Update 9/16/20 - photos of receiver groove smoothened out for possible future use if receiver remains unavailable.

Morning All,

Had a case separation or something of the like last week, blew the pmag to smithereens and left half the case in the chamber.  Rest of the case was in about 7 chunks.  I have a ton of photos and can upload as requested, but my concern is the scoring in my upper receiver caused by the BCG.  Wondering if you've seen anything like it on 308 ARs.  This one is a 6.5 CM.  Been a dream up until this incident.

Before you ask - yes these were handloads - I always shoot handloaded ammunition through this rifle.  I chrono'd the whole set and have no reason to think this was a hot round.  It came out about 2330 fps, which is about 350 fps slower than the last round with the same powder charge.  I am willing to discuss it but my reloads are headspacing well and I individually weigh every load.  I gave all of this information to Aero Precision and we have discussed replacing these parts.  Not under warranty due to handloads as expected.




I may post in AR Variants as well if you all think it would help.

Looks like your cam pin took a hell of a chunk out of the receiver.  
Personally, if I had a kaboom I'd be replacing the entire upper regardless of how it looked.  

For one, you never know exactly which parts have now been stressed/weakened/tweaked by the incident.  Unless you're planning on having them x-rayed to look for flaws.
They did the job as they were intended to do in a case failure/bore obstruction scenario...but that doesn't mean they can do it twice.  

Second, nothing precludes that possibility that, for some reason, that particular set of parts helped contribute to the kaboom in the first place.  (Tolerance stacking, out-of-spec, firing pin too long, what have you)

The price of a new upper is cheap insurance compared to your physical health or even your life.  
I'm sure Aero is going to deny your claim simply because you were shooting reloads.  But I'd buy a new upper and count your blessings.  


How does the lower look?  Any damage to the sides or the magwell?

New upper receiver was never a question.  Should have said so.  The thing is bowed out under the ehection port and the carrier dips and causes the lugs to misalign.  Fortunately the lower is not bowed.  But yes, new lower was in the works as soon as I saw the groove and the bowing.  Should have said it in the OP.  I agree with your assessment.  Thanks for responding.

I'm also not planning on a new barrel.  I have no reason to think it saw any more stress than a normal shot.  Open to that discussion tough as well.

I'd be concerned about the lugs in the barrel extension.

I would be too if there was any (ANY) damage or markings to the back of the bolt lugs, but there is none which is strange.

I'd post photos but px500 is not working for me.

Bolt lugs look fine, just one knick on the front of one lug, which seems to correspond to a knick on the face of one of the barrel extension lugs (you can actually see this in the first pic).  Strangely, this tells me the bolt may have actually tried to close by being forced into the carrier from the force of the new round stripping out of the magazine, pushing back on the bolt lug.  This combined with the damage to the cam pin, which is on the FRONT face of the pin, not the back.  This tells me these forces were slowly eroding the groove into my upper receiver, which eventually allowed the bolt to become forced into the locked position from feeding pressure alone, and it did so before the bolt lugs passed the barrel lugs.

The alignment pin is fine, and the groove for the pin in the upper is fine.  I've disassembled the upper and everything looks pretty okay except for the groove.  New BCG is here.  Aero upper is not in stock.  They did give me a good deal on the bcg and will do the same when the upper is available.  They've been pretty good to me.  If you're reading this, thanks.

In other news, the barrel is actually in the hands of a friend who does NDT for a major aerospace firm.  Won't say which, but I'm about to know if there is anything at all wrong with the steel.  Could take a couple more weeks though.

In more other news, I need a better way to host photos.  Badly!!

Id be really interested to hear about the findings

I smoothed out the groove in the upper receiver and now I'm wondering if you guys think this is a dangerous way to run this rifle.  I've seen a lot of receivers with "normal" wear in this location.  Obviously this is way worse.

