370 replies to this topic

[Commonly Hunted]

This is what I plan to do:

The plunger head right now is a CPVC endcap so I have to add a coupler and a section of 1" thin and tape in between everything.

I'm pretty sure it would work because it would be shaped like your plunger head in the SNAP.

Also the catch is like the one in Bolt's NTS and is not the final. I want a catch like Ompa's but can't have that right now.

I don't want to use O-rings because of how easy rubber washers are and how effective. So I'm not gonna do the double ring thing.

I think Ompa used a 1/2" endcap in the MEG. I was asking about the unnamed rifle he made based on the SCAR-N (AKA Ompa Special, Ompanator, Ompa Rifle). The body, 1.25" PVC, is the plunger tube so the plunger head must've been a .75" endcap. His material list shows only 1" endcaps but those are as big as 1.25" PVC, right?

[Prometheus]

To help improve seal, like Carbon said about using 2 O-rings, could you instead add a metal washer on top of the rubber washer, then add another? It would be quite a heavy plunger in the end, but a stiff spring should be able to handle it. Also, as CX once wrote up, certain types of barrels require fast or slow air delivery depending on the gun, etc. so you might not necessarily need the fastest acceleration a plunger is capable of. I guess as this is somewhat of a prototype, alot of experimentation would be fitting. Also, any chances of some pics of the assembly?

[Commonly Hunted]

I tried it but still nothing. It might be that the 1" section isn't long enough and still allows the plunger to tip but I think it is because of the space behind the plunger. There are no holes to release the pressure behind the plunger.

I'm eating right now and then I'm gonna put some holes in the body around where the trigger will be and maybe make the plunger head longer for more stability just in case.

Right now I have only three screws to put into the gun to keep it together but already I hate opening and closing the thing.

Carbon, since you have made this kind of plunger with the same fender washer and pressure chamber size, did you have these problems? Did you have a problem with the seal being too good?

And are these the only holes that are releasing the pressure behind the plunger?

[sam]

I believe Ompa used 1" PVC and 1/2" endcaps, but then nested all of that inside 1 1/4" PVC. And yes those holes are for releasing pressure.

[Commonly Hunted]

I asked if those are the only holes releasing pressure. Are they enough to let the plunger do its thing?

I tried it with my shotty, not those four holes but holes around the trigger and behind the plunger, and still nothing. It is really making me mad because I think I've covered every problem there could be. I guess I'm gonna have to change to O-rings.

[Carbon]

And are these the only holes that are releasing the pressure behind the plunger?

Yeah...it doesn't need much. And if that was your problem (plunger vacuum), your dart would be moving forward, but be getting lackluster ranges...insted of not moving at all.

Do you have any way to test a dry fire of just your plunger in your pressure chamber, to see if you're getting any kind of seal whatsoever? Fire it with your hand pressed over the end. You should be able to dramatically slow it's progress, if not outright stop the plunger's forward stroke.

If you try using a rubber washer gasket, you'll need a plunger head a lot larger than what you showed in your diagram (although I'm figurng it's not to scale). Looking at wear patterns on rubber washers I've used, it looks like the rubber gets wedged in between the cap and the PC, rather than cupping forward. If the plunger head is too small, the washer will just fold backwards and let the air past. The plunger head needs to have little wiggle room (1/2" endcaps work well...so do washers).

[Prometheus]

I would try a longer section of 1", not entirely sure it will help, but it's worth a shot. Perhaps your plunger is accelerating too slowly? Try some lube, it might improve things. Also check for any leaks around the back of the bolt, you might have a good seal with shells but poor seal with plunger tube.

[Flaming Hilt]

I'd suggest a stronger spring. If there was one thing I learned from spring strength it was that you can't really go overboard unless something breaks... yea, I gave up on a gun or two before I learned that. Came back, put in a kick-butt spring and they work great.

[Prometheus]

Makes sense, too slow acceleration, so not enough pressure build up, therefore slow velocity of dart... and no dart leaving barrel. I've also noticed the you really need to watch the strength of the spring because even though you may be able to cock it, it might still strain the PVC enough to shatter around the trigger catch. Hasn't happened to me yet, but it's something I always watch for.

