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FAR help

A quick question about spring rate and airseal

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#1 They call me Tom

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Posted 20 January 2013 - 06:57 PM

Well, I'm sure I'm not the only one, I saw the FAR and had to build my own. Unlike most people, I have access to a lathe, which makes it all the easier. I have run into a rather large problem, I'm not getting anywhere near the ranges I should be.
I have a brass barrel, and brass inserts in my shells, the caliber is micro. I'm not losing air pressure or volume, I've filled the cylinder with water to see leaks around the piston seal, there is none. All these lead to one part - the spring.
I bought my AR15 action spring online, but I cannot believe it would be so defective; I'm getting less then 20' of range.
I've tried loading rounds in the barrel directly, in case it was getting hung up on the shell to barrel lip,there was no change. I've tried thinning the rounds, to reduce friction, no change.
Any help?
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#2 roboman

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Posted 20 January 2013 - 07:14 PM

Pictures would help immensely. 9637K26 works well for most homemade springers, and it should be pretty similar to the AR-15 spring.

Of course, your dart fit could just be terrible. Is your foam snug in the shells?

Out of curiosity, what kind of a lathe do you have?
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#3 They call me Tom

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Posted 20 January 2013 - 07:23 PM

This is my last post for the day, so I'll answer any further questions by editing this post.
I will have pics asap, the blaster is not available currently.
Dart fit has been adjusted and retested numerous times, using streamlines. I have also tried different barrels, and changed the fit of darts by thinning them out (rolling in my fingers) or thickening them up (electrical tape) for the different barrels. I've tried 1/2 PVC, brass, and CPEC?
My lathe is a belt driven south bend with a 3" chuck and (I think) a 2' bed. It's WWII surplus. Only issue is I don't have a live-center.

EDIT:
So I can just order a cut-to-length spring at the same length as the action spring? Good idea!

Edited by They call me Tom, 20 January 2013 - 07:25 PM.

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#4 roboman

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Posted 20 January 2013 - 07:47 PM

South Bend makes nice things. Those old lathes are bulletproof.

Anyways, McMaster's definition of "cut to length" means you'll be getting an 11" long spring. Alternatively, I believe they offer that spring in 36" lengths. A Dremel or angle grinder with a cut-off wheel works well to trim it, since it's thick, hardened music wire.
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#5 hamoidar

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Posted 20 January 2013 - 09:54 PM

Okay, I also have built a FAR, and know a thing or two about them.

1. Are you using stock nerf darts?
2. If the answer to #1 is yes then there is your problem, as the heads of the darts will cause friction in the brass barrel. Therefore, preventing high velocities.
3. Is your plunger head to plunger tube seal good?
4. Is the shell to bolt seal good?
5. Is the shell to barrel seal good?


If you are not using stock darts then your problem is probably a seal. I had to remake the bolt/ejector 4 times before I got it to work!
Feel free to PM me!

Good luck!

Hamoidar

Edited by hamoidar, 20 January 2013 - 09:54 PM.

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#6 They call me Tom

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Posted 21 January 2013 - 12:56 AM

1) Steamlines and stefens, so
2) n/a
3) Yes, I filled the tube with water and none leaked past my seal.
4) Could be an issue, but I've tried with and without orings in the bolt face, without any change.
5) I've tested by plugging the barrel with my thumb and blowing into the rear of the shell, no leaks.
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#7 Exo

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Posted 21 January 2013 - 02:36 AM

AR15 buffer tube springs aren't all that powerful compared to [k26]'s, and they're way too damn expensive.
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#8 Guest_Argoms_*

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Posted 21 January 2013 - 08:12 AM

Remember to re-lubricate everything with a plastic safe lubricant. I don't think it would cause range drops that great, but it may have some impact on your ranges.
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#9 They call me Tom

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Posted 21 January 2013 - 02:20 PM

Many people have used AR15 springs to great effect, but I do understand a [k26] would be even better. I just cannot believe my spring would be so defective as it is...good thing it didn't end up in a firearm!
I work in the plastic industry, and I use a food-grade oil that is perfect for nerf applications. I don't even know it's name, there's no public product similar that I've seen. We just call it food-safe oil; it's a very very refined vegetable oil.

My rifle is more of a prototype/tester right now, I will need to make a new receiver once I get this problem sorted out, But it does currently fire. I can feel the air it moves, and by my calculations it should be moving the correct volume of air for the barrel size; it's the rate of the air that's the issue. I'll be heading to McMaster soon, I work close and we do will-call orders all the time.

Edited by They call me Tom, 21 January 2013 - 02:27 PM.

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#10 Zorns Lemma

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Posted 21 January 2013 - 02:33 PM

1) What volume of air are you moving? The original FAR design actually had a very small plunger volume.
2) How heavy is your plunger head and how much friction is there in the plunger chamber? I suspect your seal is in fact too good and/or your plunger head is too heavy leading to poor power delivery. Your plunger setup should move easily just with finger strength.
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#11 andtheherois

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Posted 21 January 2013 - 02:38 PM

I would say switch to silicone grease. Organic oils tend to attract pests. It could also be the issue of the plunger rod. The one in the FAR is rather bulky from what I remember. On top of that if you used ORings that may be causing a friction issue. I would suggest redesigning the plunger rod and seeing if that doesn't help a bit as well.
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#12 Meaker VI

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Posted 21 January 2013 - 04:20 PM

It occurs to me; you say you're using streamlines instead of the darts Bolt was using? Did he design that blaster to fire megas or minis? In either case, I know it's meant to fire non-streamline darts, and you've mentioned that you changed the barrel geometry. It might be that your barrel is now to long/short for the darts being launched.
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#13 They call me Tom

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Posted 21 January 2013 - 05:55 PM

I have changed the barrel size, and the ammo type. My barrel is currently cut about 5" too long, but I don't think that would cause enough friction to go from 50-70' to 10-20'.
Please correct me if that's wrong, that just seems like a huge drop off for 5" of friction.

