Help diagnosing issue in .308AR

[Quintus]

I recently put together a newish (barrel and receiver is new, GB & BCG used from previous upper) 308 upper and have been experiencing some stove piping issues. I have an SLR7 AGB on it and when set to position 6 from closed the rifle ejects near perfect 3 o'clock but will either stovepipe or jam a fresh cartridge over the partially extracted spent cartridge, causing a double feed, this happens at least once or twice a mag. If I constrict the gas down to setting 5, the extraction is a little past 3 and it functions perfectly but will not lock the BCG back. I'm using PMAGs.

3 o'clock ejection isn't over gassed, so could it be my extractor that is going bad? It's an older JP enhanced .308 with maybe 800-900 rounds on its lifetime. And setting 5 isn't under gassed either, so the BCG should be locking back? I'm using a tubbs .308 spring, which is new. Could that be the reason why it's not locking back the BCG? I didn't have the original .308 spring on hand, otherwise I would have swapped it out to try. I've never experienced this issue before, so I'm not sure if it's just the combination of parts or something else wrong.

Any feedback would be helpful, thank you.

 

[mettee]

If you made changes to the spring and it started having issues, that is likely the problem. With what you describe it really seems like a spring being more than needed.

Go back to the original spring. If you still have issues explore gas adjustments and buffer weights. It seems like you may benefit from an increase in weight, but I'm basing that only on what you described.

Also consider changes to ammo that may be loaded to differing pressures.

 

[msgriff]

Which manufacturer made your new upper receiver, and is it the same as your lower?
Can you manually engage the bolt catch and how high up on the bolt lugs does it go?
What is the distance between the bolt lugs and the bolt catch at full pull-back of the charging handle?
What is your barrel and gas system lengths, and are you using suppressor?
What length receiver extension and what is your buffer weight?

 

[mettee]

We should sticky a 308/6.5 gas gun troubleshooting questionnaire someplace.

 

[Quintus]

Which manufacturer made your new upper receiver, and is it the same as your lower?
Can you manually engage the bolt catch and how high up on the bolt lugs does it go?
What is the distance between the bolt lugs and the bolt catch at full pull-back of the charging handle?
What is your barrel and gas system lengths, and are you using suppressor?
What length receiver extension and what is your buffer weight?

Mega Arms, same upper & lower. I can manually engage the bolt catch and it goes up almost halfway up the first lug that catches. Distance between the catch and bolt is probably a half inch when the CH is pulled all the way back. 16" barrel mid length gas, no suppressor. A5 extension with an H3 buffer.

 

[Steel+Killer]

If I constrict the gas down to setting 5, the extraction is a little past 3 and it functions perfectly but will not lock the BCG back. I'm using PMAGs.

I would try a lighter buffer or trim the spring a little.

 

[longknives]

In response to metee's suggestion,, Albroswift from another forum made up a pretty good idea for a questionnaire.

Waterboarding

Got an idea that may reduce/ shorten the waterboarding procedure. I know we have a lot of fun trying to worm the pertinent information out of the OP, but maybe a check list with pictures of what exactly is meant by "Describe Your Build" would help ... green receiver with black safety lever and st...

forum.308ar.com

 

[Quintus]

I would try a lighter buffer or trim the spring a little.

I'll probably first swap the tubbs spring out for the armalite spring that I've successfully used in the past. I also have an H2 buffer I can try if it still doesn't lock open. I'm just trying to gather as many troubleshooting ideas as possible before my next range trip so I can try all solutions.

 

[Quintus]

In response to metee's suggestion,, Albroswift from another forum made up a pretty good idea for a questionnaire.

Waterboarding

Got an idea that may reduce/ shorten the waterboarding procedure. I know we have a lot of fun trying to worm the pertinent information out of the OP, but maybe a check list with pictures of what exactly is meant by "Describe Your Build" would help ... green receiver with black safety lever and st...

forum.308ar.com

I checked the link and took a second look at my gas tube inside the receiver. It looks like I could slide the GB back a tiny bit more to move the end of the gas tube closer to the center of the cam cut out, maybe 1mm or so more. I had used a borescope to line up the GB and was on the conservative side with positioning, I didn't want any chance the tube could pre-load on the key and negatively affect accuracy.

I also took out the tubbs spring and a lot of black flakes started coming off the spring as I removed it from the buffer tube, all over inside my lower receiver, inside the tube and all over the floor, it was a real mess, I had to use an air compressor to blow all of it out. I probably won't re-use it again after that. I re-installed the Armalite spring.

