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Projectle friction in fully/semi auto BB guns - discuss?

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Projectle friction in fully/semi auto BB guns - discuss?

Unread postAuthor: SPG » Wed Mar 26, 2008 10:44 am

I've been looking through all the various BB auto threads again, and I'm wondering if maybe one of the issues why a lot of these won't cycle and/or have low power is lack of friction between projectile and barrel.

Airsofters talk about getting greater muzzle velocities through fitting tight bore barrels. Pellet airguns have rifled barrels and skirted pellets which expand into these, and a pellet in no way slides down the barrel with gravity alone.

Playing about with GGDT shows that this friction can make a difference to muzzle velocity, and that velocity appears to rise as friction increases to a certain point, before levelling off and then falling again.

So, I'm wondering whether too little friction is preventing any semi/fully auto action from cycling properly. In effect the air pressure on valve opening is moving the projectile too quickly down the barrel and there is never sufficient pressure build up to cycle the "bolt" against any spring, or even potentially against the friction in the action itself.

The solution proposed in the past has always been to lighten the bolt decrease the friction, weaken the spring, but perhaps the answer is tighten the bore?
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 26, 2008 10:48 am

In the case of the blow forward breech, it functions independently of projectiles in the barrel so that's not really an issue. It would make a difference with blow back actions on the other hand, where the time interval during which there's pressure in the barrel gives the impetus to the bolt to open again.
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Unread postAuthor: sniper hero » Wed Mar 26, 2008 11:02 am

if you find out the best solution it can make all bb auto's better. :D
but do you know if the most power comes when the valve opens or a little later because if you use a ball valve, it has more power on the moment it is totally open then when it is only half but if your barrel isn't long the projectile is already out.
but I know that some guns shoot better with less pressure if they are more efficient.
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Unread postAuthor: Jared Haehnel » Wed Mar 26, 2008 11:06 am

Thats an interesting point I hadn't thought of yet thats for bring it up I'm going to have to play with some numbers now...

I would agree with Jack...but it would make a difference in say a vortex block or a cloud BBMG....but say if you had a combustion BBMG it would make a rather big difference. Its worth experimenting with
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 26, 2008 11:08 am

Having the barrel tighter at the breech is a good idea for any pneumatic launcher, as it delays projectile motion while the valve opens, giving it time to reach maximum flow before it exits the barrel - a bit like putting a burst disk between the valve and barrel.

Alternatively you could add a spring loaded detent (with the advantage of being adjustable) as I had proposed here:

Image
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Unread postAuthor: Jared Haehnel » Sat Mar 29, 2008 9:41 am

Good point...I'm going have to crunch some numbers to see if I can't incorporate that into a design...
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