Upper w/pistol length gas tube question

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wabbitzombiehunter
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Upper w/pistol length gas tube question

Post by wabbitzombiehunter » Mon Oct 08, 2012 12:04 am

I have the CMMG Upper w/pistol length gas tube and am wanting to know why is it not recommended to shoot supersonic?

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Re: Upper w/pistol length gas tube question

Post by BELT_FED » Mon Oct 08, 2012 5:12 am

Im assuming this is a 16" barrel with a pistol system on it.. if so an adjustable gas block will fix it..

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Re: Upper w/pistol length gas tube question

Post by wabbitzombiehunter » Mon Oct 08, 2012 4:59 pm

It is a 16" with a pistol length gas system will I have to open up the gas port
thanks

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Re: Upper w/pistol length gas tube question

Post by Dolomite_Supafly » Tue Oct 09, 2012 7:19 am

You will not have to open up the port for subsonics.

You will have to meter, with a smaller port, the gas for super sonic ammo though. I used a Paladin Gas block to meter my super sonic loads. Unfortunately with a .936" gas block location, which you have, the Paladin gas block will not fit under the free float tube or handguards. In order for that to happer the gas block location needs to be turned down to .750".

You DO NOT want to fire super sonic ammo on an unmetered pistol length gas system. You will damage you gun eventually.

You can also use a standard adjustable gas block. You will need to turn it all the way in then count the revolutions out until the supersonics function. This will be you supersonic setting. Then continue to unscrew teh setscrew until the subsonics cycle and this will be your subsonic setting.

For your gun, with a 16" barrel and pistol length gas system, to run subsonics the port size should be .088"-.093". And in order for your supers to run the gas port size must be .068"-.070". The only way to do this is with an adjustable gas block. The Paladin runs a wide open setting for the subsonic setting and you have a .088"-.093" gas port in the barrel. Then in the block itself it has a metered port which is .070".

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Re: Upper w/pistol length gas tube question

Post by Klem » Tue Oct 09, 2012 8:25 am

Dolomite_Supafly wrote:You will not have to open up the port for subsonics.

You DO NOT want to fire super sonic ammo on an unmetered pistol length gas system. You will damage you gun eventually.

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Why such a forceful piece of advice against using supers with pistol length gas systems.

Comparing Blackout port pressures against 5.56NATO (Quickload) the Blackout cycling pressures are far less severe. The chamber/port pressure at the 4" mark (pistol length block) with a typical 300BLK supersonic load is around 17K psi. A typical 5.56NATO at the 7" (carbine block) is much more, 27K psi. Heavier buffers are now used to take the sting out of 5.56 in carbines but they function and have done for years with normal buffers. 5.56NATO with pistol gas systems can reach over 40K psi at the block.

Compare all that against 17K psi and I am wondering what's the concern here?

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Re: Upper w/pistol length gas tube question

Post by Dolomite_Supafly » Tue Oct 09, 2012 5:36 pm

It is the amount of barrel beyond the port that is the issue not the pressure at the gas port.

When the gun unlocks there is still 12" worth of pressure in the barrel. Also there is a lot more volume of pressure than the 5.56. And that volume, along with the pressure, exerts a lot more force on the moving components of the gun.

When you compare a carbine 5.56 to a pistol 300 the 300 has 1.5x the amount of volume of gas to the end of the barrel. And that extra volume combined with the pressure cannot be good for the gun.

There is a reason that a 16" gun needs to be metered when it has the pistol gas system. With a carbine length system it isn't as critical because the amount of barrel past the block is reduced.

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Re: Upper w/pistol length gas tube question

Post by wabbitzombiehunter » Tue Oct 09, 2012 7:07 pm

Thanks for all the help I'll get an adjustable gas block an give it a try

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Re: Upper w/pistol length gas tube question

Post by Klem » Tue Oct 09, 2012 7:10 pm

I understand the length after the port concept or 'dwell time', and yes, totally agree with you there. The longer the dwell time the more the expanded case in the chamber under pressure competes against a gas system that wants to open it.

As for volume of pressure, I understand they are interrelated but am confused here. The old PV/T formula; pressure, volume and temperature. Are you saying that although the pressure at the port is lower, because the volume of a 300 barrel is larger than a 5.56 it exerts more force on every square inch of inside barrel surface, than looking at the pressure alone?

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Re: Upper w/pistol length gas tube question

Post by Dolomite_Supafly » Tue Oct 09, 2012 10:21 pm

It isn't that is is exerting more force on the barrel or at least not enough to hurt it. It is just that the more volume is going to apply the pressure to the case and in turn the bolt. Even though the pressures might be the same the surface area increases the amount pressure applied. And this is likely going to cause increased bolt speed and reliability issues. I also believe the extra bolt speed will cause damage some where because of the extra stresses associated with it. It if wasn't an issue then every gun would have a pistol length gas system and port sizes of .120" for reliability.

Ideally, at least for me, I want the bolt to come back far enough to lock on an empty without pounding the buffer into the end of the buffer tube of the gun. If I could tune it to where the buffer never touched the bottom of the buffer tube that would be great. Unfortunately with the variations between ammo that would be nearly impossible to do.

Something else. I replaced my standard buffer and spring with one made of delrin and it uses a 20 pound 1911 spring rather than the standard spring. The weight of the moving parts of the new setup weighs under 2 ounces while the old setup was a lot heavier, I think close to 5 ounces. And with the lighter setup you can feel a reduction in the amount of felt recoil as it bottoms out. So when you overspeed your gun that extra speed no doubt causes an increase in felt recoil, especially if you have a heavier buffer.

Here is the setup I built for my gun:
Image
And it does reduce the amount of felt recoil as well as the fact it seems to cycle quicker. I do not have an antibounce weight installed and it works great. If it were to be a FA gun it would need the weight though. And in that case it would be easy enough to add.

I am sure there are others smarter than I that can explain it better but that is why we don't want to shoot supers out of a unregulated pistol length gas system intended for subsonics.

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Re: Upper w/pistol length gas tube question

Post by Klem » Tue Oct 09, 2012 11:21 pm

Like the idea of the '1911' buffer, nice work!

Since posting I have been doing a little Googling on pressure and force in fluids, Bernoulli's ideas etc. The idea being that measured force is pressure over an amount of surface area. For example, snow-shoes vs. normal shoes, or a karate chop vs. and open palm strike. This being the case for similar peak chamber pressures between 300BLK and 5.56 with same bolt face/lugs will be similar recoil. With a larger volume/surface area in the 300BLK barrel however I imagine once the bullet gets going the force drops quicker and at any particular point (e.g. gas port) will be lower, requiring you to open it wider or use different powders if it doesn't cycle fully.

As far as how much force is directed through the gas system, causing the rearward motion of the carrier I'm thinking it's still the typically lower pressures caused by the 30cal barrel.

Look...I am not saying you are wrong and I value your thoughts, so maybe I'm not understanding things and that's got me stumped.

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