Does a Heavier Bolt and Less Gas => More Velocity

[Religious Shooter]

I read here and in other places that a given load was good for a bolt action but isn't good for the AR15.

My AR's have lightened bolt carriers and adjustable gas blocks.

Suppose I wanted to get max velocity out of my AR15. If I went back to the original bolt carrier and turned down the gas... does that generally mean I can push up the velocity a bit via powder charge increase or maybe even actually get more velocity from the same charge?

The way I am thinking is that if I decrease the gas and increase the bolt carrier's weight my AR... my AR becomes more like a bolt-action.

Less BCG velocity... more lock up or dwell time... => more energy/gas is used pushing the bullet out and less in the extraction?

Edited September 2, 2008 by Religious Shooter

[Patrick Sweeney]

In a bolt gun you have one variable (relatively speaking) to deal with: maximum chamber pressure at ignition.

With an AR, you have two: max chamber pressure, and port pressure/gas flow at the port location.

Assuming you load two or three loads that have the same, safe, maximum chamber pressure, powders of three different burn rates, you have the following:

The "Baby" powder (fast-burning) burns fast, but loses bore pressure as the bullet travels past the port. Low gas system pressure, short-stroking.

The "momma" powder (medium-rate) burns at the right rate, and you have a safe chamber pressure and a port pressure within the guns limits. Normal function.

The "daddy" powder (slow-burning) keeps the same safe pressure, but because the curve is extended down the bore, you get a too-high port pressure. The system is over-driven and you have difficult extraction and short parts life.

Yes, you can use a slower powder to get more velocity, but you run a higher port pressure. you can use your adjustable gas block to throttle tha gas flow back, but the problem is this: you have not way, short of testing to determine if your gas flow is too little, right, or too much.

You could bust a rifle finding out what "just right" is.

Then, how much does extra velocity get you? 2-300 fps is good for how much less drop at 300 yards? A couple inches? Not worth it to me.

[rvb]

Less BCG velocity... more lock up or dwell time... => more energy/gas is used pushing the bullet out and less in the extraction?

I'd think the bullet would be loooong gone before that bolt even comes close to un-locking. It's damn near the muzzle screaming past the gas port at 99% it's final velocity and the gas has to start accelerating the carrier and caming the bolt before the bolt can unlock. As Pat says there are lots of other considerations when loading for the autoloader but it's not like your pistol where the breach unlocks strictly based on newton's laws. The mass of the bolt/carrier effects recoil felt obviously but the breach should be locked snug as a bug until the bullet is well on it's way down range, imo.

As far as the gas systems effect on bullet velocity, pressure obviously bleeds off at the port. But how much that really effects velocity is left up to smarter folk's than I. I guess you could weld up or plug the port on a barrel to test and see...

edit: or maybe I mis-understood the question... ? Are you really asking about simply bleeding off less pressure vs trying to keep the bolt closed? I first read it that you thought the bolt was unlocking before the bullet was out of the barrel...

-rvb

Edited September 3, 2008 by rvb

[Religious Shooter]

Then, how much does extra velocity get you? 2-300 fps is good for how much less drop at 300 yards? A couple inches? Not worth it to me.

To me the drop is actually secondary. I want to smack the bejeebus out of the plate!!!! Almost to the point of being irrational about it.

Yes, you can use a slower powder to get more velocity, but you run a higher port pressure. you can use your adjustable gas block to throttle tha gas flow back, but the problem is this: you have not way, short of testing to determine if your gas flow is too little, right, or too much.

Well... if I turn the gass all the way off and turn it on a 1/4 of a turn... shoot a round... if it doesn't function turn it 1/4 of a turn... shoot another round... add 1/4 of a turn if it doesn't function... etc. until it functions.

Wouldn't this just put the force on the screw in the gas block and not subject the upper to undue battering?

[Religious Shooter]

edit: or maybe I mis-understood the question... ? Are you really asking about simply bleeding off less pressure vs trying to keep the bolt closed? I first read it that you thought the bolt was unlocking before the bullet was out of the barrel...

I guess more towards "bleeding off less pressure".

Let me ask you this... given a load ...why does a bolt action (generally) generate higher velocities than a semi-auto with the same load?

AFAIS the difference is that in a semi-auto 1) the breech opens up (with some of the gas expelled out of the breech area) and 2) gas is bled off the gas port to drive the bolt open. So instead of using as much energy to drive the bullet, some of the energy is instead being bled off to drive the bolt.

So if you delay the opening of the bolt (and therefore reduce some of the gas coming out of the breech) and if you reduce the gas being bled off to unlock and work the bolt (again increasing the gas driving the bullet)... shouldn't you get more velocity?

[rvb]

edit: or maybe I mis-understood the question... ? Are you really asking about simply bleeding off less pressure vs trying to keep the bolt closed? I first read it that you thought the bolt was unlocking before the bullet was out of the barrel...

AFAIS the difference is that in a semi-auto 1) the breech opens up (with some of the gas expelled out of the breech area) and 2) gas is bled off the gas port to drive the bolt open. So instead of using as much energy to drive the bullet, some of the energy is instead being bled off to drive the bolt.

So if you delay the opening of the bolt (and therefore reduce some of the gas coming out of the breech) and if you reduce the gas being bled off to unlock and work the bolt (again increasing the gas driving the bullet)... shouldn't you get more velocity?

I'm not sure I agree that you're loosing pressure at the bolt end. The bullet should be well on its way downrange before the bolt opens. To loose pressure at that end means the brass is not sealed to the chamber or it has blown up. There is a sequence of events... the bolt or carrier don't do a thing until the bullet passes the gas port and the pressure impulse is great enough to start accelerating the carrier. Once the carrier has moved sufficient distance and the bolt cammed out of the lugs it can start opening up. If that all happened while the bullet was still in the barrel I'd think you'd see a lot of shrapnel instead of reloadable brass.

I'm no master AR builder, just built a few for me and friends, but my engineering intuition says a lighter-weight carrier would need less force to accelerate it rearward, ie need less gas.

As to the why is velocity less and the best powders, I really dunno. I'm just getting into loading for long guns. I'd listen to Pat though and other with lots of experience loading for ARs as guys like that know what powders hit that sweet spot to make the gun run well and for a long time. With an adjustable gas block, your method sounds like what I'd do though. Find a load you like and a gas port setting that lets the gun lock back on an empty mag and you should be good to go, I'd think?

-rvb

[Para 16]

Kinda split'n hairs on this question,no offense, but I doubt you'd see any appreciable gain in speed. I think the same line of thought could be used controling the recoil. If you have a adjustable gas block try closing it all the way, fire a mag's (at least 30 rounds) worth over a chrono. Then try the same thing with the block set at its "normal" setting. Chrono/Record that data as well. Let us know what you find out. You might also try asking this question at ar15.com.