AR308 cycling problems... is this over or under gassed?

[cunnerman]

I'm having some cycling issues with my first/new AR 308 build. The casings just dribble out onto the table next to the ejection port and the action does not fully cycle. Sometimes the BCG pulls a fresh round, but it just gets jammed up against the star-chamber. Sometimes it doesn't grab a new round at all. I've read this can be a symptom of both overgassed and undergassed rigs, where the BCG is returning faster than the magazine spring can push a new round up.

Rig is as follows:
Aero M5 upper
Aero M5 lower
Aero 308 BCG
Wilson Combat 20" super sniper barrel
Rifle length gas system
AR10 gas tube
Superlative Arms adjustable gas block (tried 10-22 clicks)
Armalite AR10REKIT01 receiver extension kit (w/ H3 buffer, 5.4oz)
Geissele SSA-E trigger
Running it unsupressed right now

A deep dive into the interwebs tells me the Wilson Combat barrels have undersized gas ports and that I'm likely undergassed. I really don't want to drill out the gas port.

I've tried the Sprinco red spring with no change in results.

I'm thinking of trying H2 and H1 buffers next... or a Tubbs flatwire spring... am I headed in the right direction?

 

[thehun]

Sounds under gassed especially since you are running a 20” rifle gas setup but you can also have leaking gas somewhere … measure the gas port first…

If you do have to drill it … not a huge issue since you have an adjustable gas block.

Don’t run your gas block in bleed-off mode either …

Armalite buffer kit you have is fine. No need to change that.

 

[cunnerman]

Sounds under gassed … or leaking gas somewhere … measure the gas port …

Don’t run your gas block in bleed-off mode either …

Armalite kit you have is fine. No need to change that.

Did the best I could with my caliper and the gas port came in at 0.084"-0.089".

I think I saw somewhere that minimum 0.09" is recommended for rifle length systems?

 

[USK308]

My opinion/recommendation (FWIW):

I'd recommend measuring the gas port first; if memory serves, it should measure 0.090" - 0.096" for proper gas to the BCG. if it is not in that tolerance, you will likely have to drill the gas port with a numbered drill bit (#42 or #43, if memory serves). Once you have the gas port right, run through the standard test opening up the AGB all the way and firing 1 round, to see if the BCG locks on empty mag.

I had to drill the gas port on my 18" barrel as it was WAY undergassed (0.08375). Once I drilled out (0.094"), the rifle cycled like a rock star consistently (I too, have a Superlative AGB, which is set at 19-clicks from closed) and ejects at ~3:30 every time (I use 168gr ammo as it seemed to work best out of the various grain weights I tested).

I do think you may need to look at an H2 buffer, but check gas port and whatnot first as it does sound like your rifle is undergassed.

 

[thehun]

.095 is where it needs to be …

That said … if you are going to run it suppressed … I’d say she would be well dialed in for it.

Without a can … it needs to be opened a bit … wonder if WC is having BA do their barrels … they tend to have small ports like that.

 

[msgriff]

Does it lock back after shooting one round loaded in the mag? You should do a short stroke test either way. The barrel should be dimpled, so if you're using a set-screw AGB, alignment should be good. If you have a clamp-on AGB, check alignment. Nothing wrong with trying a lighter buffer. Many use a H3, but some, like Knights, use an H2. 0.089" gas port should be fine. Gas tube length can be an issue, are you using the ArmaLite AR-10 15.5" gas tube and where does it end in the cam pin cut-out.

 

[FredHammer]

You are overgassed. When you are undergassed nothing dramatic happens.

 

[cunnerman]

Gas port is aligned (verified with video borescope). And gas block screws torqued down to spec with a Wheeler torque limiter. Barrel is dimpled.

WC AR10 tube lands a little past halfway on the cam-pin cutout. I have a WC standard rifle length tube, but it would land 1/8” short of halfway on the cutout. Wilson Combat is annoying like that; they split the difference between the two tube lengths with their gas port placement. Annoying as hell.

FWIW, as I recall, last round does actually lock back. But I will need another range visit to verify.

