Baffle suppressors increase variance of muzzle velocity?

[dbooksta]

I took a Gemtech M4-02 to the range with my chronograph last week and was surprised to see that it consistently DOUBLES the standard deviation of muzzle velocity out of an AR-15. Is this typical of baffle suppressors?

More detailed: When I add the can to the end of the rifle I get muzzle velocities that are on average 50fps higher, but the standard deviation of velocity goes from about 22fps to 45fps!

I expected the increase in velocity, but the substantial increase in variance seems like an undesirable effect.

Has anyone else chronographed a rifle with and without suppressor and seen this?

[silencertalk]

You should try it again to make sure. It would be interesting if true.

[cyclone72]

freebore boost,


2) Freebore boost. The phenomenon known as "freebore boost" occurs in particular suppressor designs and sometimes requires a particular loading of ammunition. It is caused by a primary expansion chamber ahead of the barrel's muzzle which acts as a barrel extension as a bullet passes through it. Propellant gases continue to expand inside the chamber and push the bullet through the baffle stack at a slightly increased velocity. The suppressor can actually increase bullet velocity up to about 40 feet per second (12.1 m/s) depending on the design.

[fizassist]

More detailed: When I add the can to the end of the rifle I get muzzle velocities that are on average 50fps higher, but the standard deviation of velocity goes from about 22fps to 45fps!

What kind of statistics did you have?

[David Hineline]

My guess is first shot pushing bullet against air in silencer slower velocity, as shot string continued trapped air in heated silencer got hotter and less dense with ever shot. Thinner heated air produced less resistance to bullet flight, more velocity. Try the same test again waiting a couple minutes between each shot to wait for silencer and air in silencer to cool and regain it's density.

That's my story and I'm sticking to it.

[dbooksta]

David Hineline has a good theory. I guess we need to look at the actual speed of each bullet and the time of each shot in the string: If that theory's right then we'll see the velocities increasing when shot in close succession.

Maybe I'll even take a temperature probe to see if we can get a correlation of velocity with suppressor temperature.

But I won't be able to get to this for a week or two. Kind of hard to believe I'm the first one to look at this....

[cyclone72]

freebore boost,


2) Freebore boost. The phenomenon known as "freebore boost" occurs in particular suppressor designs and sometimes requires a particular loading of ammunition. It is caused by a primary expansion chamber ahead of the barrel's muzzle which acts as a barrel extension as a bullet passes through it. Propellant gases continue to expand inside the chamber and push the bullet through the baffle stack at a slightly increased velocity. The suppressor can actually increase bullet velocity up to about 40 feet per second (12.1 m/s) depending on the design.

i still say its freebore boost.

[Mongo]

My opinion/theory on this would be due to retained heat in the barrel/action is higher. Each round will cause the barrel to heat more with a suppressor than w/o thus giving you different velocities. the warmer a barrel is the faster the powder will burn with each round and therefore the greater the difference in velocities between shots. Residual heat in the can could also increase free bore boost.

I think the above is more likely than the change in air density in the can being the culprit.

[renegade]

freebore boost,


2) Freebore boost. The phenomenon known as "freebore boost" occurs in particular suppressor designs and sometimes requires a particular loading of ammunition. It is caused by a primary expansion chamber ahead of the barrel's muzzle which acts as a barrel extension as a bullet passes through it. Propellant gases continue to expand inside the chamber and push the bullet through the baffle stack at a slightly increased velocity. The suppressor can actually increase bullet velocity up to about 40 feet per second (12.1 m/s) depending on the design.

i still say its freebore boost.

Everyone agrees it is free-bore boost, what is being debated now is the cause of the variance in the boost, not the boost itself.

[BookHound]

What was the affect on accuracy?

Mark

[dbooksta]

Changes <50fps aren't going to affect point-of-impact within 100 yards, but I am concerned that the variance in velocity could also mean there's turbulence or other perturbations being introduced in the can that could affect accuracy.

I'll need to find a bolt gun with heavy barrel to look at the accuracy effects, since my AR-15 isn't very accurate to begin with.

[fizassist]

Maybe I'll even take a temperature probe to see if we can get a correlation of velocity with suppressor temperature.

For a string of shots, temperature should be nondecreasing with time. You could shoot a long string (10-15 shots?) and look for increasing velocity (a slope to your data points). That would tell you whether or not it's worth trying to correlate velocity with temperature.

[dbooksta]

David Hineline has a good theory. I guess we need to look at the actual speed of each bullet and the time of each shot in the string: If that theory's right then we'll see the velocities increasing when shot in close succession.

Maybe I'll even take a temperature probe to see if we can get a correlation of velocity with suppressor temperature.

But I won't be able to get to this for a week or two. Kind of hard to believe I'm the first one to look at this....

Well I finally ran this experiment. Results and chart are detailed at http://emptormaven.com/2007/03/freebore-boost-effects/ but here's the conclusion:

I went back to the range with a stopwatch and a Mastercool infrared thermometer.

What I found is that muzzle velocity is strongly correlated (R2 = .60) with suppressor temperature. I.e., once your can is hot your bullets will go faster. (Several people hypothesized that this is simply because hot air is less dense, so it offers less resistance to the accelerating bullet. Another hypothesis is that the propellant burns more completely or more rapidly in the superheated air it encounters in the baffles, which increases the rate at which it can propel the bullet.)

How hot does a suppressor get? For this experiment I shot Federal XM193 5.56mm (55gr Boat-tail ball) ammunition. On average each shot heated the 16 ounce suppressor by 10 degrees F. During one phase of the experiment I shot 30 rounds in under 90 seconds, which raised the suppressor temperature from 110 to 410 degrees F. During that extended string of shots my standard deviation was only 19fps — which is about the same variance I see when shooting without a suppressor. It took ten minutes for the suppressor to cool back down below 150F. (Ambient conditions were 62F, 25% humidity. Surface temperature in the sun was 88F. Winds were 10-15mph.)

If I may summarize liberally from the results of the experiment: A baffle suppressor essentially has two states, which we could call “Hot” and “Cold.” A suppressor is Hot when either (1) shots are fired in quick succession or (2) its temperature is above about 150F. In the first case I imagine that the baffles are still full of hot propellant vapor; in the second case the can is radiating enough heat to keep ambient air at this elevated temperature. A suppressor goes Cold as soon as ambient air seeps back in.

Using this equipment (16″ bbl and 6″ suppressor) the muzzle velocity out of a Hot suppressor is around 50fps higher than out of a Cold one. And if we look at the standard deviation of muzzle velocity when shooting Cold it comes out the same as when shooting Hot.

[BoilingLeadBath]

Aerodynamic drag in the suppressor probably isn't the cause.
If we can say that the drag is comparable to what it would be in free air, then the velocity change over such a sort distance would be less than 1 fps at STP.

Perhaps the pressure in the can is significantly over atmospheric, but the only way for that to happen would be for the gas to pass the bullet - which would:
1) Require the gas to pass the bullet - the "freebore" boost that others speak of. And so isn't even the same effect anymore.
2) Require the insertion of fresh gas, which wouldn't have had much time to change temperature, reducing the effect of suppressor temperature.

*****

By the way, how many shots were in each string?

[boatbiologist]

New perspective on a march '07 post? This wont live long... but Funny as Hell anyway

[joshsc]

Best thread I've seen in a while. A+. Would read again.