well and had a longer usage life than a FBR cap since it's a denser foam. If you really want an alternative to felt, I would suggest craft foam.

Your drawing looks almost exactly like a scout. Perhaps you could rob the cocking slide off the top and mod it to fit over your NF? you'd save a bundle on filler material and hours of sanding.