My experiences building .308 ARs from parts

[MidwestPX 172]

I have a new appreciation for what manufacturers go through in order to produce a .308 AR. Especially one tasked with operating both unsuppressed as well as suppressed without requiring adjustment to the gas system prior to changing states. I recently built a few .308 ARs using parts I stock and I suspect these large frame ARs will be increasingly popular going forward. One of them was for myself while the others were commissioned by customers. What I have learned is that in general, .308 ARs are severely overgassed. There are multiple ways to deal with it but troubleshooting can be frustrating and costly.

 

I have primarily built AR15s in the past and my experience was that if you take quality parts and assemble them, the gun will run and run well. I thought this would carry over to .308 ARs and found this to only be somewhat true. The common problem seemed to be the amount of gas bled off from the barrel to operate the action. It was always excessive and led to carrier velocity being entirely too fast. It would occasionally result in a stovepipe malfunction when unsuppressed but once the suppressor was attached, stoppages became more frequent (every 3-5 rounds). I ended up spending in excess of $500 in parts trying to address this and figured I'd share my experience with others to save them some headache (both frustration and financial pain of buying parts without knowing if it would solve the issue).

 

Each build that I've worked on used parts that I considered to be quality parts whether they were out of my store's catalog or sent in to me by the customer. Gas systems were either rifle or midlength. Both were overgassed without adjustment/tuning and ejecting between 1 and 3 o'clock. The more severe instances of being overgassed stovepiped every few rounds. To address this, I tried a number of things:

 

1. Adjustable gas block or gas key: This is a must-have, in my opinion. It will solve most users' issues. However, it is ammunition sensitive so if you dial your gas in with surplus or defensive ammo, it may not work when you change to a lighter loading. This sensitivity is most apparent when a suppressor is added and gas pressures spike.

2. Changing from a carbine extension to a rifle: The DPMS shortened buffer weighs about 3.0oz. This buffer allows the user to use AR15 extensions with their .308 BCG. The shortened rifle buffer is 5.4oz. To give you an idea of how important buffer weight is, the LMT MWS uses an H3 AR15 buffer that weighs over 5.5oz (more on using AR15 buffers next)

3. Using a MWS/SR25 receiver extension: The MWS and SR25 rifles use a different extension. It's slightly longer to accommodate the longer/larger .308 BCG but allow the use of AR15 buffers. This is neat because currently, there are more buffer options for AR15s than there are .308 ARs. This solution isn't too popular since it requires a specific extension which not many retailers carry (I do).

4. Ridiculously heavy buffers: I reached out to Clint/Slash at heavybuffers.com to get his thoughts on how best to achieve the goal of running a .308 AR both suppressed and unsuppressed with a single gas setting. His recommendation was to go as heavy as possible on the buffer. He makes a 10oz .308 rifle buffer, 6.5oz .308 AR carbine buffer, and a 8.5oz AR15 carbine buffer. His recommendation to me was to use the 10oz .308 buffer followed by using a MWS/SR25 extension and using the 8.5oz AR15 buffer. Basically, go as heavy as possible in the buffer was his recommendation. That would give me the wiggle room I needed with the adjustable gas key to dial in the gas setting that would allow the rifle to run reliably in both states. The downside to this is cost.

5. Tuned action springs: Wolff extra power springs are available and recommended by heavybuffers.com. I also found that the JP tuned springs (both rifle and carbine) helped .308 ARs run more reliably compared to generic kit springs. They cost more but they do have some impact.

 

So the great takeaway from my experience building .308 ARs is that they're generally overgassed but that's typically easily resolved by adjusting the gas with either an adjustable gas block or an adjustable gas key. Shooting a few rounds and watching the ejection pattern is a way to see just how badly overgassed the system is. Installing an adjustable gas block or key will solve most users' problems provided they aren't trying to run the rifle with and without a suppressor. With an adjustable gas key on a JP low mass carrier, I had rifles that would run beautifully unsuppressed but a few rounds into shooting with a can on, would stovepipe and ejection went to hell. That's when the other methods of adjusting the reciprocating mass of the action come into play.

 

Hopefully this is helpful in future builds. I'm happy to sell you all the parts you want but would rather you buy just the parts you need so you can get to shooting and enjoying your purchase.

[mipafox 438]

Did you try Colt?

[MidwestPX 172]

Did you try Colt?

Colt what? Buffers? Springs? Need more to go on, man!

[mipafox 438]

901s et al.

 

Colt. The Colt.

