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Observations On The Velocities Obtained
From A Direct Impingement Gas System AR-15
Compared To A “Single Shot” AR-15


I conducted a simple test that consisted of a comparison of the velocities obtained from an AR-15 with the direct impingement gas system with a standard sized gas block, to that of the same barrel with no gas system at all; that is, I completely clamped-off the gas port, turning the AR-15 into a "single-shot" rifle.

Using hand-loaded 69 grain Sierra MatchKings, I fired three 10-shot strings of this ammunition in a row over my Oehler 35-P chronograph positioned 21 feet from the muzzle using a 16” barreled AR-15 with the direct impingement gas system with a standard sized Larue Tactical low-profile gas block.

After firing the three 10-shot strings from the upper using the direct impingement system, I let the barrel cool and then removed the gas tube.  Next, I removed the low profile gas block from the barrel, turned it 180 degrees and reinstalled it on the barrel, thereby completely clamping off the gas port on the barrel.  This gave me a single-shot AR-15 in which I had to manually load and eject each round using the charging handle, giving me the means to determine just how much velocity (if any) is lost due to the auto-loading function of the direct impingement gas system of the AR-15.

Following the same procedure as previously used, I fired three 10-shot strings of the same hand-loaded 69 grain Sierra MatchKings over the Oehler 35-P chronograph positioned 21 feet from the muzzle using the single-shot AR-15.  The results are show in the table below.

As you can see in the table below, there was  a small (but statistically significant) difference of 23 FPS between the grand averages of the velocities from the direct impingement gas system and the single-shot (gas port clamped-off) system.






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good info.  Most people don't think about it but it does loose a little with the gas port.  Also mag na port quad porting reduces velocity 6% on a glock 17.

What is the standard deviation and extreme spread of velocities from your handload?

Magnaport did some really nice porting on some custom SIG P229 and HK USP barrels for me, but I find that with good form, a 9mm really doesn't need porting.

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Always useful information from Molon.

Thank you.