BCG hitting back of receiver and buffer tube

[Squad51]

I have a 308 Aero M5 and I just noticed the BCG is hitting the receiver. What can I do to slow down the BCG without spending a fortune?

Specs:
308 16" BA barrel
Mid-length gas
Superlative AGB - turned down to one click above jamming
Carbine length buffer with standard buffer spring and buffer

 

[hlee]

You probably just need a spacer to stop the buffer before the carrier hits the receiver. A quarter (maybe 2) dropped into the receiver extension before installing the buffer spring will work…

This assumes that your components are all matched to each other. Rifle buffer in rifle tube, carbine buffer in carbine tube, etc.

 

[msgriff]

If it's a parts build, it would help if you listed the exact parts used, such as the BCG, receiver extension, buffer and spring. Accurately measure the internal depth of the RE to the end of the threads and the length of the buffer. Measure the distance from the bolt lugs to the bolt catch at full pull-back of the charging handle.

 

[Milf Dots]

Call Aero for warranty support on their rifle.

 

[Squad51]

This was built by a local gun store, not a factory rifle.

Call Aero for warranty support on their rifle.

 

[6.5SH]

This was built by a local gun store, not a factory rifle.

What upper, BCG, buffer and buffer tube were used?

 

[XLR308]

Its a parts compatibility problem.

 

[kilizyrag]

The buffer is to short allowing the BCG to travel to far. You can stack quarters in the buffer tube to take up the extra space, but then the spring becomes pre-loaded which is not good. Show us some pictures of the parts. How deep is the buffer tube?

 

[XLR308]

If you bought it that way i would take it back and request a refund since it is now damaged by no fault of your own.
Is it still operable sure but new damaged isnt a good buiseness model.

 

[Magixx]

If your BCG is hitting the receiver, it could be an indication that your rifle is over-gassed.

Upgrading to a heavier buffer, such as an H2 or H3 buffer, will slow down the BCG and reduce felt recoil. This can help prevent the BCG from hitting the receiver. However, be sure to check the manufacturer's recommendations for buffer weight to ensure compatibility with your rifle.

 

[Squad51]

Unfortunately, I do not know what parts were used. The buffer and BCG do not have any markings. The buffer is 2.5" long and weighs 4oz. When I put the buffer and BCG in without the spring, it does not contact the buffer tube. The first picture below is the resting position. So apparently the buffer pad is compressing during recoil to allow the BCG to contact the tube.

 

[MK20]

So put a quarter in the bottom of the tube and learn your lesson.

Build it yourself and never trust anyone to know what they are doing these days.

 

[Squad51]

Build it yourself and never trust anyone to know what they are doing these days.

^^^ This! I went into the store to buy the receiver and the owner convinced me it would not cost much more to have him build it. I knew AR10/M5 builds were finicky so I thought I was making a good decision. I should have done it myself. It's been 4 years so I can't really take it back now.

Thank you to everyone that took the time to reply and help me out.

 

[CaptArab]

Could be totally off here but is the receiver extension supposed to have that U cut out in it like that?

I don't think mine has that- therefore mine isn't threaded in so deeply, providing additional clearence. I think that's the issue. Or at least one of the issues.

The aero M5 extensions are real cheap if you want to try that.

I'll let the rest of the gang here fact check me. I'm going off memory away from my rifle so I could be way off.

 

[Gustav7]

If your BCG is hitting the receiver, it could be an indication that your rifle is over-gassed.

Upgrading to a heavier buffer, such as an H2 or H3 buffer, will slow down the BCG and reduce felt recoil. This can help prevent the BCG from hitting the receiver. However, be sure to check the manufacturer's recommendations for buffer weight to ensure compatibility with your rifle.

No it's not. Every rifle ever made properly, should not be having the BCG hit the RE or lower. Its a parts compatibility issue because the gun shop doesn't know jack about large frame builds, and just ignorantly slapped together random large frame parts.

^^^ This! I went into the store to buy the receiver and the owner convinced me it would not cost much more to have him build it. I knew AR10/M5 builds were finicky so I thought I was making a good decision. I should have done it myself. It's been 4 years so I can't really take it back now.

Thank you to everyone that took the time to reply and help me out.

