PCC Loads and PF: blowback vs. MPX (rotating/locking bolt)

[Sdlrodeo]

Ok, So I have been shooting my Sig MPX kitted out for PCC. I have a ‘newer’ generation factory carbine barrel (13” +ringed birdcage). I have no problem making minor power factor with my normal 128-130PF loads for Carry Optics and Production. But, I’ve talked to some folks who are not able to use the same pistol round in their blowback AR style PCC.

 

So, my theory is that with the blowback, enough gas pressure is lost between ignition/extraction and the bullet leaving the barrel that it is slowing down the velocity with the increased friction of the barrel. But in a gas system with a rotating/locking bolt, the timing works so that the bullet is out of the barrel before the casing is extracted due to the time it takes to rotate the bolt and extract the case. I’m sure there is some negligent bleed of gas through the gas system itself, but since that is soooo short in the MPX, I don’t even consider it.

 

I say this because ‘normally’ most loads actually increase due to more of the powder being used in the longer barrel. In fact, in my MPX, I see a 1 or 2 point increase in PF whereas folks with blowback operated PCCs sometimes see decreases in PF.

 

Thoughts?

Steve

[Aircooled6racer]

Hello: Some barrels are faster than others and some are just slow. The JP barrels are on the slow side with the Faxon/KAW barrels being on the fast side. Wilson Arms barrels are on the fast side but a little slower than the Faxon/KAW ones. Thanks, Eric

[IntenseImage]

See my post about Chrono data ... MPX w 2 different barrel lengths plus 2 blowbacks w different lengths ... All same ammo

 

http://forums.brianenos.com/index.php?/topic/260813-Took-our-3-PCCs-out-for-some-ammo-testing-...-Taking-the-good-with-the-bad!

 

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[Sdlrodeo]

15 minutes ago, IntenseImage said:

See my post about Chrono data ... MPX w 2 different barrel lengths plus 2 blowbacks w different lengths ... All same ammo

 

http://forums.brianenos.com/index.php?/topic/260813-Took-our-3-PCCs-out-for-some-ammo-testing-...-Taking-the-good-with-the-bad!

 

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Intense, thanks for running those chrono loads and providing that data. Interesting read. I do have to admit I have only been able to chrono my loads between my SP-01 Shadows and my MPX. I have not been able to chrono the same loads out of a blowback PCC. That being said, I had increases in PF from the pistols to the MPX. I have ‘heard’ of others who shoot the loads that make minor PF out of their pistols but do not make minor PF out of their blowback PCCs. 

 

I wonder if there is a point of diminishing returns or even an opposite affect depending on the load. I’d like to try some side-by-side-by-side testing with two PCCs (MPX/Blowback) having nearly identical barrels, and pistol with the same load.

[IntenseImage]

I actually made a mistake in mine too ... The MPX test barrel was a 14.5 not a 16 like I originally thought. So that test compared a 14.5 blowback to a 14.5 mpx :-)

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[Sdlrodeo]

49 minutes ago, Aircooled6racer said:

Hello: Some barrels are faster than others and some are just slow. The JP barrels are on the slow side with the Faxon/KAW barrels being on the fast side. Wilson Arms barrels are on the fast side but a little slower than the Faxon/KAW ones. Thanks, Eric

 

Definitely truth to that. I have two SP-01 Shadows that give a 1.5 PF difference with the same load. (10 round group, avg velocities).

[Sdlrodeo]

1 minute ago, IntenseImage said:

I actually made a mistake in mine too ... The MPX test barrel was a 14.5 not a 16 like I originally thought. So that test compared a 14.5 blowback to a 14.5 mpx :-)

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Fair enough. I remember reading somewhere that SIG specifically designed the carbine barrel to be less than 14.5 as it was apparently more efficient. Could it be that the 1” change with my factory carbine barrel made the difference? At some point there is a diminishing return on a longer barrel.

[IntenseImage]

 
Fair enough. I remember reading somewhere that SIG specifically designed the carbine barrel to be less than 14.5 as it was apparently more efficient. Could it be that the 1” change with my factory carbine barrel made the difference? At some point there is a diminishing return on a longer barrel.

I'm sure there may be a point where length adds enough drag to slow the bullets but the 16" blowback was still the fastest of the group

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