 

[thehun]

You are overgassed. When you are undergassed nothing dramatic happens.

His gas port size is saying otherwise …

 

[FredHammer]

WC AR10 tube lands a little past halfway on the cam-pin cutout. I have a WC standard rifle length tube, but it would land 1/8” short of halfway on the cutout. Wilson Combat is annoying like that; they split the difference between the two tube lengths with their gas port placement. Annoying as hell.

FWIW, as I recall, last round does actually lock back. But I will need another range visit to verify.

Which gas tube are you using?

 

[cunnerman]

His gas port size is saying otherwise …

So. Here’s the one odd thing that makes me think possibly overgassed… she kicks like a MOFO! The recoil is harsh. I know 308 is quite a jump up from 556, but this thing really bucks the shoulder.

Is that normal? Or am I just a wimp?

 

[cunnerman]

Which gas tube are you using?

WC AR10 tube installed for all tests thus far.

WC rifle length tube is in packaging on the bench.

 

[cunnerman]

OK, that ain't right.
What weight buffer? 5.3Oz

Currently H3 that weighs in at 5.4oz on my scale.

Aero BCG, bolt, and firing pin complete combo comes in just a little over 17oz.

 

[FredHammer]

Currently H3 that weighs in at 5.4oz on my scale.

I would run the SA GB in bleed off (settings 30 ish) and see if this excessive recoil impulse goes away. I have built many AR10/M5 and a few with WC barrels. None have given me that holy fuck recoil.

 

[thehun]

Yeah. A well tuned AR10 is an easy shooter.

 

[TheOE800]

None have given me that holy fuck recoil.

I tried one of those slash heavy buffers once when running suppressed and boy did I not enjoy shooting that gun. It was even 6.5 CM, but couldn’t stay in the scope from shot to shot. Terrible.

 

[thehun]

Currently H3 that weighs in at 5.4oz on my scale.

Aero BCG, bolt, and firing pin complete combo comes in just a little over 17oz.

You might try just a regular CAR buffer since your gas port is so small ...

 

[cunnerman]

Thanks for the input everyone!

Sounds like I just need to get back to the range with some springs, buffers, and the key for my gas block, and just run a bunch of tests and take a bunch of notes. I'm going to try and record some slow-mo video of the action as well.

I'll try to get out this weekend and will report back.

 

[mettee]

You answered my question, it's recoiling hard.

You are overgassed.

How does the brass look? Take some pics and save a few.

 

[mettee]

I tried one of those slash heavy buffers once when running suppressed and boy did I not enjoy shooting that gun. It was even 6.5 CM, but couldn’t stay in the scope from shot to shot. Terrible.

What spring were you using with it?

P.E. and K.E.

P.E. can generate K.E.

 

[thehun]

You answered my question, it's recoiling hard.

You are overgassed.

How does the brass look? Take some pics and save a few.

How does something overgas when his gas port is under sized? That doesn’t compute.

Chamber out of spec?

 

[TheOE800]

What spring were you using with it?

P.E. and K.E.

P.E. can generate K.E.

Tubb 308 flat wire.

 

[FALex]

I’ve been hearing/reading nothing but shit coming from WC in regards to their AR’s.

 

[isofahunter]

I had similar issue albeit with a carbine gas system. I was running 178 gr pills and it was unreliable as you described. Put in ADJ gas block, better but still a problem child. Shot up the 178 and loaded up 150 gr pills and it was night and day in reliability and performance. Runs cleaner (suppressed) and no cycling issue since the change. YMMV but something you might consider before becoming a part changer.

 

[cunnerman]

I had similar issue albeit with a carbine gas system. I was running 178 gr pills and it was unreliable as you described. Put in ADJ gas block, better but still a problem child. Shot up the 178 and loaded up 150 gr pills and it was night and day in reliability and performance. Runs cleaner (suppressed) and no cycling issue since the change. YMMV but something you might consider before becoming a part changer.

Hmmm... Interesting.