[Commonly Hunted]

Your responses are helpful but I most of them are things I know.

Carbon, I am going to use a .75" endcap like the one in your SNAP if lubing my current set up doesn't work. It will have wiggle room if yours did because I have a 1.25" body/plunger tube and a 1.5" rubber washer just like you. And since my plunger is inserted through the front of the gun, the washer cups forward and doesn't let any air past it. No loss of air around the plunger head at all. That might be slowing it down so I'm gonna lube everything up real nice. I know I need to because I don't even have to have the front of the plunger tube plugged to slow or stop the plunger, it is slow all by itself.

And the spring I'm using, FH, is of course the AR spring that Boltsniper uses. It is half of the spring and is longer than it needs to be to make sure that it gives enough force. I think the spring is strong enough so I don't need any other kind.

Promy, I was going to do the longer 1" section but now I'm just switching to the endcap plunger head. And the bolt carrier doesn't have a seal behind it yet because I haven't decided on how I want to attach it (another 1.5" rubber washer with the inner hole made bigger for air flow). Maybe nail it, screw it, or glue it. I prefer the first two so that it can be replaced but I don't think making holes in the carrier would be a good idea. And I don't think that the slightest of space between the outside of the carrier (1" thin PVC) and the inside of the body/plunger tube (1.25" PVC) is enough to lose all pressure made by the plunger.

And if all fails, I'm going to have to change to O-rings. I really don't want to.

I have to take pictures of my stuff today for my presentation and lab report so you guys will probably see them by the end of tonight. And my SketchUp design which is mostly complete to the functional level (I only have the barrel, bolt, shell, and general shape of the body and receiver of the shotgun).

EDIT: I tried more lube and the first shot got half way through the barrel. Then, I super lubed it and the dart only moved an inch down the barrel, don't know why. Finally, I added a few layers of electrical tape around the back of the bolt carrier for a temporary seal and the stefan went 30 feet! Yes!

I know that it isn't much but with the improved plunger and carrier seal (neither will be done today, I think) I can improve the range to a much more acceptable one. And with the rate of fire that comes with pump-actions, it will be great.

Then, when I got 30 feet I showed my little brother the whole process of loading for now (about 45 seconds long) and shot it. The gun embarrassed me and I found out that the stefan got out of the 12" brass barrel but stopped in the barrel sheath, the .75" thin-wall PVC. I also found that it was because for some reason the stefan got chubbier and was harder to fit into the shell and it was the same stefan that I used for the previous three shots.

At first, the first time I ever shot it (Friday), I did put lube on the plunger head but not in the tube. And on the head I only used a slightly lube paper towel and wiped it on the rubber washer. Definitely not enough, but I didn't want to make the gun so messy.

I might be going to Lowe's to buy the endcap to replace the plunger head today but I doubt I will get it onto the plunger.

Still gotta take those pictures, though.

Edited by Commonly Hunted, 09 April 2007 - 08:51 PM.

[Prometheus]

Okay, sounds like the same problem my FAR bolt carrier had. I only had E-tape on it, but some spot I could see a sliver of light through, and my plunger seal was immaculate. I was losing alot of pressure when I plugged the one end and blew through the other. Then I added some glue, and now no seal problems at all. I'm thinking this is what happened to you, so glad you got it fixed.

[Carbon]

And since my plunger is inserted through the front of the gun, the washer cups forward and doesn't let any air past it.

I was doing the same thing, but used washers show that it doesn't stay cupped forward when you fire. This isn't a bad thing at all, it just means that the plunger head needs a slight bit of taper to allow the washer to wedge and make a seal. I'm thinking that a square cut plunger head wouldn't work at all.

[Commonly Hunted]

Yeah, I have a square cut head so I'm changing to the end cap today.

And I didn't take the pictures because it was late and I wanted to watch House instead.

Also, does any one have an idea for attaching the rubber washer on the back of the bolt? I can't decide on nail, screw, or glue. Other suggestions would be nice.

Edited by Commonly Hunted, 10 April 2007 - 08:58 PM.