OP, you have a couple options:

1.) Buy the proper Armalite buffer tube kit that is 3/4" longer and uses a regular 3.25" buffer thats MINIMUM 5.3oz. Another option that is the same thing is to buy the Vltor A5 receiver extension which is the same length as the Armalite, and then buy a STANDARD 3.25" buffer(not the A5 buffer). You can find the PWS steel body buffers that are ~5.2oz (you can also get a 6.8oz one). Then either get an Armalite rifle buffer spring, or a proper Sprinco spring.

2.) Buy a better 2.5" buffer that is 5oz or heavier. I recommend a 6oz+ buffer if you have an adjustable gas block. I think KAC makes a heavy 2.5" and so does heavybuffers.com

3.) Buy a different DPMS pattern BCG and see if it still does it. (probably not the best starting point).

4.) Put a quarter in the bottom. Not the best choice long term, but will probably solve your issue for now.

 

[1911hombre]

That tube is threaded WAY too deep into the lower. I don't understand how it could even get threaded in that far with the buffer retention pin present. Some goober depressed that retention pin WAY down in the bore to allow that tube to get in that far. At least two turns too far.

 

[Gustav7]

Could be totally off here but is the receiver extension supposed to have that U cut out in it like that?
I don't think mine has that- therefore mine isn't threaded in so deeply, providing additional clearence. I think that's the issue. Or at least one of the issues.
The aero M5 extensions are real cheap if you want to try that.
I'll let the rest of the gang here fact check me. I'm going off memory away from my rifle so I could be way off.

It is hitting the lower receiver as well. If you screwed the RE out further, I bet it'd make a nice chunk into the lower receiver.

 

[XLR308]

It is hitting the lower receiver as well. If you screwed the RE out further, I bet it'd make a nice chunk into the lower receiver.

^^ This if the tube is backed out anymore the damage would-be worse as it would have more reward travel when cycling.

 

[CaptArab]

^^ This if the tube is backed out anymore the damage would-be worse as it would have more reward travel when cycling.

Just cause that's wrong, don't make the receiver extension right

 

[XLR308]

Just cause that's wrong, don't make the receiver extension right

I totally agree thats why my first comment was its a parts compatibility issue.

 

[Kadams1563]

Does it function?

Just keep shooting it and it will remove whatever is in the way and be custom fit.



Lol don’t do that.

 

[msgriff]

That looks like an Aero Enhanced Carbine Buffer Tube, which has the U notch. It does not look to me to be improperly installed, and if it was unthreaded 1 turn, it would increase the BCG travel. There have been some reports recently of similar situations, and some believe that a batch of polymer bumpers are compressing too much. You can use a spacer at the bottom of the RE, but you should verify that there is enough distance between the bolt lugs and the bolt catch for LRBHO to function. There is a simple way to measure that distance. There are some specific BCG construction details that are pertinent and I believe that @rpoL98 has some info on that.

 

[mtrmn]

That tube is threaded WAY too deep into the lower. I don't understand how it could even get threaded in that far with the buffer retention pin present. Some goober depressed that retention pin WAY down in the bore to allow that tube to get in that far. At least two turns too far.

This is backwards to the way it actually works. The tube is threaded in correctly from what I can see in the pic.

 

[Cascade Hemi]

2A Armament sells delrin buffer tube spacers. AR308 parts don't always interact perfectly, even when they're sourced from the same vendor. Spacers will help limit the BCG from over traveling which tends to cause the BHO to fail prematurely.

 

[6.5SH]

Unfortunately, I do not know what parts were used. The buffer and BCG do not have any markings. The buffer is 2.5" long and weighs 4oz. When I put the buffer and BCG in without the spring, it does not contact the buffer tube. The first picture below is the resting position. So apparently the buffer pad is compressing during recoil to allow the BCG to contact the tube.

View attachment 8116084
View attachment 8116085

Compare your BCG with Aero's, theirs is tapered where yours is making contact:

.308 / 7.62 Bolt Carrier Group, Complete - Black Nitride

Our .308 / 7.62 Bolt Carrier Group is the perfect firing mechanism for your big-bore rifle build. This high quality BCG features a Black Nitride finish and now includes a laser engraved Aero Precision "A" logo!

www.aeroprecisionusa.com

While they are "mostly" DPMS AR-308 pattern receivers, they state they are not 100% the same. Even that BCG they list as "M5" platform that fits AR-308, not the other way around.