 

[mtrmn]

In response to metee's suggestion,, Albroswift from another forum made up a pretty good idea for a questionnaire.

Waterboarding

Got an idea that may reduce/ shorten the waterboarding procedure. I know we have a lot of fun trying to worm the pertinent information out of the OP, but maybe a check list with pictures of what exactly is meant by "Describe Your Build" would help ... green receiver with black safety lever and st...

forum.308ar.com

I was just about to post the same thing. Thanks

 

[mtrmn]

I'll probably first swap the tubbs spring out for the armalite spring that I've successfully used in the past. I also have an H2 buffer I can try if it still doesn't lock open. I'm just trying to gather as many troubleshooting ideas as possible before my next range trip so I can try all solutions.

Sounds like a good start. Only change 1 thing at a time for proper troubleshooting.

 

[mtrmn]

I checked the link and took a second look at my gas tube inside the receiver. It looks like I could slide the GB back a tiny bit more to move the end of the gas tube closer to the center of the cam cut out, maybe 1mm or so more. I had used a borescope to line up the GB and was on the conservative side with positioning, I didn't want any chance the tube could pre-load on the key and negatively affect accuracy.

I also took out the tubbs spring and a lot of black flakes started coming off the spring as I removed it from the buffer tube, all over inside my lower receiver, inside the tube and all over the floor, it was a real mess, I had to use an air compressor to blow all of it out. I probably won't re-use it again after that. I re-installed the Armalite spring.

The tube needs to be as close to exactly in the middle of the cutout as possible. In my experience, you shouldn't be able to move the gas block back any more if it's in the proper position. You may need a longer tube. Mine required the 12-1/16" length (Armalite 308 carbine length) gas tube. It's something like 5/16" longer than the "AR15 mid length" tube that seems to be what you normally get in LR308 parts kits.

Also, MEASURE, don't guesstimate the gap between the bolt catch and front of the bolt at full pullback. Before and after you swap springs or buffers. IIRC, the gap should around 1/8-1/4". If it's more than that, you can remove the spring and drop a quarter into the bottom of the tube to take up some space.

 

[longknives]

Post 11 & 12 is solid advice!

I would give the Spinco Red spring a go.

 

[mettee]

I checked the link and took a second look at my gas tube inside the receiver. It looks like I could slide the GB back a tiny bit more to move the end of the gas tube closer to the center of the cam cut out, maybe 1mm or so more. I had used a borescope to line up the GB and was on the conservative side with positioning, I didn't want any chance the tube could pre-load on the key and negatively affect accuracy.

I also took out the tubbs spring and a lot of black flakes started coming off the spring as I removed it from the buffer tube, all over inside my lower receiver, inside the tube and all over the floor, it was a real mess, I had to use an air compressor to blow all of it out. I probably won't re-use it again after that. I re-installed the Armalite spring.

That flat wire spring is more than likely scraping the black anno off the inside of the receiver extension. If its doing that its also inhibiting the BCGs movement. Shine a light in there and see?

I am running a 7 5/8" internal receiver extension(or A5) with the standard 308 rifle spring even with a TBAC suppressor, I dont see a reason to up the spring pressure, an adjustable gas block would be a better mod(if needed). Extra spring pressure is going to cause unnecessary forces in compression and rebound of the spring. You only need to control the BCG movement, not overcontrol it.

I would keep the buffer you have because you are running mid length gas vs rifle.

 

[mettee]

What can we add, subtract, and or modify from this list?


1.) Are you shooting suppressed, if so what suppressor?
2.) What BCG do you have and what is the weight?
3.) What is your buffer weight?
4.) What is your buffer length?
5.) What is your Receiver Extension/Buffer Tube internal Depth?
6.) What is your Buffer Spring, Length, Coils, Brand, Color, etc?
7.) What is your Gas System Length- Rifle, Midlength, Carbine, Pistol, Other?
8.) What is your Gas tube Length, number 9 can help answer this?
9.) Take a Photo looking directly into the magwell that shows gas tube protrusion distance and post it.
10.) Take a Photo of the Bolt Face and post it.
11.) What is your BCG Clearance from the bolt stop under full compression?
12.) What is your Gas Journal Dia and gas Port Dia measured via wire drill size (If you can get it)?
13.) Photo and Description of the Malfunction or Issue.
14.) Any photos that are helpful, spent brass, etc.
15.) What ammo are you using, factory, hand loads, list specs?
16.) What lubrication are you using, grease, oil, etc?

 

[msgriff]

That flat wire spring is more than likely scraping the black anno off the inside of the receiver extension. If its doing that its also inhibiting the BCGs movement. Shine a light in there and see?