I will cut it down to see if there's any change. I'm currently at around 21.9 cubic inches of cylinder volume, and while I'm running a foot of barrel, it should be just under 7".
As for weight;
I've used a light and a heavy piston, neither made much of a diffence. I don't remember the weight now, but I do know it weighed less then bolt's.
I used o-rings, but then I switched to a rubber cup seal. It does move with a finger push.
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#14 They call me Tom

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Posted 21 January 2013 - 05:55 PM

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The shell isn't final, it just has that ring to align it in the chamber.
The receiver will be rebuilt with trigger catch in correct location.

Edited by They call me Tom, 21 January 2013 - 06:19 PM.

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#15 andtheherois

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Posted 21 January 2013 - 06:19 PM

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The shell isn't final, it just has that ring to align it in the chamber.


Your problem, right away, is that rubber washer. I can tell you right now it's too thick. Switch to a 1/16" neoprene washer, you should be much better off.
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#16 They call me Tom

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Posted 21 January 2013 - 06:53 PM

Um, I'm about 85% sure it is 1/16.
Cut-down barrel gave a 14" range.

Edited by They call me Tom, 21 January 2013 - 07:22 PM.

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#17 snakerbot

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Posted 21 January 2013 - 07:52 PM

I'm currently at around 21.9 cubic inches of cylinder volume

No you're not. That's more than double what a typical snap or rainbow nowadays has.

While you seem to have covered all your bases, I think the bolt to shell seal could use another look, as could the shell to barrel seal and the plunger seal. Also, how is the dart fit? You said you tried a few different things, but some more info could help us. Do you need to twist the darts into the barrel? Can you push them in? If you blow down the barrel, do the darts move?
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#18 They call me Tom

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Posted 21 January 2013 - 08:07 PM

diameter in place of radius, duh
5.5 cubic inches. So, 1.375 cubic inches of barrel volume. Still 7 inches of barrel.
Dart fit is tight, but I don't need to twist them in. I can blow them through rather easily, and I will get orings on the two joints as soon as possible.
I just retested the plunger seal, and a small amount of water leaks past the rubber seal.
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#19 Guest_Argoms_*

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Posted 22 January 2013 - 12:57 PM

Just something to try:

Prime the blaster with an empty (no dart) shell in it, and pour water down the barrel. You sound like you're having a bolt/barrel seal problem, but it might not be immediately obvious where this is.
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#20 Daimler

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Posted 20 June 2013 - 04:52 PM

If you are still encountering issues, I have built a version of the FAR I called the BANR and I believe I can help you out... So here are a few things to try or look for.

1. You have previously stated that your seal from the barrel to the shell and shell to the plunger tube/bolt are water-tight. This is excellent and I do not believe that the rubber washer or the M16 spring (as someone previously named as the main issues) are the reason for your depreciated range, as I used an almost identical setup. However, I have to ask you if, when you fire the gun and release the plunger covering the end of the barrel with your hand, does the plunger advance slowly forward or advance like there is nothing stopping the air. If it advances slowly forward over the couse of a few seconds, then you have a good plunger seal and have nothing to worry about. Also, make sure it is properly lubricated.

2. You stated that you are using streamlines for ammunition. If you are using 0.5 inch brass tubing, these actually work very well aside from the randomness of their flight pattern. I have found that they expand at the hallow end from the compressed air and grab the barrel, producing an excellent seal. If you are using 0.5 inch cpvc you have to use micro stephans almost exclusively. Streamlines are slightly too large for the size of the cpvc, which does not allow the dart to easily leave the barrel. I would also recommend not using cpvc, as the material has a higher coefficient of friction than brass tubing. If you are using regular 0.5 inch pvc, as in Boltsniper's original plans, I would suggest adding brass tubing to the barrel and the shells, as I have found the inner diameter of this tubing is too large for the plunger volume and getting it to seal well with stock nerf darts or macro stephans is quite a challenge.

3. Have you checked the alignment between the shell and the barrel? Look down the barrel into the chambered shell (without a loaded dart) and see if there is a lip or non-parallel alignment at the junction. If there is such an occurrence, it will grab the dart and impart friction, or if it is a more severe difference it will let the dart just leave the barrel and fall to the floor. If you had access to a lathe, this should not be an issue however, unless the tape you used to hold the tubing in the barrel was non-concentric to 0.5 inch pvc.

4. A lesser known issue that has not been mentioned previously was the issue of bounceback encountered with the bolt. This issue arises from the plunger hitting the end of the plunger tube, resulting in a bounceback that overcomes the spring force from the bolt spring as used on the FAR. Boltsniper remedied this with a complex locking mechanism in his SCAR model as did I in a much simpler way. I would say that a locking mechanism is almost essential as it prevents the parasitic effect of bounceback and at the same time provides a secure seal between the shell and the bolt and barrel.

5. You stated that you shortened the barrel and the range was reduced. Therefore, did you create a new one to the dimensions as specified by Boltsniper's equation? If not, I would explore this possibility as well since barrel length is critical to the function of the gun.

If you still encounter problems with your version of the FAR, feel free to ask questions. ~Daimler
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#21 Carbon

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Posted 20 June 2013 - 07:45 PM

That was...a good post. However, this thread hasn't seen any action in half a year, and OP hasn't even signed in since March. Check the dates before posting to avoid raising the dead.

Closed.
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