What pattern AR308 is the the M5?
The M5 shares compatibility with several DPMS Gen 1 parts and components. However due to different variances within that category we do not guarantee that all DPMS Gen 1 style components are compatible.

 

[bfoosh006]

Not all 2.5" buffers are actually 2.5".

Measure yours... also measure the internal depth of your buffer tube.

Same goes for the recoil spring. I have seen some that were flat out wrong.
It could very well be a tolerance stacking issue.

I have used Tubbs 308 Flatwire springs and KAK 2.5" 5.3oz in all my DPMS pattern buffer setups for years, never had any lower impacts.

 

[hlee]

Does it function?

Just keep shooting it and it will remove whatever is in the way and be custom fit.



Lol don’t do that.

The AKM approach.

Does it need to be fixed?

Nyet, rifle is fine.

 

[Nebulous]

While I agree with what everyone has been saying, I'm curious what the brass looks like coming out of this gun? OP says the agb is basically closed, I'm curious if he's got a tight chamber exacerbating the light buffer issue.

 

[rpoL98]

That looks like an Aero Enhanced Carbine Buffer Tube, which has the U notch. It does not look to me to be improperly installed, and if it was unthreaded 1 turn, it would increase the BCG travel. There have been some reports recently of similar situations, and some believe that a batch of polymer bumpers are compressing too much. You can use a spacer at the bottom of the RE, but you should verify that there is enough distance between the bolt lugs and the bolt catch for LRBHO to function. There is a simple way to measure that distance. There are some specific BCG construction details that are pertinent and I believe that @rpoL98 has some info on that.

 

[6.5SH]

I prefer the bolt carriers where the back end of the fat part is beveled.

regarding your AGB tuning, "one click above jamming", I believe a better set-point is one click above last-round bolt hold open. load 1 round in the mag, shoot it, does the bolt face lock back behind the bolt catch? if not, increase the gas incrementally until it does, then 1 clock more, just for operation margin.

if at the outset the bolt face does lock back, then load 1 round, decrease the gas incrementally until it doesn't lock back, then increase 1 click to verify that it does again lock back, and then (optional), 1 click more.

make sure it's the bolt face locking back behind the bolt catch, not the lower edge of the bolt carrier, and not with any part of the BCG getting caught on the mag follower.

If you look closely at OP's pics can see where the top of the BCG hit the back of the lower as well.

 

[Squad51]

While I agree with what everyone has been saying, I'm curious what the brass looks like coming out of this gun? OP says the agb is basically closed, I'm curious if he's got a tight chamber exacerbating the light buffer issue.

The brass looks good. I was shooting suppressed (for the first time). I initially turned the AGB down but had extraction issues so I turned it back up. It ended up back where it was without the suppressor. As for the brass, the primers are a little flat but this seems normal for Winchester primers. The spent cases fit back in my case gage but the shoulder is pushed out by ~.002.

 

[Yondering]

That tube is threaded WAY too deep into the lower. I don't understand how it could even get threaded in that far with the buffer retention pin present. Some goober depressed that retention pin WAY down in the bore to allow that tube to get in that far. At least two turns too far.

No it isn't. The buffer tube appears to be installed correctly, it's just a different buffer tube than your rifle has. The enhanced tubes from Aero and several other places have an extended section that the back of the BCG rides on, and it's notched out for the retention pin. It wasn't some goober who installed that, looks to me like they did that part right.

 

[Jgault]

You probably just need a spacer to stop the buffer before the carrier hits the receiver. A quarter (maybe 2) dropped into the receiver extension before installing the buffer spring will work…

This assumes that your components are all matched to each other. Rifle buffer in rifle tube, carbine buffer in carbine tube, etc.

This is correct. The problem is pretty common when using a carbine sized buffer in a 308, the 1 or 2 quarters until the bolt face is just behind the port. Another option is to use the sprinco orange 308 spring, it’s stiffer and a little longer, on one of mine it replaced the need for quarters.

