308 build issues

[M1911a1g17]

Hello all. I am having issues with a 308 build. It does not want to cycle with Winchester 7.62x51 m80 ammo or federal .308 150gr. It cycled Winchester 150gr and 180 gr powerpoint 308 ammo fine. I was hoping for any suggestions you guys might have. It’s a dpms pattern lr308.

 

[PappyM3]

Going to need a whole lot more details than that.

What is your barrel (brand/gas block/gas length)? BCG make and type? Receiver extension length, buffer spring and weight?

7.62x51 and 308 loads can have a pretty wide range of pressures, so people often recommend using an adjustable gas block. But there are a lot of other factors too and an AGB won’t help if your gas port is too small. But gunsmith can open up the gas port if you need it.

 

[M1911a1g17]

Barrel Faxon pencil 18in barrel extension LR-308
Rifle length gas system
Kak industries gas block
Kak industries bcg LR-308 double ejector spring slick side
LR-308 carbine heavy buffer kak
LR-308 carbine spring kak
Expo arms gas tube

 

[SKumar]

Rifle length gas is likely the issue. The difference in powder between the brands is likely what caused one to cycle and the other didn't. I think a lighter buffer spring will help compensate for weak gas.

 

[PappyM3]

Barrel Faxon pencil 18in barrel extension LR-308
Rifle length gas system
Kak industries gas block
Kak industries bcg LR-308 double ejector spring slick side
LR-308 carbine heavy buffer kak
LR-308 carbine spring kak
Expo arms gas tube

Hmm. Your “failing to cycle”, is it failing to eject, failing to feed the next round?

Faxon can be hit or miss for their gas port sizes. But I wouldn’t have the barrel’s gas port opened any without an AGB.

Some Faxon barrels (usually their match barrels) can have really tight chambers. I know my Faxon 6.5CM match barrel is hard to extract fired brass. I’ve had to kick start the charging handle. A tight chamber can sap energy from cycling.

I’d keep your BCG as is. Going lighter reduces reliability in feeding the next round as there isn’t as much mass going forward.

If your issue is with short stroking, my first place to try would be a lighter buffer and/or spring. But I’m hesitant with carbine length receiver extensions for 308. The top manufacturers all seem to use A5 length extensions and use normal AR15 carbine-length buffers.

 

[ReluctantDecoy]

Faxon can be hit or miss for their gas port sizes. But I wouldn’t have the barrel’s gas port opened any without an AGB.

I noticed on my last build with Faxon barrel that the gas port was located on the journal in a way that required the gas block (Aero) to be pulled off ever so slightly from sitting flush with the barrel shoulder. Like a playing card distance away from the shoulder for a perfectly lined up gas connection. Lots of people told me this is typical when mixing and matching parts, but it seemed very unusual to me. Made me wonder how many people mount it flush assuming both parts share the same spec and don't get full gas.

Not saying that's the problem here. Just something that I thought was weird.

 

[PappyM3]

I noticed on my last build with Faxon barrel that the gas port was located on the journal in a way that required the gas block (Aero) to be pulled off ever so slightly from sitting flush with the barrel shoulder. Like a playing card distance away from the shoulder for a perfectly lined up gas connection. Lots of people told me this is typical when mixing and matching parts, but it seemed very unusual to me. Made me wonder how many people mount it flush assuming both parts share the same spec and don't get full gas.

Not saying that's the problem here. Just something that I thought was weird.

That’s actually the way all are supposed to be. It is to account for the metal handguard cap behind a FSB on A1/A2 handguards.

But you bring up a great point that the OP should double check the gas block location and that it’s secure.

Also OP, check to ensure the gas key on the carrier is tight.

 

[M1911a1g17]

@SKumar do you have a spring recommendation?

@PappyM3 it’s not picking up another round. I believe short stroking. Do you recommendation for a spring? I have a carbine buffer I have tried and is a bit better but seems to launch brass into space.

@ReluctantDecoy i will give that a try because I think I noticed that.

 

[ReluctantDecoy]

That’s actually the way all are supposed to be. It is to account for the metal handguard cap behind a FSB on A1/A2 handguards.

But you bring up a great point that the OP should double check the gas block location and that it’s secure.

Also OP, check to ensure the gas key on the carrier is tight.

That makes a lot of sense. Wonder if there are any Gucci parts companies out there sizing gas blocks to work flush considering the A1/A2 handguards seem to be more of a specific throwback build these days.

 

[PappyM3]

@SKumar do you have a spring recommendation?

@PappyM3 it’s not picking up another round. I believe short stroking. Do you recommendation for a spring? I have a carbine buffer I have tried and is a bit better but seems to launch brass into space.

@ReluctantDecoy i will give that a try because I think I noticed that.

