AR15 pistol length location and port diam?

[pogo]

I am finishing a 300/221 fireball project, and have chambered, profiled, and test fired the barrel.

I've decided to go with the pistol length gas port, but I've never worked with a pistol length gas block rifle before.

I've sure looked, and asked around, but I can't find references for how to fit the a barrel for the pistol length gas block.

1) So, how far from the barrel extension shoulder to the gas block shoulder on an AR barrel?

2) Assuming a non-adjustable gas block, what is a good starting gas port diameter for the pistol length 300/221 fireball?

[tp555]

Length from bbl ext to gas block cut 3.6" . Gas port is .295" from gas block cut
forward. For gas hole what length bbl and mfg? For 16" try about .080" or .110"
for 10" bbl.regards.

[pug]

Everybody is so far ahead of me it isn't funny. I just chopped a blank and did a test threading for the extension so I still have a bit to do while I wait on the reamer. Gonna try a 10.5" pistol barrel first.

[jarhead1086]

I'm further behind than you Pug. I haven't even got my 1:6.5 PACNOR blank yet. I just ordered it a week ago. I've been looking at the MGI adjustable gas tubes and I'm wondering how hard it would be to shorten by swapping the carbine tube with a pistol tube? I will call MGI tomorrow and see if they would make a pistol batch. I preferred the larger adjustment knob I had on my SSK gas tube, but they wouldn't sell them alone. I'm planning on a 10 or 12" for my M-16 with enough blank left over for a 16" for an AR upper. I think the MGI would work great by making a knob that reached down to the adjustment screw so you could adjust with your fingers externally between off-super-sub. Retain the knob to the f/f handguard with a c-clip/o-ring contraption. Armalite is making a neat block for their suppressed AR-10 and it has 2 positions that adjusts and looks similar to a FAL gas plug, but the positions are only 45 degrees apart. I haven't got a chance to disassemble one of those yet. I would like to build on that idea with one more position (off).

[pug]

The MGI pistol length is doable since I just shortened one and reground it to put on my Oly but there appears to be two different pistol lengths out there so be sure of your parts before anything permanent is done. I am with you on having off, low and high positions on the gas manifold. Actually there is right much bypass of gas even with the MGI turned all the way down so wouldn't really say it has a off position and not quite as pleased with its performance as I had hoped.

[pogo]

Thanks for the information, guys. I am all ears.

What was that about two pistol length standards?

A bit more information on my build:

It is a 1/10 (I know) twist on a 24" blank of unknown lineage (I forgot, it has been a long while since I bought it). I cut it down to 18", and I will cut it back to 16.1" when all is said and done. I chambered it with a reamerrentals reamer (*fantastic* reamer), and profiled it to .875" under the handguard to carbine length, and .75" the rest of the way to the muzzle. Balance is pretty good with the heavy stock I have on that lower.

I planned to shoot either the 175 SMKs subsonic or the 125 Speer JHP varmint bullets as I have plenty of these bullets and likely always will.

I planned to put a carbine length gas system on it per the 300 fireball FAQ advice. I will want to cut it down if I go the short barrelled rifle route, and then put a suppressor on it, so I decided to go with a shorter gas system.

I was going to cut 2" off the carbine gas length and cut down a rifle gas tube. I was going to start with a small gas port and work up until it functioned with a 125 and 175 grain load.

I wanted an adjustable gas block as recommended by the same FAQ. That is interesting information regarding the MGI block. I was looking at that, but it was not available in my diameter and I didn't want the 1913 rail interfering with a free float handguard. I'll make my own somehow.