I have been doing all of my testing with 178gr and 185gr Federal Terminal Ascent or Sierra Match King factory loads. Trying to use this for longer range shooting; any advice for making it run reliably with the hotter stuff?

@FredHammer recommended 30-clicks... I am going to give that a go.

 

[isofahunter]

Hmmm... Interesting.

I have been doing all of my testing with 178gr and 185gr Federal Terminal Ascent or Sierra Match King. Trying to use this for longer range shooting; any advice for making it run reliably with the hotter stuff?

Mine is a 16 inch tube so different than yours but with the switch to 150 I picked up velocity and saw better performance out to 500 yards which is as far as I am taking a shot with this set up. The other thing I did with the load was switch to a the fastest burning powder I had on hand with that bullet. My thought process was the 178 load was so slow it was staying in the barrel longer keeping the pressure up as the action started to cycle at a higher pressure. By switching to a 150 and going to a faster burning powder it allowed for a better burn and the bullet leaves the barrel faster dropping pressure quicker allowing the bolt to cycle under less pressure. I do not have evidence other than it went from a messy, unreliable pig to something I use around the ranch without a worry post the load change.

 

[mettee]

Tubb 308 flat wire.

The poundage is increased with that spring.

The poundage for that spring is listed on Tubbs website.

Slash provides the recipe on his page, if that isn't followed the recipe is off and your cookies turn out bad...

 

[mettee]

How does something overgas when his gas port is under sized? That doesn’t compute.

Chamber out of spec?

He's describing super heavy recoil, so for me that gives the answer, nothing else can cause recoil like that.

Hot loads, chamber, tight bore, many things can lend to this, not sure what it is yet.

Need to see brass too. Look for ejector and extractor markings, where the carbon starts if at all.

 

[BLEE]

Make sure the gas port is in spec and run a Sprinco Orange. Tune your AGB, accordingly. I run the orange and a 4.7oz buffer, with a carbine tube, on my Aero M5, 6.5CM and it consistently and vigorously ejects at 4 o'clock. Recoil is similar to a good running AR15 in .223.

 

[mettee]

Hmmm... Interesting.

I have been doing all of my testing with 178gr and 185gr Federal Terminal Ascent or Sierra Match King factory loads. Trying to use this for longer range shooting; any advice for making it run reliably with the hotter stuff?

@FredHammer recommended 30-clicks... I am going to give that a go.

It probably won't run reliably if you pass a certain point pressure wise. Hunting rounds are typically set up for bolt guns and don't get along well with gas guns.

With the gas port potentially being on the small side( your measurement wasn't the best) and the feedback on recoil, now adding in hot ammo, it makes more sense.

A gas gun should run federal gold medal match 168 and 175s. I believe many use it as the gold standard.

 

[thehun]

He's describing super heavy recoil, so for me that gives the answer, nothing else can cause recoil like that.

Hot loads, chamber, tight bore, many things can lend to this, not sure what it is yet.

Need to see brass too. Look for ejector and extractor markings, where the carbon starts if at all.

He measured the gas port and it’s under the nominal gas port size … meaning the gas port is undersized not oversized …

But yea. It can be hot loads for sure.

 

[msgriff]

Hmmm... Interesting.

I have been doing all of my testing with 178gr and 185gr Federal Terminal Ascent or Sierra Match King factory loads. Trying to use this for longer range shooting; any advice for making it run reliably with the hotter stuff?

@FredHammer recommended 30-clicks... I am going to give that a go.

You should really follow the tuning procedures that Superlative recommends, loading one round in the mag and adjusting until you get LRBHO, then add a bit more for reliability. I've found that it's easier to tune in restrictive mode (1 - 18 clicks/4.5 rotations) until the firearm breaks-in, then re-tune in bleed-off. You should know how the bleed-off works, and that bleed-off has limitations with severely over-gassed rigs.

This table can be confusing, particularly the variation in charge weights, but it might help some see the correlations between projectile weight and port pressure using the same powder type.

 

[msgriff]

He measured the gas port and it’s under the nominal gas port size … meaning the gas port is undersized not oversized …

But yea. It can be hot loads for sure.