First thing to do is double check gas key and the gas block location. It is standard for there to be a tiny gap between the gas port and journal shoulder. I usually install gas blocks by having them upside down and look down the gas port through the set screw hole. Set screw holes are almost always lined up exactly with the gas port hole on the top. So I line it up upside down, then just rotate while maintaining the appropriate gap next to the shoulder.

Honestly, my recommendation for buffer/springs is to go with the A5/Armalite length receiver extensions that KAC, LMT, Armalite, and V-Seven use on their large frame ARs.

But aside from that, I would go just slightly lighter with the buffer (look up your buffer weight and try to find one an ounce lighter or so). For buffer spring, you could try a Sprinco Orange or Tubbs flat wire spring.

 

[PappyM3]

That makes a lot of sense. Wonder if there are any Gucci parts companies out there sizing gas blocks to work flush considering the A1/A2 handguards seem to be more of a specific throwback build these days.

I’m sure there are. But I couldn’t name one.

Centurion Arms uses a custom indexing gas block with their barrels. But I’m not sure if they kept that spacing since you can still use other gas blocks.

 

[whatluck]

308 should use an h1 or h2 buffer. if it's short sroking on an h1 buffer and rifle length gas system you probably have a different problem, gas port/tube related. check the staking on your bcg, youll short stroke if that's not tight

 

[paul73]

Barrel Faxon pencil 18in barrel extension LR-308
Rifle length gas system

sorry to tell you - i had same exact issues with faxon on rifle length. it did not cycle on 7.62 150gr ammo i had - using standard non adjustable gas block. all was aligned fine. but it would cycle on stock 175gr .308 and my own reloads. the mid length barrel would cycle fine.

spring change is easy, but you need to make sure if with softer spring the heavier 308 loads would not produce a buffer smack into the end of the tube.

 

[Tallahassee]

It's already been said, but go with a lighter buffer. It should cycle. It may run hard, and throw the brass around.

Faxon QC can be a little lack luster at times as well, I would check the gas port sizing and location.

 

[paul73]

Faxon QC can be a little lack luster at times as well, I would check the gas port sizing and location.

For latter- take a ruler and sharpie and draw a line on a barrel from gas port. It may or may not be centered properly. Look where hole is on a gas block. Align it without a tube first, blow to check it did align.
It all may sound stupid but it is all about giving proper time to check for unexpected.

If you feel adventurous (and have tools) - you can try to enlarge the port to get more gas.

 

[M1911a1g17]

@PappyM3 ok thank you. I will check that out. I am gonna use the carbine and see if that helps at all.

@whatluck I will check that out. Thank you.

@paul73 ah man. Ok. That stinks. Did you ever get the rifle length running well? I will do the sharpie to double check. I did run air through it after installed with compressor and got a lot though. If I was to enlarge it. What would be an appropriate size to enlarge to?

@Tallahassee I am gonna throw the lighter carbine in and see how she does.

 

[paul73]

@PappyM3 ok thank you. I will check that out. I am gonna use the carbine and see if that helps at all.

@whatluck I will check that out. Thank you.

@paul73 ah man. Ok. That stinks. Did you ever get the rifle length running well? I will do the sharpie to double check. I did run air through it after installed with compressor and got a lot though. If I was to enlarge it. What would be an appropriate size to enlarge to?

@Tallahassee I am gonna throw the lighter carbine in and see how she does.

i did not get good accuracy from my reloads on faxon and just returned it. and it was not critical for me to make it run properly on light 7.62 ammo, as i run only 168gr and 175gr bullets.
overall the 308 build is more picky to a particular ammo type and is more difficult to be 'universal' the way almost any .223 build can be.
if you want something that would run on anything you shove into it - i would look at medium length port barrels. that is what i got back to, and it cycles everything for me.
a carbine would be too short.

enlargement of the gas port in the barrel is a tricky task, as, obviously, it is not reversible. and it really will require an adjustable gas block as well to control the gas.
there are videos on youtube on that topic, but it is not something you can do with a hand drill, and, it is, again, something that may end up with you needing a new barrel.

plus, again, the whole deal is - you may get enough gas to cycle on the weaker ammo, but the heavier 308 175gr ones will smack the hell out of the gun. i would try to play first with different buffer and spring. but i have to say - i had a rifle and carbine buffer length lowers, none of them would cycle with that faxon, so i concluded its port was a bit too tight.

seriously - where did you get that faxon? just return it if it is an option. primary arms still has medium gas ballistic advantage builds. grr, they sold out on 18", only 16" are left. it would still cycle on anything, but, a 16" on 308 is a bit short for my taste.