I am running a 7 5/8" internal receiver extension(or A5) with the standard 308 rifle spring even with a TBAC suppressor, I dont see a reason to up the spring pressure, an adjustable gas block would be a better mod(if needed). Extra spring pressure is going to cause unnecessary forces in compression and rebound of the spring. You only need to control the BCG movement, not overcontrol it.

I would keep the buffer you have because you are running mid length gas vs rifle.

My understanding is that Tubb changed the alloys that they use a couple years ago. The new versions are coated with dry film lube, which have reported flaking issues. Some say the flaking is a non-issue, I'm not sure that I'm in that camp.

 

[Ex E6]

MY $0.02 worth , I like advice change one component per attempt to rectify issue . That said cartridge extraction isn't clearing the gate before BCG is pulling up another feed aka short stroking . BCG is NOT fast enough ,H2 buffer would have been 1 St. order of change , second order Spring in that order . Stiffer springs are wonderful ,provided A; One is running Hefty charges or heavy bullets with full loads . B; Gas charge is sufficient to full cycle

IF problems persist Gas issue is more likely problem . Everyone assumes if the gas tube is in and BCG is active gas isn't an issue . Gas pressures are ALWAYS an issue . Take a gander at this sample run of ammo and issues regarding different powders . Fail to lock on empty fail to cycle ,yet other powders weren't an issue . I also run 7.62x51mm ,as well as 7.62x63mm ,so Gas issues and I are old acquaintances . This target happens to be with one of MY 6.5 CM . Yet runs near everything else perfectly , Powder pressure Peak and secondary MATTER .

​

 

[mtrmn]

If you remove the gas block don't reinstall it before you get a reliable measurement of the gas port diameter. If your gun is actually UNDERgassed due to too small a port adjustable gas will not help you at all. What diameter is the gas journal? Most are .750 but not all.

 

[Quintus]

The tube needs to be as close to exactly in the middle of the cutout as possible. In my experience, you shouldn't be able to move the gas block back any more if it's in the proper position. You may need a longer tube. Mine required the 12-1/16" length (Armalite 308 carbine length) gas tube. It's something like 5/16" longer than the "AR15 mid length" tube that seems to be what you normally get in LR308 parts kits.

Also, MEASURE, don't guesstimate the gap between the bolt catch and front of the bolt at full pullback. Before and after you swap springs or buffers. IIRC, the gap should around 1/8-1/4". If it's more than that, you can remove the spring and drop a quarter into the bottom of the tube to take up some space.

I did some measurements with calipers and feeler guages, the distance between the bolt face and bolt catch when bottomed out is 5.7mm, just under 1/4. And I have about 2.6mm of space between the back of the GB and the barrel shoulder, so I have should have plenty of gas tube to move it back.

 

[Quintus]

If you remove the gas block don't reinstall it before you get a reliable measurement of the gas port diameter. If your gun is actually UNDERgassed due to too small a port adjustable gas will not help you at all. What diameter is the gas journal? Most are .750 but not all.

I would have to eyeball it with calipers, I don't have any pin gauges. Its a .750 journal.

 

[mtrmn]

I did some measurements with calipers and feeler guages, the distance between the bolt face and bolt catch when bottomed out is 5.7mm, just under 1/4. And I have about 2.6mm of space between the back of the GB and the barrel shoulder, so I have should have plenty of gas tube to move it back.

I've always butted the gas block against the shoulder, but some are made to allow for the thickness of the triangular handguard retainer. Just make sure the hole is aligned with the gas port when you move it.
Use the opportunity to measure the gas port while it's loose. If you have a set of drill bits you can probably get close. I've used a set of cutting torch tip cleaners before. Found one that fit and measured it with calipers.

 

[Quintus]

I've always butted the gas block against the shoulder, but some are made to allow for the thickness of the triangular handguard retainer. Just make sure the hole is aligned with the gas port when you move it.
Use the opportunity to measure the gas port while it's loose. If you have a set of drill bits you can probably get close. I've used a set of cutting torch tip cleaners before. Found one that fit and measured it with calipers.

I was able to fit the caliper upper jaw tips in the port and get a measurement of .082, eyeballing on the outside with the lower jaws, it looks about accurate.

With the GB butted against the shoulder the end of the gas tube is dead center in the cam channel. I'm just going to verify with the borescope that the GB isn't covering any part of the gas port.

 

[Quintus]

I was able to fit the caliper upper jaw tips in the port and get a measurement of .082, eyeballing on the outside with the lower jaws, it looks about accurate.