Not everyone here agrees with your assessment that the gas port is too small. The OP may very well decide to open up the port, and it may solve his issue, but it is in no way guaranteed. There are a couple of ways to test for under-gassed/short stroking conditions before bringing out a drill. There are also other factors which can cause short stroking other than an undersized gas port.

 

[thehun]

Not everyone here agrees with your assessment that the gas port is too small. The OP may very well decide to open up the port, and it may solve his issue, but it is in no way guaranteed. There are a couple of ways to test for under-gassed/short stroking conditions before bringing out a drill. There are also other factors which can cause short stroking other than an undersized gas port.

I didn’t say that is solely the issue but it points to it.

He can also have a gas leak somewhere …

Or hot loads …

 

[cunnerman]

FWIW... I didn't test solely with hunting loads.

My first range trip was with a box of Browning 308win 168gr Long Range Pro w/ Sierra Tipped MatchKing. I went through the entire box of 20 without getting the rig to cycle once. That being said, I also never went past 20 on the SA AGB.

As for recoil, yes, it is harsh. Definitely can not stay in the scope between shots.

 

[tylerradke]

This sounds exactly like my rifle which is nearly identical build. Aero receivers Wilson combat 20" SS superlative gas block, Wilson BCG, rifle buffer.
My rifle is doing the same thing, right down to the harsh recoil. Fgmm will not cycle in mine but lake city m118 will 90% of the time. I have turned gas block down and it only cycles worse. My rifle also beats the shit out of my brass denting case mouth hard and it looks like the dual ejectors are putting too much pressure on case or its cycling too slow because I have 2 gouges in case neck that appears to be from the barrel extension lugs. I will say my ejection is about 330-4 but it launches the brass 10ft. I thought about trying my suppressor to see if it helps cycling, that or has anyone tried an AR15 rifle spring in their AR10? Also what's the deal with the gas tube? Mine looks a little short in the upper receiver.

 

[longknives]

Also what's the deal with the gas tube? Mine looks a little short in the upper receiver.

That is a problem, WC drills the port in a different location so you need to get a longer gas tube to reach the center of the cam pin cutout. I think that length is apx. 15 1/2".

 

[cunnerman]

Also what's the deal with the gas tube? Mine looks a little short in the upper receiver.

That is a problem, WC drills the port in a different location so you need to get a longer gas tube to reach the center of the cam pin cutout. I think that length is apx. 15 1/2".

Wilson is annoying like that... the gas port isn't placed perfectly for either the AR10 tube or the standard rifle length tube. The picture below is of my build with Wilson's AR10 rifle tube. I also have the Wilson standard rifle tube, but if I install that, it will come up short on the cam pin cutout. But maybe that will reduce the gas in the gas-key just enough to slow the cycling?

Here's a stove pipe from my last range visit. Note the smashed-in casing...

 

[cunnerman]

Update for those asking about chamber... I did test 308 go/no-go gauges, and they worked (or didn't work) as expected.

 

[thehun]

Wilson is annoying like that... the gas port isn't placed perfectly for either the AR10 tube or the standard rifle length tube. The picture below is of my build with Wilson's AR10 rifle tube. I also have the Wilson standard rifle tube, but if I install that, it will come up short on the cam pin cutout. But maybe that will reduce the gas in the gas-key just enough to slow the cycling?

View attachment 8284249

Here's a stove pipe from my last range visit. Note the smashed-in casing...
View attachment 8284250
View attachment 8284251

Your gas tube should not be that far back ... it should be right in the middle of the cam track ... wonder if that can cause the bolt not being fully locked in ...

Here is a pic of my AR10 with a 20" Criterion 308 barrel with rifle gas ... I am also using an Aero M5 receiver set ...

 

[mettee]

He measured the gas port and it’s under the nominal gas port size … meaning the gas port is undersized not oversized …

But yea. It can be hot loads for sure.

Well yes and no, he used calipers, that isn't going to give a real number.

He needs to use drill bits or something, only saying that because gauge pin sets aren't cheap.