Aero Precision M5 Barreled Upper .308 Mid-Length Atlas S-ONE Black - 16"

DetailsAero Precision M5 Barreled Uppers are changing the way you shoot large calibers out of a semi-automatic AR-10 platform. The AR-308 platform is not as plug and play as the AR-15 platform and can be a headache when building a gun from the ground up from different manufacturers. Solve that...

www.primaryarms.com

Ps. It was same exact barrel btw, a pencil 18” faxon, and it closed on a 308 no go gauge as well, that did add to my happiness. So it went back to opticsplanet.

 

[M1911a1g17]

@paul73 well that does suck hahah. I like the barrel because of its weight and length. But if it’s not good then it’s not good. I bought it from primary arms. I am gonna look into returning it. I am looking to run 168 gr mostly as well but like to have options as well. I don’t like the idea of only being able to run very specific ammo. I guess worst case scenario I can’t return it and then I try drilling it out anyway. At that point there’s no real loss if it gets ruined. I would like to stick with a 18 inch barrel and the lighter the better. Do you have any recommendations for one? Doesn’t have to be Faxon.

 

[milktree]

I usually install gas blocks by having them upside down and look down the gas port through the set screw hole. Set screw holes are almost always lined up exactly with the gas port hole on the top. So I line it up upside down, then just rotate while maintaining the appropriate gap next to the shoulder.

The set screw hole is how they get access to the gas port hole. Set up the mill once, drill, tap, drill. I think it's safe to say, "Set screw hole is lined up with the gas port"

 

[PappyM3]

I guess worst case scenario I can’t return it and then I try drilling it out anyway. At that point there’s no real loss if it gets ruined. I would like to stick with a 18 inch barrel and the lighter the better. Do you have any recommendations for one? Doesn’t have to be Faxon.

If you have the gas port enlarged, you’ll probably want to do a lot of research on forums with specific large frame AR knowledge. I doubt a local gunsmith would be able to recommend an appropriate size. Many will be able to ream out a larger hole, but knowing a good size is a different matter, especially with 308 instead of 5.56.

But if it’s too large. That’s what an adjustable gas block is for.

Regarding other barrel options, you’re not likely to find another barrel anywhere close to as lightweight as the Faxon pencil. Criterion is a good choice, but it their 18” hybrid offering is ~10oz heavier. The lightest non-Faxon that’s reasonably priced, that I know of, is the Wilson combat fluted hunter. I think it’s 8oz heavier than the Faxon. Both options will be much better quality than Faxon. I say this as once being a fan of Faxon, but they’ve gone downhill during the pressure of the 2020 market.

 

[paul73]

n combat fluted hunte

If you have the gas port enlarged, you’ll probably want to do a lot of research on forums with specific large frame AR knowledge. I doubt a local gunsmith would be able to recommend an appropriate size. Many will be able to ream out a larger hole, but knowing a good size is a different matter, especially with 308 instead of 5.56.

But if it’s too large. That’s what an adjustable gas block is for.

Regarding other barrel options, you’re not likely to find another barrel anywhere close to as lightweight as the Faxon pencil. Criterion is a good choice, but it their 18” hybrid offering is ~10oz heavier. The lightest non-Faxon that’s reasonably priced, that I know of, is the Wilson combat fluted hunter. I think it’s 8oz heavier than the Faxon. Both options will be much better quality than Faxon. I say this as once being a fan of Faxon, but they’ve gone downhill during the pressure of the 2020 market.

i have fluted wilson arms 18" barrel, medium gas, it is 170 grams heavier than faxon, shoots also not stellar, but, a bit better, around 1-1.5moa. i am not crazy happy with it, but, it works.
at least it runs on everything i tried on it so far. wilson combat and criterion should be better, but i think they are heavier as well.

both faxon and this wilson arms heat up like crazy after 10 shots.

 

[PappyM3]

i have fluted wilson arms 18" barrel, medium gas, it is 170 grams heavier than faxon, shoots also not stellar, but, a bit better, around 1-1.5moa. i am not crazy happy with it, but, it works.
at least it runs on everything i tried on it so far. wilson combat and criterion should be better, but i think they are heavier as well.

both faxon and this wilson arms heat up like crazy after 10 shots.

Good to know.

I am in the process of switching my Faxon 16” 6.5 CM match gunner with a 16” Criterion 6.5 CM because of Faxon’s chamber tightness. The extra weight is a downside, but it’s not too bad given the barrel’s profile and weight distribution. I wish I just went Criterion first thing, but at the time Faxon was the only 16” 6.5 CM game in town. (Aside from having a custom barrel cut)

Interestingly, opposite of you, I’m actually going from mid-length to rifle length for the 16” 6.5 CM. But. 6.5 CM has a higher pressure than 7.62x51, so it should be fine. And I trust Criterion to gas it correctly.