With the GB butted against the shoulder the end of the gas tube is dead center in the cam channel. I'm just going to verify with the borescope that the GB isn't covering any part of the gas port.

Verified with borescope that the GB is fine against the shoulder, plenty of breathing room!

 

[mettee]

That's all positive news. I say run that 308 spring and re-adjust your gas. You could try both springs and use the one that works the best. The gas tube being just that slight bit more centered could be that extra push of gas to help it achieve lock back.

 

[Top O' Texas]

To save the 2nd round from being beat up, reuse it until issue is fixed. Most tune gas with an empty mag for lrbho? Knowing if it's short stroking or overriding the bolt catch can be confusing! If you know what setting the block was on before (or when last used on correct cycling) set it back + 2-4 more clicks open. I like tuning with 3-5 rounds in the mag before charging 1. When a round doesn't extract after fired the extractor (depending on how many rings are used) can be to tight and will cause stove-piping (This was the case on an AP bcg I had). Now if it's too loose it won't hold onto it and extract from the chamber and look like double feeding.

Back to the A5! Are you using an AR10 rifle spring in conjunction with a 3.25" buffer (AR15 carbine buffer) or just a standard flat-wire spring not for this setup? I haven't tried any flat-wire springs yet in an A5 tube, so it's unfamiliar territory.

You stated it will hold open when the chargehandle is used? Is it held by manually pressing on the catch or activated by the mag follower?

Is this an armalite setup or dpms platform? Not sure what (if any) length difference there is between armalite and dpms springs?

 

[Bobby Ball]

I don't think I saw this reading through everything so far, but does your extractor have both the spring and an o-ring installed? I've had a few occasions with multiple rifles where simply removing the o-ring has solved similar problems completely. There is a lot of good advice in this thread, just thought I'd throw this in if all else fails it's a very quick and simple thing to test.

 

[Quintus]

I'm fairly confident that the GB being set too far forward is the likely culprit given that there are no other indicators of anything else being out of spec. I am using a clamp on block, so positioning isn't fixed like a set screw would have been with the first dimple as an indexing point. I won't be able to test it out again until maybe after Christmas, but I will definitely post an update once I do.

And now that I know that the tubbs flaking is just dry lube, I'll test both springs, but for now I'll just leave the Armalite in place.

 

[mtrmn]

I'm fairly confident that the GB being set too far forward is the likely culprit given that there are no other indicators of anything else being out of spec. I am using a clamp on block, so positioning isn't fixed like a set screw would have been with the first dimple as an indexing point. I won't be able to test it out again until maybe after Christmas, but I will definitely post an update once I do.

And now that I know that the tubbs flaking is just dry lube, I'll test both springs, but for now I'll just leave the Armalite in place.

Probably your best bet until you get everything figured out. You now have the basic recoil system with the proper combination of A5 length tube, Armalite spring, and H3 buffer that's known and proven to work. ( at least without a suppressor) The same system Armalite puts on their .308 carbines.
You do whatever you think is necessary, but if the problem persists, I would fully open the gas block as my next step. You'll essentially be running it like it has a normal non-adjustable gas block. If it works but seems overgassed, adjust down from the wide open position until it changes. Then decide if the change is better or worse. Adjustable gas is a blessing and a curse when it comes to troubleshooting. It introduces too many variables at once, and every time you change something else you may have to go back to square one on the gas adjustment and start over.
Also, on large frame ARs, many times the ejection angle may never get to the perfect angle without other malfunctions showing up. If it has strong ejection, not damaging the rim of the brass, cycling correctly etc then who cares if it's ejecting at 2:00 if that's the best it gets? Take plenty of ammo and several mags with you

 

[mtrmn]

I was able to fit the caliper upper jaw tips in the port and get a measurement of .082, eyeballing on the outside with the lower jaws, it looks about accurate.

With the GB butted against the shoulder the end of the gas tube is dead center in the cam channel. I'm just going to verify with the borescope that the GB isn't covering any part of the gas port.

The port in my Faxon 16" pencil bbl is .093, .750 journal, requiring the 12-1/16" gas tube.

 

[Ex E6]

Port sizing is a cluster fuck of info ,IMO . I've seen 16" tubes with 0.065 holes and Ideally their 16" Bbl. length in .308 is 0.081 !. Ditto with AR 15 Platform . Again according to manufacturers longer the tube greater gas port hole Diameter . 20" Bbl. .308 0.096 .

I've got a 22.5" tube in .308 which eats everything with equal gusto and it's 0.088 ,so go figure . One thing is certain though ,adjustable gas blocks are MARGINAL on smaller ports .