Velocity loss from gas port question

[hermosabeach]

Ok- I'm a nerd. I saw a 24" 300 win mag AR and my brain jumped to, how much velocity does it lose using a standard gas system designed around an18" barrel versus engineering a power closer to the muzzle?

I then started to wonder, how much velocity do you you in a standard semi auto due to the gas port bleeding off gas before the projectile exits the muzzle?



In the FN FAL you have an adjustable gas block from zero to open.

I seem to recall the M14 had a similar set up for launching lines off of ships with blanks.

So do you know how many FPS you lose with a gas port?

[hermosabeach]

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One answer


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https://www.shootingillustrated.com/content/ar-15-gas-ports-impacts-on-accuracy-and-velocity/

[jamesweed]

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One answer


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https://www.shootingillustrated.com/...-and-velocity/

The article is a great write up. It makes you wonder if he would have had the same results gaining velocity with the gas block fully open on the Grendel if it had a rifle length gas tube like used for the 6.5cm and 308.

[hermosabeach]

I understand harmonics are an issue.... so a free floated bolt gun is different that a semi auto with gas tube attached to the barrel

I'm curious as to how a gas port would impact the 24" semi in 7 mag.

[Featureless]

[jamesweed]

I understand harmonics are an issue.... so a free floated bolt gun is different that a semi auto with gas tube attached to the barrel

I'm curious as to how a gas port would impact the 24" semi in 7 mag.

I believe it would be a similar result to the 6.5cm if a rifle length gas system was used or possibly a tad greater because of the slightly longer barrel length.

Being the rifle length system is the longest of them it's port is further down the barrel trapping pressure just that much longer before it starts to relieve any and it's still going to be building some velocity down a "24 barrel

[divingin]

Easy way to tell is to find a rifle with an adjustable gas port. Fire and chrono a dozen rounds in the normal configuration, then close off the port and fire and chrono a dozen more. Should get you fairly representative data.

[FNGGlock]

It seems the differences are marginal at best, here is another data point for you - using 308.

https://www.youtube.com/watch?v=FQ9rr1zBLvY

[ar15barrels]

When I did gas port sizing experiments for 6.8spc on the AR-15 platform, I did chronograph some barrels before and after drilling the gas ports.
I saw an average velocity loss between 3fps and 7 fps after drilling the gas port on the same exact barrel.
As a side note, I was also seeing 30fps extreme spreads so the before and after velocity differences are smaller than the extreme spread across a 10 round string of fire with factory ammo.
Enough testing could likely get you to find a barrel that got faster after drilling the gas port simply because you could have 1-2 rounds of the 10 round string with higher velocities effecting the average velocity of the string.

Ok- I'm a nerd. I saw a 24" 300 win mag AR and my brain jumped to, how much velocity does it lose using a standard gas system designed around an18" barrel versus engineering a power closer to the muzzle?

I then started to wonder, how much velocity do you you in a standard semi auto due to the gas port bleeding off gas before the projectile exits the muzzle?

In the FN FAL you have an adjustable gas block from zero to open.

I seem to recall the M14 had a similar set up for launching lines off of ships with blanks.

So do you know how many FPS you lose with a gas port?