For a lightweight upper utilizing a rifle-length gas system, do you prefer a 17" or 18" barrel, and why?

-z

I prefer the 18" for balance and reliability.

What reliability problems have you had with the 17"?

I haven't used a 17".  The shorter you go the greater the chance of having a problem, so I only went down as far as 18"

Quoted: I haven't used a 17".  The shorter you go the greater the chance of having a problem, so I only went down as far as 18"

Hoplophile and I have similar setups.  Chopped FN 1/7 Gov't back to 18".  His is naked and mine has an A2 flash hider on it.  Gas ports were not changed.  He is using a rifle buffer, I am using a carbine buffer an a M93A stock.

I don't know if he has had any problems with his or not.  I know that when I first got it, I tried some low powered 55 grain russian ammo, and it wouldn't work if you let it free recoil.  It would eject the empty, but not get far enough back to pick another one off the mag.  Worked fine from the shoulder, I just couldn't fan the trigger with it!  I am sure that it would work now that it is all slicked up and broken in.

I did notice that once when I was using 45 grain Wally winchester varmint with a rifle buffer to try a RRA 2 stage, it seemed a little sluggish.  I don't know if it was or not, it just *seemed* that way.  With my non-dominant eye, I could see the empty come out of the ejection port and do a 180 degree turn  about 6-8 inches out of the ejection port.  I don't know, maybe I could see that all the time if I focused, but it felt sluggish too.  Like the bolt came back and went forward sooner and not as hard.  Rifle was pretty new then too, but I would think it would have been broken in at 450 to 500 rounds.

Of course, you are probably talking about a custom SS barrel, so any gas port issues are off.

How does this sound?  17.5 or 18 inches, Gov't profile except that the part under the handguards is .675 instead of .65 tapering to .60 at some points.  Follow all the Gov't  style contours except chop 2 inches off of the front, and beef up the part under the handguards a smidge.

I would either go with a 17.5 or an 18.  18 has a good visual balance as well as actual balance and I think that 17 would look a little weird.  Of course, I doubt it would matter much in use.  I doubt that you would use a front sight tower either and use a gas block, so it wouldn't look quite so weird.

I would show you mine, but the old link is dead.  I might be able to post pics later.

Ask MSTN what they use on their 3 gun uppers.

Forgot the pic.  Now that the ban has gone away this is getting threaded for a flash hider.


The only reliability issues I've had have involved Bernaul ammo with the painted cases getting glued to the chamber during extended rapid fire.  I'm pretty sure that isn't caused by barrel length.  

You're getting close.  The idea for this upper is something lightweight and "handy", but also with the "mild gas" advantages of a rifle-length system.  The barrel needs to be sub-MOA and stiff enough to not "lose it" shooting, say, 500 rounds in 20 minutes -- something I don't do regularly, but one of the more intense tactical 3Gun matches throws stuff like that in at the Army's request.   Since I already have a heavier 20" JP gun, I think it makes sense to go 17" on this one.

-z

Quoted: You're getting close.  The idea for this upper is something lightweight and "handy", but also with the "mild gas" advantages of a rifle-length system.  The barrel needs to be sub-MOA and stiff enough to not "lose it" shooting, say, 500 rounds in 20 minutes -- something I don't do regularly, but one of the more intense tactical 3Gun matches throws stuff like that in at the Army's request.   Since I already have a heavier 20" JP gun, I think it makes sense to go 17" on this one. -z

Damn, that is a lot of rounds per minute.  I remember you telling about that.  Gas blocks were working their way down the barrel.  Did steel gas blocks have any problems or was it just aluminum blocks?  Well, I guess it is only 25 rounds a minute and max sustained rate of fire is 12-15 rpm, so it sounds like it wouldn't be that bad for a 10 or 12 minutes at that pace, but 20 minutes would be smoking.

I think I remember mathmatically figuring that my barrel would weight very close to a 20 inch A1 profile.  I think that you would want a little more barrel mass than that to make it balance a little better.  Maybe .70 or .725 back under the handguards.  I personally wouldn't want to go any heavier than a .70 profile, but then again, I don't shoot 3 gun.

What type of handguards are you going with?  You need something that would let the barrel cool.  Maybe something that can act as a heat sink.  Let the handguard pull the heat out of the barrel and then let the handguard deal with getting the heat out.

I think you'd have less trouble from an 18".  I would bet you'd have to mess with the gas port on a 17".  If you're going to do that might as well go 16.5" and get it a bit shorter.  That way you could add in the flash supressor and not go over your 17" length.

I heard reports of at least 2 or 3 aluminum gas blocks moving, but no steel gas blocks.  If the gas block is the same material as the barrel, and it's physically blocked from moving, I think that's good enough.  Set-screw gas blocks can have a hole slightly countersunk into the barrel for one of the screws.

The barrel will be a Krieger 0.700" under the handguards and 0.740" forward of the gas block.  

Handguards will be PRI GenIII, and we'll use an aluminum barrel nut.

-z

Quoted: I heard reports of at least 2 or 3 aluminum gas blocks moving, but no steel gas blocks.  If the gas block is the same material as the barrel, and it's physically blocked from moving, I think that's good enough.  Set-screw gas blocks can have a hole slightly countersunk into the barrel for one of the screws.

I kinda wondered if that was the way it worked but I was not sure.  Now I know.

The barrel will be a Krieger 0.700" under the handguards and 0.740" forward of the gas block.   Handguards will be PRI GenIII, and we'll use an aluminum barrel nut. -z

Excellent choice, have you decided what  length to use yet?  I would strongly recommend against going less than 17 inches.  I think that going with less would make the gun a little on the whippy side.  Plus it would make the gun a little less tolerant of different types of ammo.  Not that you would have reliability problems, just that in theory it would make the gun less tolerant of different amounts of pressure and different pressure curves of different ammo types.

There are some that are recommending no shorter than a 18" but I am not sure that they understand that you are building an upper, not modifying a current one.   The gas system on an AR is by design pretty tolerant of different ammo the corresponding pressure curves, but the rifle length gas system seems to be particularly so.  I think it might have something to do with the fact that the pressures are lower out that far on the barrel and that the gas ports are larger.  Not that having a large gas port is what makes it run better, just that if you are tuning one, the next largest reamer isn't as big a step area wise as what it would be on a carbine.

Of course, whoever you have do the barrel will probably know what size of gas port you will need.

Who are you going to for the barrel?  I'm planning a Krieger for a custom AR-10 build in the future and need some options as to where I should go.

I'll try to answer all those questions at once.

With regard to the gas blocks..  My JP CTR02 has their aluminum gas block (set screw type) without any countersunk holes.  Dousing it with water periodically throughout the burner stage was enough to keep it from moving.  I had no malfunctions during that entire stage.  (I used the SS JP bolt carrier, not the Al one.)

I've decided on the 17", rifle length gas.   A shorter barrel will be slightly stiffer than a longer one, everything else equal.  The Krieger SS is supposed to be stiffer than other SS barrels, so that should help even more.

I'm having MSTN build this upper for me.   They've been building 17"'s similar to this one for 3Gun use for some time with good success, so I have no doubt the gas system will be set up right.   I can always switch out the bolt carrier for one that's lighter or heavier.   I don't know his source for Krieger barrels.. but I'd be surprised if it wasn't Krieger themselves.

CC, for the AR10/308 barrel, I highly recommend the JP Lightweight 20" barrel.   Get it threaded 5/8-24 and you're ready to go.  Mine is extremely accurate - shoots half MOA pretty consistently with Fed GMM 168gr.  Plus, you can use that sexy KAC gas block, or even a PRI flip-up for an AR10/SPR clone.

-z