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Accuracy Issues?

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Accuracy Issues?

Unread postAuthor: vng21092 » Tue Dec 30, 2014 1:43 pm

So I finally got around to finishing my old .177 build. I noticed my shots are really inconsistent, and I mean REALLY bad. At 10 yards I'm getting like 2-3 inch groupings and that is absolutely terrible. The barrel is in a fixed position so it doesn't move, which makes it all the worse. As far as I'm concerned, the barrel is straight, smooth and clean on the inside, so here are my assumptions:

(Muzzle loaded, 18'' barrel, 0.18'' ID, 4.5 mm BB (It's actually 4.4 mm, checked with digital caliper))
1) Barrel is slightly crimped at the end so BB won't fall through, might affect the way air flows in?
2) Magnetic detent, the attraction isn't strong by any means, a slight shake would free the BB. Would this affect the rotation of the BB as it flies out?

Image

Picture of terrible grouping (10 Shots), 2 hit the same place and 1 went god knows where, so you only see 8 holes. The 9th hole is because one bounced back out, you can tell by the way the hole is formed.

Image

Picture of barrel, epoxy covered so I'll show whats underneath

Image

BTW, saw this suggested by JSR in older thread, bought it, http://www.amazon.com/Serfas-FMP-500-Bicycle-Floor-Pump/dp/B000UV8I94/ref=pd_sbs_sg_5, GREAT pump, now operating at 260 psi (100 psi upgrade woop woop). Only worry is I noticed after each fill, a tiny bit of water forms inside the pump tube/ head, hopefully it doesn't enter my gun and rust the inside... O_o. I ordered a new barrel, same ID but made from aluminum, Amazon states "Seamless construction for a smoother bore than welded tubing" so I'll keep you posted.

So what are your thoughts on this issue?
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Re: Accuracy Issues?

Unread postAuthor: bravootome » Tue Dec 30, 2014 2:41 pm

dont know what to say but this is what i done after my gun was shooting exactly the same, and now i got better groups to 40 m than you got to 10...
most of the times the barrel is not streight even you think it is, and the main cause of the inaccuracy is a rotation of the bb ( i suggest you to push a bb trough the barrel with a stick and feel if the bb goes smooth all across the barrel with out any little restrictions - do this in first place )

So i cuted out few cm from the muzzel end of the barrel and recrown it . The first 10 shoots to 25 m was incredible accurate , i mean i could of shoot a fly by that distance. Anyhow now is shooting fine ( not shooting a fly, but close to).
so something was wrong with the end of my barrel, either it was not straight or i was fct up....cause of the hundrets of steel bb's i use. I did this to many of my oldest barrels and it fixed the problem for most of them. but first try a deep recrowning of the barrel.
Usualy aluminium barrels get bent very easy just by shooting and they are not good for more then 200-300 steel bb..after that they shoot like that...

Dont cut your barrel unleast it is the only solution . So happy new Year !!
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Re: Accuracy Issues?

Unread postAuthor: vng21092 » Tue Dec 30, 2014 3:08 pm

I don't really have the tools for recrowning so I'm gonna take a sheetrock blade and try to rim out the edges as smooth as possible. The barrel is spaced every 4 inches throughout the PVC pipe so hopefully it's as straight as can be, but thanks for the tip tho. Happy new year =]
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UPDATE

Unread postAuthor: vng21092 » Tue Dec 30, 2014 6:07 pm

THANK YOU BRAVOOTOME, after cutting the barrel tip off (4'' =\) and recrowning using a dremel sanding bit, accuracy became sooo much better. I should mention the barrel wasn't mounted this time so I was actively aiming, probably explains the loner... Check it:
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Re: UPDATE

Unread postAuthor: bravootome » Wed Dec 31, 2014 2:01 am

You had moved fast, still if you have a aluminium barrel it will be destroyed pretty soon. Find a steel barrel and shoot lead bullets.. and try 600 psi ..:"

a way to know if th barrel is bent is to now shoot at 15 or 20 yards, if you got same grouping but away from where you aim, than th barrel is a little bend ....
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Reconfigured...