 

[1911hombre]

No it isn't. The buffer tube appears to be installed correctly, it's just a different buffer tube than your rifle has. The enhanced tubes from Aero and several other places have an extended section that the back of the BCG rides on, and it's notched out for the retention pin. It wasn't some goober who installed that, looks to me like they did that part right.

Yeah, yeah, I took a closer look at OP. The extra threading out into the sloped part of the lower looked like damage from the BCG hammering on it.

 

[r.tenorio671]

...it's the BCG that is the problem. rpoL98's post above shows the cause, the sharp 90* edges of that rea portion portion of the BCG that the gas key is mounted to, which is also shown in the OP's 3rd post with the BCG at the entrance point to the RE & buffer tube.

...not knowing how many rounds have been thru the rig yet, OP's pic of buffer, BCG and spring upon closer inspection does not show "mushrooming" of the plastic buffer piece, which is a common indicator of if the buffer tube was too short or the BCG was traveling too far rearward. Also, if the BCG was traveling too far rearward, there would be impact marks on the center of the upper portion of the receiver from the gas key hitting it. The marks that are present, slight as they are, indicate the upper portion of the untapered BCG is making contact at times, most likely due to misalignment being caused by the lower portion of the BCG as it enters the RE/tube, pushing the BCG upward.

...the greatest danger in continuing to use that particular BCG is the "unseen" effects on the RE. There could be cracks developing underneath the anodizing. Due the "wobbling" effect of the BCG's passage into the tube, a quick inspection by removing the buffer tube and checking the inside of the RE would be a good idea as that is where the cracks may form first.

...change out the BCG for one that has the tapering in that area of the BCG that rpoL98's photos indicate. The exiting bolt can be continued to be used, but the cost of a new complete BCG is more economical than just a BCG minus bolt. Doing a complete BCG replacement gives you a spare bolt for those "gotcha's".

 

[XLR308]

I have a 308 Aero M5 and I just noticed the BCG is hitting the receiver. What can I do to slow down the BCG without spending a fortune?

Specs:
308 16" BA barrel
Mid-length gas
Superlative AGB - turned down to one click above jamming
Carbine length buffer with standard buffer spring and buffer

View attachment 8115703

One other thing to consider if it hasnt been mentioned alldeady is if you ever decide to change things up and want to replace the extension it will more than likely gall the threads and seize up.

 

[mtrmn]

The easiest, most economical solution was offered up long ago in this thread. 2 minutes and 25-50 cents in quarters. Provided the BCG travels far enough past the bolt catch. If he tries to unscrew the buffer tube with that damage, the lower receiver threads will suffer.

 

[Yondering]

...it's the BCG that is the problem. rpoL98's post above shows the cause, the sharp 90* edges of that rea portion portion of the BCG that the gas key is mounted to, which is also shown in the OP's 3rd post with the BCG at the entrance point to the RE & buffer tube.

...not knowing how many rounds have been thru the rig yet, OP's pic of buffer, BCG and spring upon closer inspection does not show "mushrooming" of the plastic buffer piece, which is a common indicator of if the buffer tube was too short or the BCG was traveling too far rearward. Also, if the BCG was traveling too far rearward, there would be impact marks on the center of the upper portion of the receiver from the gas key hitting it. The marks that are present, slight as they are, indicate the upper portion of the untapered BCG is making contact at times, most likely due to misalignment being caused by the lower portion of the BCG as it enters the RE/tube, pushing the BCG upward.

...the greatest danger in continuing to use that particular BCG is the "unseen" effects on the RE. There could be cracks developing underneath the anodizing. Due the "wobbling" effect of the BCG's passage into the tube, a quick inspection by removing the buffer tube and checking the inside of the RE would be a good idea as that is where the cracks may form first.

...change out the BCG for one that has the tapering in that area of the BCG that rpoL98's photos indicate. The exiting bolt can be continued to be used, but the cost of a new complete BCG is more economical than just a BCG minus bolt. Doing a complete BCG replacement gives you a spare bolt for those "gotcha's".

Or just drop in a quarter or two and be done with it. Problem solved.

 

[Cascade Hemi]

Or just drop in a quarter or two and be done with it. Problem solved.

