Caliber choices for AR builds

Capt Skup · Mar 8, 2015

I like many panicked when the sky was falling and bought a handful of AR lowers. I have those built and am now starting the process of building the uppers. Half will be 5.56, one will be the .300Blackout, and I am thinking be nice to have a 6.8SPC rifle. Now here may be a silly question for those that have built the 6.8 but here goes. Can the 5.56 PMags be used for the 6.8? I haven't seen any 6.8 marked PMags as the stores or shows. Just curious. And if you can give any pointers for someone building the 6.8 I am all ears. Such as barrel twist rate. Any concerns to be aware of. Thanks for your help.

Boom Boom · Mar 8, 2015

Dedicated magazine.
http://www.stagarms.com/10-rd-6-8-spc-magazine/
http://www.stagarms.com/25-rd-6-8-spc-magazine/

my69rox · Mar 8, 2015

762 x 39 cheap , fun , easy to shoot love mine. Would chouse it over my 556 any day.

Overboost44 · Mar 8, 2015

Check out www.bisonarmory.com. They have many items dedicated to the 6.8. Most of my research has found barrels to be 1:11 or 1:12 twist. Also, everything I have read says that you can use pmags for 6.8. I have no experience with it yet, but have been researching parts and threads on this and other forums.
Precision Firearms had some 6.8 things the last time I checked.
www.68forums.com
https://www.saodind.com/uppers-specialty/6-8-spc-uppers

Capt Skup · Mar 8, 2015

Thanks guys, I really appreciate the information.

Capt Skup · Mar 8, 2015

First .30 caliber I thought of building for one of the AR's was the 7.62x39. Than I started reading and talking to many. Most told me better choice is the .300 Blackout. Most complained of feeding and accuracy issues with the Ruskie round. Price of ammo sure is attractive.

jimbobborg · Mar 8, 2015

I second Bison Armory for 6.8 stuff. I suggest if you're using it for hunting to get PRI mags, as they will let you load out longer than standard 2.600" OAL.

As for the 300 Blackout, make sure you mark the mags that they're 300. The 300 will chamber in a 5.56 upper with bad results.

Mike3888 · Mar 8, 2015

I just finished a 6.8. I used aero upper and lower. Used a 18" blackhole weaponry barrel.
Rock River 2 stage trigger. Surplus Arms carries the barrels and are about 100 bucks cheaper then buying through blackhole plus no wait. Added bonus is they give a free stripped upper with those barrels. I didn't know that till package came. Ordered ASC magazines through 44mag.com. This is my second blackhole barrel and couldn't be more satisfied. Enjoy your build. Let us know how you go.

FlatsFlite · Mar 10, 2015

My 20" 6.8 ARP is the most accurate rifle I own, better than a couple cut rifled barrels I have. Ask around on 68fourms. You can't go wrong with any of the above, but a melonited ARP is the best bang for your buck, and you lose nothing to the others.

FlatsFlite · Mar 10, 2015

my69rox said:
762 x 39 cheap , fun , easy to shoot love mine. Would chouse it over my 556 any day.

I have a 10.5" pistol that puts a smile on my face every time I dig it out of the safe.

No mistaking it for a 5.56.

photoracer · Mar 13, 2015

Capt Skup said:
I like many panicked when the sky was falling and bought a handful of AR lowers. I have those built and am now starting the process of building the uppers. Half will be 5.56, one will be the .300Blackout, and I am thinking be nice to have a 6.8SPC rifle. Now here may be a silly question for those that have built the 6.8 but here goes. Can the 5.56 PMags be used for the 6.8? I haven't seen any 6.8 marked PMags as the stores or shows. Just curious. And if you can give any pointers for someone building the 6.8 I am all ears. Such as barrel twist rate. Any concerns to be aware of. Thanks for your help.

The mags are basically the same but the follower is different because the case width is different than a 5.56/300Blackout. The PRI 6.8 mags also allow a longer OAL which is important only if you intend to load heavier and longer bullets in the 130-140g area. As far as I know the only 6.8 mags are metal ones, and I don't think anyone makes a follower you could use in a PMag.
There is a new PMag 6.8 but it is a special mag designed for the LWRCI Six8 rifle which has non-standard mag dimensions and won't fit in a standard AR. And you are asking for trouble down the road loading 6.8 rounds in a 5.56 PMag. The follower does not match the dimensions of the case which means the mag is being stressed in directions it was not designed for. In this case the follower which is designed to stagger the rounds properly and push them up is in reality pushing them to the side and will eventually cause the feedlip area of the PMags to crack. Remember there are idiots born every minute.

jimbobborg · Mar 13, 2015

photoracer said:
The mags are basically the same but the follower is different because the case width is different than a 5.56/300Blackout. The PRI 6.8 mags also allow a longer OAL which is important only if you intend to load heavier and longer bullets in the 130-140g area. As far as I know the only 6.8 mags are metal ones, and I don't think anyone makes a follower you could use in a PMag.

I use the PRI mags for my 110gr Accubond loads to run 2.300" COAL to reduce pressure on some RL10x loads I have. I've gotten 2740 fps with that bullet out of my 20 SPR build.

MikeTF · Mar 13, 2015

I use PRI mags (25 rnd) on my 6.8 spc. I bought some at $44 a piece, and others were on sale for $22. Not cheap!

kenpo333 · Mar 14, 2015

I have a 6.8 LWRC and love it. Different mags for it than 556

boss66tcode · Mar 14, 2015

Ive had good luck with both ar stoner, and barret mags, but they are distinct from standard 5.56 mags. I have an asc mag 10 rounder that is fine too.

The angle curve of the mag is different, and the feed lips are also different.

Caliber choices for AR builds

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