Erratic FPS out of 16" but consistent out of 18"

[joelogic]

Two similar AR15s.  JP LMOS BCG, JP SCS, SLR adjustable gas block.  

 

1)18" JP .223 barrel, tank brake, and rifle length gas.

2)16" Faxon .223 barrel, surefire brake, mid length.

 

Load 77gr TMK, 23.5gr R15, mag length.

 

Gun 1 has been my long time go to match gun. I wanted to build a lighter local match gun and gun 2 was the result.  

 

Gun 1, MOA or better.  20 FPS ES and 7 SD.  Average V 2550fps

Gun 2, Inconsistent MOA or better.  186 fps ES and 70 SD. Average V 2308fps

 

What gives?  Ammo is from 1 batch.  I am loosing a lot of velocity.  Does 2 inches make that much of a difference in burn?  Elevation and Temp were consistent.  

 

Just for another data point Gun 3, 14.5' carbine length gets 2330 average fps, 97 ES and 35 SD.

[TonytheTiger]

Every barrel is different. This isn't too uncommon.

[dillon]

More likely it is the gas tube length affecting this. More powder is burnt before the gases enter a rifle-length gas tube, which probably reduces ES. This would be a good experiment.

[gmantwo]

Seems I remember a test someone did where they cut the barrel back by 1/2” at a time. I believe each inch generally accounted for 50fps muzzle velocity 

but my memory isn’t what it was