Exactly. I have a AR308 and the JP BCG I used (plus two other brands I sub'd) contacted the lower like the OP's. I put a couple of Delrin spacers behind the buffer spring and the rifle runs perfect. Also, AR308's are known to break the bolt hold open. Limiting rearward travel reduces how hard the BCG hits the BHO lever. Going from memory I set the last one at 1/8" or a little more, static between the bolt and BHO. Some of the responses in this thread make it out to be a huge problem when in reality it is both commonly known and easily fixed. The bonus is you can fix the BCG contacting the lower and the excess stroke length at the same time.

 

[Gustav7]

Exactly. I have a AR308 and the JP BCG I used (plus two other brands I sub'd) contacted the lower like the OP's. I put a couple of Delrin spacers behind the buffer spring and the rifle runs perfect. Also, AR308's are known to break the bolt hold open. Limiting rearward travel reduces how hard the BCG hits the BHO lever. Going from memory I set the last one at 1/8" or a little more. Some of the responses in this thread make it out to be a huge problem when in reality it is both commonly known and easily fixed. The bonus is you can fix the BCG contacting the lower and the excess stroke length at the same time.

I think the BHO space is very commonly overlooked.

The last thing I’d be doing in this situation is buying a new BCG though. Spacer followed by buffer system changes are the easy answer

 

[TonyTheTiger]

The easiest, most economical solution was offered up long ago in this thread. 2 minutes and 25-50 cents in quarters. Provided the BCG travels far enough past the bolt catch. If he tries to unscrew the buffer tube with that damage, the lower receiver threads will suffer.

Or just drop in a quarter or two and be done with it. Problem solved.

Just to add a possibly irrelevant data point, in a blowback gun (not sure about a locked breech AR10) those quarters will eventually turn into little bowls resembling engine block freeze plugs. I still have a few lying around here somewhere.

 

[hlee]

Just to add a possibly irrelevant data point, in a blowback gun (not sure about a locked breech AR10) those quarters will eventually turn into little bowls resembling engine block freeze plugs. I still have a few lying around here somewhere.

Remember kids. Rotate your quarters every 1000 rounds...

 

[TonyTheTiger]

Found one.

 

[Yondering]

Found one.
View attachment 8121908

Are you sure that was a quarter, not a punched out copper (or something) disc? Seems like a quarter would at least have something left of the ridges on the edges, all I see is rough tear marks, usually that's from punching a disc out of plate.

 

[rpoL98]

Found one.
View attachment 8121908

wow, that's some serious long-term overgassed.

 

[TonyTheTiger]

Are you sure that was a quarter, not a punched out copper (or something) disc? Seems like a quarter would at least have something left of the ridges on the edges, all I see is rough tear marks, usually that's from punching a disc out of plate.

Absolutely positive. It still has remnants of its past life if you look hard enough. I've made a few of them.

 

[Gustav7]

Absolutely positive. It still has remnants of its past life if you look hard enough. I've made a few of them.

How many rds to get to that point?

 

[mtrmn]

Found one.
View attachment 8121908

How does that not beat the end out of the buffer tube? You said it was a blowback-- pistol caliber? Does it even have a recoil spring? Is this a late April fools joke?

 

[TonyTheTiger]

How does that not beat the end out of the buffer tube? You said it was a blowback-- pistol caliber? Does it even have a recoil spring? Is this a late April fools joke?

So to add to my original post and explain my caveat that it may be irrelevant here, the cupped quarters come from my blowback 9mm PCC, and it has an unconventional buffer that I'll include a pic of. It takes about 3-4k to kill a stack of quarters( @Gustav7 ), but it didn't do any damage to the buffer tube in about 10k rounds with no quarters.
I was pretty alarmed the first time I found said quarters, but multiple threads on forums dedicated to USPSA PCC tuning (where short stroking with quarters standard practice) indicate it's a very well known and common phenomenon with all sorts of different buffers, including regular rubber bumpered ones.
So again, maybe not pertinent to locked breech rifles, just pointing out that quarters aren't necessarily an invincible short stroke solution.

 

[TonyTheTiger]

wow, that's some serious long-term overgassed.

Technically no, as it's in a blowback gun, and doesn't take very long, but it does prove that PCC's are hard on components. I always scratch my head when people say their 9mm AR is soft shooting, like, how bad does your DI AR suck for you to think a blowback gun is soft?