 

[paul73]

But. 6.5 CM has a higher pressure than 7.62x51

my 6.5CM AR is on 20" barrel, rifle length, it is a aero build with a generic for them ballistic advantage barrel - it cycled on any of 6.5cm ammo i had, and on all my weak and hot loads as well.
that i run it on now is same load i made for bergara with a 41.2gr R16 and it does 2667fps there with 140gr ELD bullet.
never had any cycling issues with it, and it run fine on both carbine and rifle buffers.

 

[M1911a1g17]

@PappyM3 thank you! I have some research to do.
@paul73 thank you. I am going to look into those for sure

I shot it the other day and it was weird. I got it to run fine with 150gr federal fine. But it would not run 168 gr federal match and then I ran Hornady black 168z it ran a couple cycling fine but then wouldn’t after. No matter the buffer and spring combo I have. Could this be a mag issue possibly? Going to get an adjustable gas block.

 

[paul73]

@PappyM3 thank you! I have some research to do.
@paul73 thank you. I am going to look into those for sure

I shot it the other day and it was weird. I got it to run fine with 150gr federal fine. But it would not run 168 gr federal match and then I ran Hornady black 168z it ran a couple cycling fine but then wouldn’t after. No matter the buffer and spring combo I have. Could this be a mag issue possibly? Going to get an adjustable gas block.

the 150gr ammo - was it saying 308 or 7.62 on the box? the 7.72 is weaker - but it should cycle on a heavier grain. dunno, i had no issues on mine with 160+.
it refused to run on a 7.62 'adrenaline' 149gr - an ok lighter hunting round.

does it eject all the time? what do you have precisely, as it gets interesting. with me it would not eject it, the used brass would be either stuck or not out at all, as bolt would not go back far enough.

 

[M1911a1g17]

@paul73 it was 308. It ejects every time with that. It ejects even if it doesn’t pick up another round. But with the 7.62x51 it would not fully chamber another new round. I got it to cycle 180 gr federal 308 fine. No issues at all.

 

[mac1911]

Rifle length gas is likely the issue. The difference in powder between the brands is likely what caused one to cycle and the other didn't. I think a lighter buffer spring will help compensate for weak gas.

Should not these rifles be built around 7.62 NATO Ammunition specs?
I cant see all but the fastest powders being much of a issue.

 

[paul73]

Should not these rifles be built around 7.62 NATO Ammunition specs

there is no such thing as set 'specs' for AR10 builds in 308. it`s just none. so you go where you want to go.
there are ways to simplify process a bit - like sticking to known components that merry more happily than some others - like aero M5 lower/upper, etc., or you can venture off to do something else.
and, 7.62!=308 neither pressures nor chamber size wise.

SAAMI 308Win:
GO 1.630"
NO GO 1.634"
FIELD 1.638"

FN FAL:
GO 1.6325" (FN & Brit/commonwealth) (Canadian is 1.6315")
NOGO 1.638"
FIELD 1.640"

7.62NATO (M14 US MILSPEC):
GO 1.6355"
NOGO 1.638"
FIELD 1.6445"

PS. and that faxon pencil 308 barrel we spoke before did close bolt on the saami 1.634" nogo gauge, btw.

 

[mac1911]

there is no such thing as set 'specs' for AR10 builds in 308. it`s just none. so you go where you want to go.
there are ways to simplify process a bit - like sticking to known components that merry more happily than some others - like aero M5 lower/upper, etc., or you can venture off to do something else.
and, 7.62!=308 neither pressures nor chamber size wise.

SAAMI 308Win:
GO 1.630"
NO GO 1.634"
FIELD 1.638"

FN FAL:
GO 1.6325" (FN & Brit/commonwealth) (Canadian is 1.6315")
NOGO 1.638"
FIELD 1.640"

7.62NATO (M14 US MILSPEC):
GO 1.6355"
NOGO 1.638"
FIELD 1.6445"

PS. and that faxon pencil 308 barrel we spoke before did close bolt on the saami 1.634" nogo gauge, btw.

Gene Stonersn Armalite AR-10 7.62 NATO battle rifle.......but not adopted but surely you can get where Im coming from. These "AR-10" rifles in "308/7.62" should easily function on 7.62 NATO ammunition.

Problem with comparing NATO/SAAMI pressure specs is the different types of chamber pressure testing.
As for actual chamber cut....well we need to know what reamer the manufacture actually used.

Its all fun stuff.

I know I would want to know why my "308" AR does not function on plain jane M80 ball right from the box , other than manufacture defect.

 

[mac1911]

Barrel Faxon pencil 18in barrel extension LR-308
Rifle length gas system
Kak industries gas block
Kak industries bcg LR-308 double ejector spring slick side
LR-308 carbine heavy buffer kak
LR-308 carbine spring kak
Expo arms gas tube

I have to ask
How heavy is the carbine heavy buffer ?