 

ATF recently said you can put RDIAS in them as well if you are into S&M

[Dan 177]

Ty, I'm in the midst of building a PSA PA-10. I already have a gas block but after reading your article here (thanks for that), I'm thinking of moving to an adjustable block.

 

I see your adjustable block on your site and am about to order it. The only think I want to know from you is if there is anything else to expect with an adjustable block, like in terms of life. As you know here in New Jerkistan, my brake will be pinned on, so removing a problematic gas block is a no-go.

 

Also, the PSA kit came with what looks to be their Heavy carbine buffer along with a PA-10 specific buffer tube. Do you sell the heavy buffers that you mention above, or should I go right to heavybuffers.com?

[MidwestPX 172]

901s et al.

 

Colt. The Colt.

 

ATF recently said you can put RDIAS in them as well if you are into S&M

Nope, haven't messed with 901s.

[MidwestPX 172]

Ty, I'm in the midst of building a PSA PA-10. I already have a gas block but after reading your article here (thanks for that), I'm thinking of moving to an adjustable block.

 

I see your adjustable block on your site and am about to order it. The only think I want to know from you is if there is anything else to expect with an adjustable block, like in terms of life. As you know here in New Jerkistan, my brake will be pinned on, so removing a problematic gas block is a no-go.

 

Also, the PSA kit came with what looks to be their Heavy carbine buffer along with a PA-10 specific buffer tube. Do you sell the heavy buffers that you mention above, or should I go right to heavybuffers.com?

Gas blocks aren't a consumable item so it shouldn't require replacing.  I don't sell the heavy buffers as they don't wholesale them.  Wish they would but that's the way the cookie crumbles.

[Dan 177]

Gas blocks aren't a consumable item so it shouldn't require replacing.  I don't sell the heavy buffers as they don't wholesale them.  Wish they would but that's the way the cookie crumbles.

Thanks, I didn't think they were. I checked out those buffers, pretty hefty price tag. I'll try the H buffer with the adjustable block and go from there.

[MidwestPX 172]

Thanks, I didn't think they were. I checked out those buffers, pretty hefty price tag. I'll try the H buffer with the adjustable block and go from there.

If that doesn't work, you can also try changing out the extension for one of the LMTs and grab a H2 or H3 AR15 buffer.  That's a cheaper solution than the 8.5oz buffer by itself.  The 8.5oz is probably overkill but I did grab an extra one for a shop build that's being put together (full auto .308 AR).

[Dan 177]

If that doesn't work, you can also try changing out the extension for one of the LMTs and grab a H2 or H3 AR15 buffer.  That's a cheaper solution than the 8.5oz buffer by itself.  The 8.5oz is probably overkill but I did grab an extra one for a shop build that's being put together (full auto .308 AR).

 

The PA-10 buffer tube is proprietary. The spring is also longer and has a different wire/coil weight/length.  I just learned that the tube uses regular AR15 buffers, so now I can look into H2/3 AR15 buffers for some savings.

[MidwestPX 172]

The PA-10 buffer tube is proprietary. The spring is also longer and has a different wire/coil weight/length.  I just learned that the tube uses regular AR15 buffers, so now I can look into H2/3 AR15 buffers for some savings.

If it's like the LMT/KAC units, it's a rifle length spring.  

[Displaced Texan 11,731]

Nope, haven't messed with 901s.

The LE901 is lovely......

[DargZ 5]

Nice write up, Ty.

 

I built mine with the JP adjustable gas block.  I also went with the JP AR-10 silent captured spring unit.  Like you, I had tons of tuning but I can't say that it runs reliably.  I initially tuned it so that there is just enough gas to lock the bolt back on an empty mag.  That worked for the day.  I'm pretty certain that was tuned for AE .308 factory rounds.

 

On another range day, I needed to up the gas when I switched to LC 7.62 rounds.  First round was crushed by the bolt coming back before the round was even ejected.  I had to open up the gas quite a bit.

 

While living in NJ, I don't have to worry about running suppressed but having an adjustable gas block is going to help.

 

I want to build another one with a shorter barrel and I am leaning towards a carbine buffer tube.  I didn't realize there were so many different options and I was always curious about the buffer.  What you posted helped a lot!  I just need to do more research.

[Old School 611]

This stuff sounds like a nightmare.

 

I just hope my 6mm AR runs.

[MidwestPX 172]

Nice write up, Ty.