[SHADOW HUNTER ALPHA]

Also, does any one have an idea for attaching the rubber washer on the back of the bolt? I can't decide on nail, screw, or glue. Other suggestions would be nice.

Glue it. The repeated force from the plunger impact might worsen the cracks or holes caused by nails and screws, and you don't want the bolt breaking apart when you fire the gun.

[Flaming Hilt]

I would actually suggest screwing it. With a small screw and a clean hole. Clean holes don't crack. And materials (ie screws) almost always hold better than glue.

[Meaker VI]

Ummm... Well, 2 things; first: nice job on continuing this project despite nearly all of the rest of our having fallen off.

Second, I'd also suggest gluing the seal to the bolt (if it's the part I am understanding it to be). You'll have shells being rammed up against it and an ejector running through it and nowhere to put screws that won't be under constant impact. Beside that, BoltSniper's FAR does fine and he glued the seal down.

[Commonly Hunted]

With this brass system, I don't need that kind of seal. I'm talking about the black line behind the bolt carrier.

I have the rubber washer for it but need to know how to attach it.

I don't think screws would fit because there are only two layers of PVC that are stationary in the bolt carrier. Same goes for nails because the things would just dig into the space between the layers and tear them apart.

I guess I have to go with glue. Does any one know any glue that can hold strong but has some kind of remover? Like how rubber cement can be removed with paint thinner I think.

[SHADOW HUNTER ALPHA]

Fishin' Glue is supposedly the "strongest shit you will ever get your hands on," according to FA24. However, that means that it is also a bitch to remove. Epoxy and Loctite glues may work. Also, Guardsman's Goof-Off is an excellent glue remover.

[Commonly Hunted]

What kind of glue does it remove? Any?

I tried the .75" endcap plunger head and it isn't working so I'm going back to mine. And it might be because the cap is too round since they don't sell the square ones anymore.

[Prometheus]

Acetone removes most super glues and Amazing Goop for PVC, but being a ketone I'm thinking you might need to replace the O-ring afterwards...

[Meaker VI]

Ah. I see what you're getting at now, and as far as I can tell, you're not going to get anything strong enough to hold up for that part, without re-routing the air from the plunger around it or something. I'd suggest putting the o-rings around the back of the bolt, seeing as you could put several since weight and friction don't matter for that part and you'd like to keep the seal as tight as possible; and flat out ditch the neoprene washer plan. Unless you want to drill very small holes in your bolt (probably 3-6 in ring fashion), and then put either nails or screws through your washer into the bolt. For additional strength you could super glue the tips (not getting any glue on your washer) and secure it that way, but you could test it without the glue.

And an issue that I'm just noticing - you'd need to seal around the hole in that washer so that air doesn't go around the sides of the bolt through the gap between the washer and bolt.

[Commonly Hunted]

I tried gluing the washer on with hot glue but the glue kept drying before I even touched the parts together. When I finally got it, I found that the washer will not work because it gets pushed over the lip of the the bolt carrier and that doesn't fit. So I have to change to O-rings.

Also, I can't get it to fire anymore. I went back to the way I had it when it was firing and it doesn't work. Tomorrow, I'm making a new shell and dart because I believe that they are the problem.

[Commonly Hunted]

DP

What do you guys think of a foam rubber seal? I'm thinking of cutting a craft foam circle to use as the seal behind the carrier because it is much softer than the rubber washer and should be able to form around the carrier and create a seal.

And are there any other suggestions to improve the overall seal? I have the shotgun back the way I had it when it first fired but with a new shell and the dart just barely reaches the end of the barrel. I'm still gonna make a new dart but I have a new reason for that. When I have it in the barrel, I can see light going around it, so air must be escaping around it.

[Prometheus]

Yeah, that's definitely a problem. You might even want to try a progressive barrel, or a new barrel, or new foam. If you can see light around your dart, that is probably the biggest culprit.

[Ronster]

How long is your berrel. That could be part of the problem.
The barrel is too long, thus the overall friction of the barrel against the dart is too high for it to even leave the barrel. Last I saw, your barrel was huge. Cut it down to maybe a foot at most, then do tests to find the effective barrel length for your gun.