Barrel and gas system lengths

[kfreeze]

So I've got a pair of AR10s and I use that term on purpose.
One is an Armalite AR10 that was originally a 300 SAUM. I rebarreled it to a 6.5 PRC with a 24" barrel and a +2 gas system. I've just about got it dialed in to cycle perfect...but the Armalite mags are dog-spit and I think I'm gonna end up having to 3d print a self leveling follower for their Gen2 pattern mags.
The 2nd is a Matrix upper lower set that is still the Armalite pattern, but takes SR25 mags...that one is 22" with a +2 gas system. Pmags don't work 100% but so far ACS mags have been doing great. Seems to be tuned and feeding/cycling but with both systems...I'm thinking I should have just made them 16 or 18? from the jump.

Has anyone ever taken a barrel with a +1 or +2 gas system and shortened the barrel down to 16 or 18"??
I assume this is going to drastically reduce the gas and potentially start short stroking, but with the buffer, springs and gas block I have...I assume getting it tuned to run suppressed again would be fairly easy.

Really thinking I should have gown shorter.

View attachment 640486

Shortening that barrel will give you problems if you don't open the gas port up. The dwell time gets way to short to give enough gas. Better off to use a good click adjustable gas block and tuning it right in most cases instead of going to a longer gas system unless using slower burning powders.

As a side note, I sold a bunch of dpms/sr25 pattern ar10 mags on year a while back. We could take those and and remove some of the follower and they worked for saum stuff. Maybe bend the feed lips out a little if needed.

On the armalite factory ar10 300 saum I had for a while, the follower was modified by having the rib that sticks up in the follower modified to not touch the fatter case was the only difference that I could see.

 

[liber]

I rebarreled it to a 6.5 PRC with a 24" barrel and a +2 gas system.

The part I'm confused about still is how you handle the magazine? A 6.5 PRC won't fit in a short action mag, which most all LR-308s are. How did you overcome that on a semi-auto gun?

 

[ARlife4me]

The part I'm confused about still is how you handle the magazine? A 6.5 PRC won't fit in a short action mag, which most all LR-308s are. How did you overcome that on a semi-auto gun?

getting the correct mag is key! most will be 2.830" and shorter, but larue, acs and a few others get you past 2.860" problem i have is the follower with both short mags i have. i've tried 10-25 round mags as i know when fully loaded you can get and average of 70-80% of 308win ammo capacities. what's worse is keeping them from tilting which limits it even more to 40-60% of 308win capacities. say my 20 round mag could hold 14 rounds it won't fit into the magwell due to the mag swelling which limits capacity even more (maybe down to 8-10). the poly doesn't swell as much as steel or aluminum mags does, but cartridge length is lessened by .040" or more. if i'm staying under 2.830" magpul m118 mags are the ticket as they give me more capacity with less swelling and/or tilting. using the 6.5prc in conjunction with m118 mags holding 8-7-8 rounds in a 25 round mag feeds the factory norma ammo of 143golden targets which is around 2.830". i need a 3d printer to make followers that utilize short mag cartridges. i might totally remove the cartridge portion of the follower and somehow use a dummy round in its place? i've talked to places that offer short mag ar firearms and their response to my mag question(s) is they use standard 308win mags.

 

[kfreeze]

getting the correct mag is key! most will be 2.830" and shorter, but larue, acs and a few others get you past 2.860" problem i have is the follower with both short mags i have. i've tried 10-25 round mags as i know when fully loaded you can get and average of 70-80% of 308win ammo capacities. what's worse is keeping them from tilting which limits it even more to 40-60% of 308win capacities. say my 20 round mag could hold 14 rounds it won't fit into the magwell due to the mag swelling which limits capacity even more (maybe down to 8-10). the poly doesn't swell as much as steel or aluminum mags does, but cartridge length is lessened by .040" or more. if i'm staying under 2.830" magpul m118 mags are the ticket as they give me more capacity with less swelling and/or tilting. using the 6.5prc in conjunction with m118 mags holding 8-7-8 rounds in a 25 round mag feeds the factory norma ammo of 143golden targets which is around 2.830". i need a 3d printer to make followers that utilize short mag cartridges. i might totally remove the cartridge portion of the follower and somehow use a dummy round in its place? i've talked to places that offer short mag ar firearms and their response to my mag question(s) is they use standard 308win mags.

You are right. I forgot to mention the swelling. Removing the cartridge portion on the follower will help you. That's what dpms did on the 30 rem ar mags to allow for the fat case. You only have to remove enough of it though. Not all of it as they did on those mags.

