SpudFiles

So I've been brainstorming about automatic bb/pellet guns.




Red is rubber/o-rings, green is the hammer, the pink thing is the sear.

Here's the idea: you've got a hammer that slides along the breach of the barrel, pushed by a spring. The hammer strikes the valve pin, which consequently closes the end of the hammer, so that no air can get into the barrel. At the same time, the forward motion of the hammer exposes a hole in the breach for the ammo to enter the barrel from the mag/hopper. The pressure in the space between the hammer and the reservoir rises as the valve vents into it, untill the hammer starts sliding backwards. This closes off the loading port, and then the valve closes, and then the valve pin unseats from the hammer and allows the high pressure air stored between the hammer and reservoir to escape down the barrel.

Whaddya think? Will it work? It could be a lot of machining if you wanted it to be. Can I get the hammer to slide far enough to cover the ammo port? Will the hammer get enough power to cock itself? Will I loose efficiency b/c the effective chamber volume is just that space there, rather than the air coming directly to the pellet from the high pressure source, as in a normal hammer vavle?

Wow, it looks good. I actually had to turn my music off and tell my gran to shut up to concerntrate and understand the mechanism. It all looks fine, except how fast do you think the hammer will unseat from the pin to allow air into the barrel?

Can I get the hammer to slide far enough to cover the ammo port?

You should be able to with a long, but not too strong spring behind the hammer

Will the hammer get enough power to cock itself?

Why shouldnt it. There is no real pressure against it sliding far enough back to load the BB and hit the pin.

Will I loose efficiency b/c the effective chamber volume is just that space there

Just use high pressure in a small volume instead of low pressure in big volume, or you could add a T and a chamber adjacent to the valve for more volume. It doesnt really matter if its direct or not, because compressed air doesnt go "stale" or lose any "oomph" if its stored in a secondary resivior.

All this is just opinion though, so I hope its generally correct

Updated design, with simplified sealing face.



The bolt/hammer is green. The cycle is the same as before.

So the bolt opens the valve but locks off the air from entering the barrel, air pressure moves it forward blocking off the ammunition slot then opening to allow the air through. Pressure drops - spring moves bolt back - cycle is repeated?

You got it!

I'm thinking the bolt will have to be pretty massive, so that A) it plunges the valve wide open and thus gets a long power stroke on the way back B) it shoots forward enough to allow good airflow into the barrel.

I can see at least two flaws. One, there may not be much of a power stroke; the piston won't move far under full pressure before the barrel is opened, because the hammer won't open the valve very far, because as soon as it opens, the full force of the air is already going to hit the hammer. In fact, I think that pretty much means that the thing won't work. There's a small chance it would, though, b/c the projectile will keep some backpressure on the chamber for a little while.

Another flaw is that the deadspace created by the hammer/piston operates basically as secondary firing chamber, where a normal hammer uses the air directly from the reservoir, which increases efficiency.

JSR, your hammer design gets around the first flaw in mine, but I think we are both stuck with the second one. The blowback on the caselman avoids the second one too!

The blowback on the caselman avoids the second one too!

The Caselman though is using 3,000 psi to start of with, that's almost 30 times more pressure that I have to play with, so replicating that mechanism isn't really an option. The blowback area of the bolt can't be the same diameter as the barrel, at least from what blowback experimentation I've done that's what I concluded.

Have you ever tried using a large blowback diameter?

While fiddling with blowback here I proposed this modification, but never actually made it:



I'm more inclined to go for blow forward designs however, the way I see it blowback systems risk opening the ammunition feed port before the projectile leaves the barrel, something which the blow forward design is unlkely to do.

I don't think it is possible... (opening the ammunition feed port before the projectile leaves the barrel)

even with heavier projectiles, like marbles, the bolt is likely to be much heavier and would have more friction to overcome than ammo

POLAND_SPUD wrote:even with heavier projectiles, like marbles, the bolt is likely to be much heavier and would have more friction to overcome than ammo

Yes, and it's heavier too, but it has a much shorter distance to travel - you might have to play around with spring strength to get it right.

yeah you just have work out how to seal the bolt on jsr blowback so air doesnt leak out in to the barrel. hmmmmmmmmmm ill stick with my single shot bolt action for a while un till i find a good semi plan thats easy to make or even auto but the problem i face is that getting an ammo that fits perfectly in the barrel and is round with no bumps or oblong shapes like the ones i can find from like the cheap shops here they sell really cheaply made necklasses but the roundness of the plastic is crap but hey they work for now untill i find a shope that sells christmas decorations during the middle of the year lol oh wells ill find something later