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BBMG detent alternative

This area is for the discussion of BB or airsoft sub machine guns, and or other small arms such as pellets.
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BBMG detent alternative

Unread postAuthor: jackssmirkingrevenge » Mon Aug 22, 2011 5:41 am

As some people are already aware, adding an airtight detent to the breech of a BB machinegun is a good way of boosting power as it allows pressure to build up in the BB chamber before the projectile is fired, translating into higher muzzle velocities.

The downside is that in most cases, a second BB tends to block the detent before the first has left the muzzle, resulting in erratic firing patterns and even barrel collisions which can damage the launcher.

Here's a theory on how to achieve high BB chamber pressure without the need for a detent. The breech is blocked off by a spring loaded piston which essentially acts as a pop-off valve. This gives a minimum firing pressure before the breech is opened and BBs can be fired.

Another alternative would be to have the BB chamber permanently pressurised, with the breech closed off by a balanced spool valve which is directly connected to a trigger, ensuring there will always be maximum pressure behind the BBs.

Thoughts/comments?
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bbmgdetentalternative.png
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Unread postAuthor: POLAND_SPUD » Mon Aug 22, 2011 6:11 am

bbs need time to agitate... something tells me the pressure will drop significantly before the first bb enters the barrel

so in my opinion you need to find a way to block the next bbs from entering the barrel before the previous one leaves it

remember the design with an air cylinder that blocked the flow of bbs ? that seems promising
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 22, 2011 7:10 am

POLAND_SPUD wrote:bbs need time to agitate... something tells me the pressure will drop significantly before the first bb enters the barrel


I see your point but in this case if the pressure drops the piston will re-close...

remember the design with an air cylinder that blocked the flow of bbs ? that seems promising


You mean something like this? With a weak spring on the piston, in theory it should work well enough, but there's a bit too many essential o-rings for my liking...
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Unread postAuthor: wyz2285 » Mon Aug 22, 2011 7:23 am

As always jack, your designs work in theory, but hard to do in reality :roll:
there are too many parts that need to be done precisely to be airtight :shock: I like bbmgs because they are simple, if I really want power I would build a qev+pop off and magazine feed
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 22, 2011 7:37 am

wyz2285 wrote:As always jack, your designs work in theory, but hard to do in reality :roll:
there are too many parts that need to be done precisely to be airtight


Don't forget I have a lathe/mill now, so I can't use that excuse anymore ;)
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Unread postAuthor: wyz2285 » Mon Aug 22, 2011 7:40 am

so why don´t you make them, I would love to see them in action :D
(even through I don´t think that´s going to happen :D )
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 22, 2011 7:41 am

wyz2285 wrote:so why don´t you make them, I would love to see them in action :D


That's the point of this thread, trying to do some public brainstorming to see what will work best before I start making swarf ;)
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Unread postAuthor: wyz2285 » Mon Aug 22, 2011 7:48 am

I have more faith in the frist one :)
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Unread postAuthor: POLAND_SPUD » Mon Aug 22, 2011 7:50 am

but there's a bit too many essential o-rings for my liking
naaah not really...

remember that you have a lathe... if you're really worried about leaks then you might as well put a small equalisation hole between the inside of the air cylinder and the outside of the gun
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 22, 2011 8:25 am

Here's another simpler variation that should work well without o-rings apart from the detent:
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edit: variation of the airsoft blow forward valve
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Unread postAuthor: POLAND_SPUD » Mon Aug 22, 2011 9:45 am

nahhh try harder... :D

too many moving parts that can be blocked by bbs
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Unread postAuthor: saefroch » Mon Aug 22, 2011 9:53 am

I doubt the first one you posted will work. I see a stepped piston and a pop-off valve, but I find it unlikely that the piston will retract far enough to let a BB agitate and load before it closes again.

The second design seems a more likely candidate, but I'd re-orient the BB chamber and air inlet, so the BBs are held in a thinner, more vertical chamber, so as to give good agitation when the valve triggers and blasts a lot of air out the barrel. However, this design again suffers from the problem that you must vent a lot of gas from the chamber to get flow and agitation for feeding.

Third design doesn't seems too complex and not serviceable at all.

I'm a big fan on the second-to-last design. Slap an o-ring onto the bolt piston thinger and it should work. My only concern would be finding an appropriate spring.

On the last one, the piston area ratios seems a bit small and thus prohibitive of rapid cycling, but it has the same merits as the one previous to it, with the possible issue of running out of space to cut an o-ring to provide sealing between the bolt and its housing. It's also not adjustable, which could make it a PITA compared to the previous one when getting it initially set up.

Any questions? :P

EDIT: In response to Poland's comment, you could just put a vertical BB chamber below (not directly below, but lower in altitude) the mech so that the BBs will settle down and away from it while the BB is being forced through the detent.
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Unread postAuthor: POLAND_SPUD » Mon Aug 22, 2011 10:12 am

bbmgdetentblocker.png is really promising... I don't see any need to modify it
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Unread postAuthor: warhead052 » Mon Aug 22, 2011 11:01 am

Honestly Jack, do what you think will work best. I agree with poland, bbmgdetentblocker.png is pretty promising.
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 22, 2011 1:19 pm

POLAND_SPUD wrote:too many moving parts that can be blocked by bbs


I don't think so, considering it will only move when a BB gets stuck in place, and one that happens, flow stops.

I doubt the first one you posted will work. I see a stepped piston and a pop-off valve, but I find it unlikely that the piston will retract far enough to let a BB agitate and load before it closes again.


The idea is to have a high flow valve that will allow a long enough time interval for the BBs to go through. I also envision setting a pop-off pressure significantly lower than the feed pressure.

The second design seems a more likely candidate, but I'd re-orient the BB chamber and air inlet, so the BBs are held in a thinner, more vertical chamber, so as to give good agitation when the valve triggers and blasts a lot of air out the barrel. However, this design again suffers from the problem that you must vent a lot of gas from the chamber to get flow and agitation for feeding.


You're always going to vent some gas before a BB reaches the breech with any design so it's pointless worrying about it ;) In the case of clouds a verically oriented chamber is indeed a better idea in terms of space distribution.

Third design doesn't seems too complex and not serviceable at all.


Much as I hate cutting threads, attached a more plausible version.

On the last one, the piston area ratios seems a bit small and thus prohibitive of rapid cycling, but it has the same merits as the one previous to it, with the possible issue of running out of space to cut an o-ring to provide sealing between the bolt and its housing. It's also not adjustable, which could make it a PITA compared to the previous one when getting it initially set up.


What will govern rate of fire is flow into the barrel, the pistons only serve as blockers that actuate when there is pressure in the barrel and release when pressure is released.

Honestly Jack, do what you think will work best.


The attached should work beautifully, I'm trying to figure out if there's a simpler way op putting it together.
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