 

[liber]

getting the correct mag is key! most will be 2.830" and shorter, but larue, acs and a few others get you past 2.860" problem

I guess I've been living under a rock. I've always used magpul mags in my LR-308 with 308. I just got a Lancer mag I'm anxious to try. I have never had any that I haven't had to bend the feed lips. Have had feed issues with 6.5 Creedmoor and 8.6 blackout. Seems odd as 308 has always fed fine in that lower. I had some AR Stoner mags in 6.5 Grendel / 6.8 SPC, but sent them back as they didn't feed well. Different cartridge, so could be different issues.

 

[ARlife4me]

I guess I've been living under a rock. I've always used magpul mags in my LR-308 with 308. I just got a Lancer mag I'm anxious to try. I have never had any that I haven't had to bend the feed lips. Have had feed issues with 6.5 Creedmoor and 8.6 blackout. Seems odd as 308 has always fed fine in that lower. I had some AR Stoner mags in 6.5 Grendel / 6.8 SPC, but sent them back as they didn't feed well. Different cartridge, so could be different issues.

IF, you're lucky enough to find any knights armament company (kac) it'll go 2.880ish from what i've read. key to knowing and finding mags that'll give you more oal is how the mag is welded. most are lap welded so you loose 1 thickness of the metal due to this. some are laser/tig welded together on 1 side or the other by the bolt catch part of the mag, so you gain 1 thickness of the metal. poly is just due to thickness needed for enough strength which keeps them under or at 2.830ish give or take .005-.015".

 

[blksno]

Shortening that barrel will give you problems if you don't open the gas port up. The dwell time gets way to short to give enough gas. Better off to use a good click adjustable gas block and tuning it right in most cases instead of going to a longer gas system unless using slower burning powders.

As a side note, I sold a bunch of dpms/sr25 pattern ar10 mags on year a while back. We could take those and and remove some of the follower and they worked for saum stuff. Maybe bend the feed lips out a little if needed.

On the armalite factory ar10 300 saum I had for a while, the follower was modified by having the rib that sticks up in the follower modified to not touch the fatter case was the only difference that I could see.

I have plenty of gas, there will be no reason to open the port. My adjustable gas blocks are nearly closed the way it is.
I just got a 3d Printer and my intention is to 3d print a self leveling follower for the Armalite Gen2 mag that will better accommodate the PRC Case girth.

I do still have my origional Gen 1 Armalite mag that I used with my 300 SAUM. I cut the front out for longer OAL, but other than that...It fed 190 bergers fine and dandy. It just doesn't handle the PRC case quite as well

 

[liber]

IF, you're lucky enough to find any knights armament company (kac) it'll go 2.880ish from what i've read.

Still short of 2.955. What type of velocity would one get at 2.880?

 

[blksno]

Still short of 2.955. What type of velocity would one get at 2.880?

I'm not really worried about velo. I have plenty of other rifles for that...especially if I'm going to cut to 18" anyway.
My hunting rifle is an 18" 6.5 PRC bolt gun and I run 130 TGKs at 2960 so if it's anywhere near that, I'll be happy as a clam.

 

[ARlife4me]

Still short of 2.955. What type of velocity would one get at 2.880?

you can get roughly the same velocity at both lengths (within 30fps), but the heaviest might be seated to deep to secure the bullet when ogive is in the neck. that's why using up to 135's and some 140's the boat-tail isn't to deep in the body (or just barely).

 

[liber]

you can get roughly the same velocity at both lengths (within 30fps), but the heaviest might be seated to deep to secure the bullet when ogive is in the neck. that's why using up to 135's and some 140's the boat-tail isn't to deep in the body (or just barely).

I normally use BTs, although Barnes does make some FT, like in 160 grain 338 TTSX. The 185s are BT. I like the BT for 2 reasons. 1) it's just easier to seat in a non-chamfered case, although I still do not like to do that, I most always chamfer. The other thing is it usually brings the section density up higher so there's not chance of a donut. All of that is not required for the shooting/hunting I do, but it's how I've been doing it. Even though the Barnes 6.5mm 127gr LRX is a BT, you might be pointing out that, that's the reason they don't produce higher weight LRX bullets???

It seems the PRC should allow bullets to extend farther out. I don't have any experience with 6.5 PRC yet, so don't want to comment on loading for it, or what results I may get. Also, if that's the case (no pun intended), what about those Bore Riders that Barnes came out with, maybe another specialized for the 6.5 PRC specific, cause 140-147 seems common, and many target shooters using those Berger 156s. I'm looking for copper though.