 

I built mine with the JP adjustable gas block.  I also went with the JP AR-10 silent captured spring unit.  Like you, I had tons of tuning but I can't say that it runs reliably.  I initially tuned it so that there is just enough gas to lock the bolt back on an empty mag.  That worked for the day.  I'm pretty certain that was tuned for AE .308 factory rounds.

 

On another range day, I needed to up the gas when I switched to LC 7.62 rounds.  First round was crushed by the bolt coming back before the round was even ejected.  I had to open up the gas quite a bit.

 

While living in NJ, I don't have to worry about running suppressed but having an adjustable gas block is going to help.

 

I want to build another one with a shorter barrel and I am leaning towards a carbine buffer tube.  I didn't realize there were so many different options and I was always curious about the buffer.  What you posted helped a lot!  I just need to do more research.

Was your rifle dirty?  I've noticed that as the actions foul, the guns get even more sensitive to gas levels.

This stuff sounds like a nightmare.

 

I just hope my 6mm AR runs.

It's not too bad if you're just looking to run unsuppressed (which the majority of people are).  An adjustable gas block or key with a heavy buffer should square most away.

[Dan 177]

This stuff sounds like a nightmare.

 

I just hope my 6mm AR runs.

 

I call it an adventure.

[Old School 611]

Was your rifle dirty?  I've noticed that as the actions foul, the guns get even more sensitive to gas levels.

It's not too bad if you're just looking to run unsuppressed (which the majority of people are).  An adjustable gas block or key with a heavy buffer should square most away.

 

Yeah I wouldn't want to suppress a 24" barrel any way...LOL

[DargZ 5]

Was your rifle dirty?  I've noticed that as the actions foul, the guns get even more sensitive to gas levels.

It's not too bad if you're just looking to run unsuppressed (which the majority of people are).  An adjustable gas block or key with a heavy buffer should square most away.

 

Nope, I tend to clean my stuff after every outing as I take different things out at different times.  I'm not sure when the next outing is so I thought it is best to just clean it.

 

I'm thinking the day was colder although I'm not sure if that changes anything.  A few turns on the gas block fixed everything.

[Bully 749]

Retracted. Gonna start another thread.

[MidwestPX 172]

Bumping this because I just had another .308 AR come through to be fixed for being overgassed but this one takes the cake. It was quite the hot mess.

 

1. Overgassed so badly, the BCG would fly back with such velocity, it would knock the charging handle loose. This rifle had a 16" barrel with a carbine length gas system. Fixed with an adjustable gas key with the gas nearly completely shut off. If the customer wants to shoot a hotter load than the Remington ammo he bought, he'll need to swap out the receiver extension with one from LMT so I can use AR15 buffers and drop a H3 or heavier in. If you take anything away from this post, it's to not buy a 16" .308 barrel with a carbine length gas system.

2. Tried to fix the overgassed situation originally with one of my house-branded adjustable gas blocks but discovered the gas seat OD was 0.758" and not 0.750" as it was supposed to be. That would explain why a YHM clamp-on gas block was used. Oh and the gas block was clearly crooked as well as not against the shoulder (about a 1/16" gap).

3. Timney FCG wouldn't stay put. If you rested the rifle on its side, the hammer and trigger pins would drop out. Turns out whoever built this rifle (it was won at a raffle) removed the screws that are used to keep upward pressure on the pins. Solved with KNS anti-rotation pins.

4. Castle nut worked loose after three rounds. Torqued and staked.

5. General condition of the parts would suggest the parts were either demo/sample parts or used parts. For a supposedly brand new gun with 15 rounds fired by the current owner, it sure had a lot of wear. Bolt lugs were damaged (exposed metal) and cerakote on the carrier (yes, the builder cerakoted the carrier...) was worn on the races.

6. "Matched" billet receiver set had a substantial gap between upper and lower at the rear. I suspect this was a blem receiver set.

7. Flash hider was literally finger tight. I threw a crush washer on and torqued it down.

 

The customer had a second upper that came with the rifle, this one with a 24" bull barrel. The carrier was out of spec and would bind when trying to get it in battery. Fixed it with a new carrier. I didn't have time to figure out where exactly it was binding but my guess would be the Cerakote was a little thick on this one.

[Shane45 807]

Sometimes free AINT!

[MidwestPX 172]

He was into it for $20. I charged him $85 for parts and $75 for labor (two hours at half shop rate since his wife watches my kid during the day). $125 should get his other upper running properly so not too bad.

[MidwestPX 172]

Oh, I forgot the lower receiver was too thin. The FCG pins stuck out about 1/32" on both sides.