velocity changes with adjustable gas blocks

[taliv]

shot my new AR15 upper with an adjustable gas block over the chrono this afternoon. was fairly surprised with the results.

i'm shooting some cheap reloads that are reasonably accurate but have never had a great SD. so i shot them today, 40 degrees, and got a mean velocity of 2889 and standard deviation of 43 fps. this was for 15 rnds, so it's a fairly reasonable sample size... not like i just shot 3 rounds or something.

then i shut off the gas completely so that the bolt didn't cycle at all and got the following: 2869 mean and 15 SD.

that makes absolutely no sense to me. i would have guessed turning off the gas might have added a few fps, not drop 20. and it seems the only way the SD would improve so much would be if the gas system itself were responsible for quite a bit of deviation. that seems improbable.

anyone ever tested this before. any ideas?

 

[Lone_Gunman]

I think an SD of 43 fps on a round going nearly 2900 fps is pretty good.

 

[Dr. Dickie]

What was the range of the first set you shot?
What was the range of the second set?
Most likely you are the victim of the statisics of small numbers.
Next time try it the other way around. Start with the gas port closed and then open. You may even get the same results.

 

[taliv]

max: 2981
min: 2819
spread: 162 fps

with gas shut off
max: 2885
min: 2841
spread: 44 fps

 

[Owen]

What is the distribution of the velocities? Is it normal? If the curve isn't normal, the SD doesn't mean much.

I'm betting you have a few hot rounds in the lot, and were lucky enough to get them all in the "gas on" sample.

 

[taliv]

here's the "gas on" values
2819
2837
2840
2845
2858
2871
2878
2886
2900
2903
2906
2912
2912
2926
2963
2981

 

[dispatch55126]

Here's an offbeat thought. The rifle has the small hole in the barrel for the gas syphon. If you have the gas block turned off, there is still an increase in pressure in that tube as the bullet travels down the barrel and it may cause a slight air pressure "speed bump" as the bullet passes over the hole.