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.

[DonT]

Here is another option for an adjustable gas block:
http://www.jprifles.com/1.4.6.php

I have a JP on one of my rifles and it is nice quality. He even makes a low profile for floating handguards.

Just a thought....

DonT

[pug]

I guess I just threw confusion into the whole thing but OA (if using any of their parts) does things their own way sometimes and you should get the specs off the parts you are actually going to use. You can buy the low profile adjustable gas block and pistol length gas tube and save yourself a lot of hassle. I believe OAs tube may be a little shorter but M1S lists a pistol length tube also. The MGI is an adjustable tube and still needs a gas block. I went the MGI route because I needed some additional info that their tube design would provide otherwise I would have gone the JP route all the way.

[pug]

And just to give me peace of mind tonight I pulled the barrel nut to double check what I had written down and the OA 300 Fireball is 3.2" from the shoulder. Port is still located 0.295" from breach edge of gas block band. There were some posts mentioning the shorter length and the Oly cycles the lighter bullets well so I personally would use the 3.2" myself if I decide to put the gas system on my pistol project. I see a big difference in cycling 150s reliably and shooting 180gr full power loads if I don't change the gas setting. The buffer goes thump giving a jar if you don't turn the gas down on the full power 180s. My gas port is .101 now but start small. Hope the info helps.

I would like to point out to anyone coming along reading these threads that the Olympic Arms 300 Fireball ran flawlessly as delivered and it was my own needs and ideas that led to modifying it.

[jarhead1086]

When shooting such a diverse range of bullets as you can in the Whisper you really need an adjustible system. I forgot to de-tune going from 220's to 125's and it was stiff on the rifle and it launched my brass like an AK to the point it would bend the rim a bit from the violent extraction. I will look at the JP a little closer. I also thought about bushing a 9mm bolt and going straight blowback to ditch the gas system. Form 1 a cantilevered suppressor that also acted as the barrel nut without all the gas tube BS in the way. You could get a high volume can in a very short NFA package. The full house light bullets would have an RPM like an AM-180 though huh? Turn a jack rabbit into a skin frisbee : )

[pug]

The JP block is the easiest fix. Whether is is the best I'm not sure. I have seen some complaints from people who actually do turn theirs up and down a lot say the screw gets loose. I can't speak to that because I haven't tried it. I went with the adjustable gas tube partly because I thought the tube was split in two pieces with the valve in between which would have worked good I think. It actually is a one piece tube with the valve slipped over it and pinned. The valve pin is screwed down inside the gas tube and since the pin can't be the same size as the tube it can't completely cut off the gas. I am still thinking on a three position selector as well as one with changeable inserts. Probably just wasting time but got to waste it on something . Right?