Unread postAuthor: vng21092 » Mon Jan 05, 2015 5:34 pm

So I reconfigured the setup and took out the slide valve. At higher pressures the slide valves become less efficient and... ergonomic? I lose air when I pull it as a trigger, and I lose air when I push it forward to feed, higher pressure = more air lost. Also, since it gets harder to pull, it throws off my aim because I end up yanking the gun a bit. So here is the new setup:

Image

The "button valve" is just a 1/4'' NPT plug with a hole through the center. The holes on the side help increase the flow and redirect the air away from my eyeball. The stem is a 6/32'' screw with a o-ring on the inside, on the outside is just a nut. Behind the screw is a spring which also pushes the piston up against the barrel. Modified the piston to enable feeding through chamber. In case some of you don't know how this is done, I'll save you some searching. Two things must be done to enable filling through chamber.

1) Spring must be added behind piston so it pushes on the barrel sealing it (Not too strong)
2) Piston must be punctured to allow air to get behind piston, just make sure not to puncture any area of the piston that covers the barrel. Look here:

Image

Chamber got cut about .5'' and pilot volume definitely went down, almost non-existent. And without the wasted air the slide valve was causing, I'd say efficiency definitely improved quite a bit.

Old Stats:
250 - 180 PSI : 5 Shots
180 - 160 PSI : 10 Shots

New Stats:
250 - 230 PSI : 5 Shots
230 - 210 PSI : 10 Shots

Image

New Grouping: 10 shots, 6 holes

Image
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Last edited by vng21092 on Thu Jan 08, 2015 11:46 am, edited 2 times in total.

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Re: Accuracy Issues?

Unread postAuthor: Ianbuckwell » Tue Jan 06, 2015 8:48 am

Hi, sorry its off topic but what did you use to fix your scope to the gun? Thanks Ian
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Re: Accuracy Issues?

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Re: Accuracy Issues?

Unread postAuthor: vng21092 » Tue Jan 06, 2015 11:18 am

http://www.amazon.com/gp/product/B001WJ4BB2/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1
Barrel goes through one hole, and another piece of pipe through the other hole so they're parallel. They're both Schedule 40 1/2'' PVC. On the top pipe I used a dremel to cut grooves so the scope rings could clamp on. Like so:

Image
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Re: Accuracy Issues?

Unread postAuthor: Ianbuckwell » Tue Jan 06, 2015 1:44 pm

Thanks for detailled reply. It looks nice.
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Pilot Valve

Unread postAuthor: vng21092 » Thu Jan 15, 2015 9:29 am

Just to show how it works...

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Re: Accuracy Issues?

Unread postAuthor: tetra » Fri Feb 27, 2015 11:30 pm

Is there a chance you can re-upload the video?
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Re: Accuracy Issues?

Unread postAuthor: Doodmens » Mon Aug 17, 2015 5:04 pm

I don't quite get how you pilot it yet.. any chance you have a schematic?
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Re: Accuracy Issues?

Unread postAuthor: Brian the brain » Tue Aug 18, 2015 7:13 am

I'm guessing ...with that ghetto button valve..
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Re: Accuracy Issues?

Unread postAuthor: vng21092 » Tue Aug 18, 2015 4:16 pm

Brian the brain wrote:I'm guessing ...with that ghetto button valve..

Yah man...
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Re: Accuracy Issues?

Unread postAuthor: topelitemarine » Sat Sep 26, 2015 5:27 am

Maybe you should consider using airsoft barrel because they are made for accuracy and the less gap between the bullet and the barrel will enhance the velocity and the accuracy of the bullet. u can also install a system that most airsoft gun use. when the bullet is pushed from the barrel a hop up will make the bullet spin and create higher pressure under the bullet and allows it to increase ranges significantly.
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