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New semi auto build

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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New semi auto build

Unread postAuthor: Datsun » Mon May 12, 2014 4:44 am

Hey guys,

I'd like to build a semi auto air rifle, however I have a few question.

I'm planning on using a qev as my main valve and a slide valve to pilot it.

As for the ammo I'm going to machine a tee piece loader with a dent in it.

For the barrel I'm looking to go 6mm.

I'm wondering for the best flow rate and fps should I use a bigger qev perhaps 3/4''? Instead of 1/4'' which seems to be commonly used. Same goes for the slide valve, should I use a bigger 3/4'' one instead of its 1/4'' counterpart?

I'm happy to spend a few hundred on parts, so let me know what way you think I should go.


Kind regards,

Datsun.
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Re: New semi auto build

Unread postAuthor: Ragnarok » Mon May 12, 2014 8:08 am

Datsun wrote:I'm wondering for the best flow rate and fps should I use a bigger qev perhaps 3/4''?

Absolutely no point. The barrel itself can only support so much flow.

A slightly larger valve diameter than the barrel can sometimes be worth it if the flow is then carefully and smoothly transitioned into the barrel, but most of the time, reducing back down to the barrel diameter is just as much of a flow restriction as the smaller valve would have been in the first place.
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Re: New semi auto build

Unread postAuthor: jackssmirkingrevenge » Tue May 13, 2014 8:10 am

Ragnarok wrote:Absolutely no point. The barrel itself can only support so much flow.


Yes and no... for an exhaust valve, a larger valve can equal the barrel blow in less time, so you might see a marginal increase in power.
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Re: New semi auto build

Unread postAuthor: Ragnarok » Tue May 13, 2014 11:11 am

That's of negligible concern. Once a valve opens on a timescale of less than a few milliseconds (as QEVs do), it means that the projectile is only at less than maximum acceleration for only a tiny portion of the barrel length (given that the projectile is starting from a stand-still and isn't moving very far in that time).

~~~~~

To grab some simulated figures for Timberwolf (which has needed rethinking after you lot decided it needed a resetting valve), it's not important that the valve takes 1.5ms to open, even if that's a fifth of the 7.5ms a 15 gram projectile is in the barrel for, because that same 15 gram projectile has moved only about 7 millimetres along the one metre barrel in that time.

That's less than a percent of the "distance" in the equation of "energy equals force times distance", and even then it's had an average acceleration of nearly 4000 G on it over that distance, which is actually more than the average along the entire barrel length.

Even if I could make the valve open instantly, a muzzle velocity of ~275 m/s would improve by less than a tenth of a percent - completely imperceptible amongst natural shot to shot variability.

~~~~~

With that in mind, the added cost of a larger valve and the extra reducing fittings is completely unneeded. You'd almost certainly improve muzzle velocity more just by cutting the barrel length half an inch longer.
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