 

[thehun]

Well yes and no, he used calipers, that isn't going to give a real number.

He needs to use drill bits or something, only saying that because gauge pin sets aren't cheap.

True.

 

[mettee]

Your gas tube is wrong as already mentioned...that will keep the key "in the gas" longer than intended. It's only milliseconds but that's all it takes. So your BCG is being pushed for a longer duration.

The stove pipe also tells me the bolt is traveling forward faster than it should, likely after hitting the bottom of the extension.

The flat case mouth will likely go away once you sort the other issues out.

Great pics

 

[thehun]

I guess my question is ... what is WC thinking spec'ing out a gas tube this way ... makes no sense ... it just further confuses AR10 builders ...

So is WC using Armalite gas length specs?

 

[cunnerman]

Your gas tube should not be that far back ... it should be right in the middle of the cam track ... wonder if that can cause the bolt not being fully locked in ...

Here is a pic of my AR10 with a 20" Criterion 308 barrel with rifle gas ... I am also using an Aero M5 receiver set ...

Your gas tube is wrong as already mentioned...that will keep the key "in the gas" longer than intended. It's only milliseconds but that's all it takes. So your BCG is being pushed for a longer duration.

The stove pipe also tells me the bolt is traveling forward faster than it should, likely after hitting the bottom of the extension.

Great pics

OK. So I had thought about this, and was wondering if it was overgassed due to extended gas key dwell time (on the tube).

The problem is there is literally no way for me to get the gas tube at the corrent position (exactly halfway) on the cam-pin cutout. Wilson Combat is r*tarded and splits the difference between AR10 rifle-length and standard rifle-length with their gas port location (on the barrel). So no matter which tube I use, I'm either going to be too short, or too long.

My photo shows the 15.5" AR10 rifle-length tube. If I install the 15.125" standard rifle-length tube, losing that 3/8" means it will fall short on the cam-pin cutout (see pic below). I hope this is all making sense.

Should I just go ahead and try the "too short" option at this point?

 

[thehun]

Have you tried using the WC gas block vs the SA?

 

[mettee]

If you go shorter I can't help but assume that that will also create its own problems. You'll be turning the gas up to achieve function.

Modifying an existing tube could be a solution. "Fix" the current gas tube by drilling a new port hole and roll pin hole, cut it shorter and replace the plug? What I am thinking, is that you make it so you can install the gas tube deeper into the gas block. This mod along with setting the gas block could be the solution.

The other option is ending the gas key sooner but that's not the best idea either. I wonder what Wilson would say.

 

[tylerradke]

So wilson doesn't have the correct length gas tube for their barrels? That's the dumbest thing I have heard in a while. So does the longer gas tube work as well? Maybe that would fix my issues, maybe not, who the hell knows at this point.

 

[cunnerman]

If you go shorter I can't help but assume that that will also create its own problems. You'll be turning the gas up to achieve function.

Modifying an existing tube could be a solution. "Fix" the current gas tube by drilling a new port hole and roll pin hole, cut it shorter and replace the plug? What I am thinking, is that you make it so you can install the gas tube deeper into the gas block.

The other option is ending the gas key sooner but that's not the best idea either. I wonder what Wilson would say.

Looks like this chap had the exact same problem...

I'm now seriously considering just buying a different barrel. It seems like Wilson Combat doesn't take their engineering very seriously when it comes to this sort of stuff.

What are the recommendations out there for standard gas-port position 20" 308 barrels <$500? Doesn't have to necessarily be match grade, but I'd like something that will be a solid shooter at 200-600 yards.

 

[tylerradke]

Looks like this chap had the exact same problem...

I'm now seriously considering just buying a different barrel. It seems like Wilson Combat doesn't take their engineering very seriously when it comes to this sort of stuff.

What are the recommendations out there for standard gas-port position 20" 308 barrels <$500? Doesn't have to necessarily be match grade, but I'd like somethign that will be a solid shooter.

No kidding. I even went with their bcg to help try to avoid any issues and look where we are at. Does your long tube allow the bolt to go all the way into battery?