I'm answering my own question as I play with the bcg in the disassembled receiver.  The cam pin WILL rotate prematurely by the force of stripping a round from the magazine.  This is what caused my bolt lug to slam into my barrel extension lugs during a few of the rounds which failed to fire before the kaboom.  Lug damage photos below

Finally I have effective image hosting

There may also be tiny stress fractures on that bolt that are not visible.  It should be Magnaflux tested if you even think about using it.  Machine shops can do it.  But, given the obvious visible damage to the front of the lugs, I'd not use it, even if Magnaflux test was negative.

A new mil-spec bolt is not very expensive.

Oh my.  I've never heard of this or knew this could happen.  If I'm reading this right, the bolt rotated closed from the simple engagement forces of the cycling, so basically the bolt stopped on the ouside of the lugs, rotated, and thus allowed the firing pin to engage upon trigger pull?

Holy crap.

I guess my first question is who's gun/parts were this?  

You mention this is a 6.5 CM.  I will say, the broad shoulder of the 6.5 CM in my opinion makes for a less smooth feeding system.  Shoulder hang or long bullet engagement on a non-radiused BBL there stopping the round entering, and causing the massive BCG to hang-up and drag, which then tries rotating the bolt, with the only thing stopping that being the cam-pin engaging that soft aluminum?  Oh my.  I do note AR10 Cam Pins have the flange portion in-line with the BCG, unlike an AR15, where the flange of the pin is pointed to the sides.  Which means the AR10 campin maybe doesn't have as much margin of engagement with the upper to keep it from incorrectly closing.  And so the errosion pattern you show, might not let an AR15 bolt close, but maybe it will an AR10 bolt.   No way in bloddy Hell would I reuse that upper, nor should you sell it.  That thing needs to be destroyed.

Really want to know who made that barrel and upper.  I've seen others post issues with bullets hanging on non-radiused 6.5CM BBLs.

As to reusing your parts, it kind of depends on the above.  You might have a defective barrel or other part design that lead to this.  Maybe more than just "might".  Also, what bullets were your running and what OAL?

Also, what magazines were you using?  

Thanks for posting this.

I've had a new BCG for this gun for weeks.  I thought I mentioned that.



Yep, pretty crazy right?  I wholeheartedly agree that the 6.5 puts more stress on the cam pin during feeding.  I can not say the extent to which this action eroded the receiver because I was not aware of it until it fired out of chamber and that caused way more damage in that same place, making hard to know how much erosion would cause the bolt to turn enough to allow firing.  Now onto your questions:

1) the BCG and upper were Aero Precision M5 parts and have both been replaced.  I have no intention of reusing either.  BCG is in hand, new upper is en route.

2) I will not name the barrel manufacturer, as I built this upper myself from hand selected parts, and there is no indication that the barrel contributed to this malfunction.  The barrel has fired flawlessly (and accutarely too) the 200 or so rounds I have fired through it thus far.  It has been x-rayed by a friend who works in NDT.  The barrel steel made for a difficult scan and we're still awaiting results but I don't suspect damage.

3) Rounds used at the time were hornady 140gr American Gunner to warm up, and then was halfway through a handload workup when the malfunction occured.  Those rounds were not halfassed.  Please don't question my ammunition if you are not familiar with my process.  My chrono data has ruled out a "hot" round.  Brass was once fired hornady, 143gr eldx, superformance powder, winchester large rifle primer, loaded to safe mag length.

4) Magazine used at the time was a 20 round magpul 308 Pmag.  Standard stuff.  That magazine is no more, but has been replaced.

Also, looking into the causes of wear in this location for standard ARs, I am under the impression I need to keep this upper wet as HELL and inspect the upper after each few rounds when I get it operational again.  The reason this was not covered under warranty by Aero was my admission to using handloads.  This time I will exclusively use factory ammo until I am sure the upper is not eroding during feeding.

Actually quite the opposite - I think you made good quality ammo.  That's actually not good news; because that you could fix on your end.  

So no, not questioning your ammunition quality at all, appologies if it looked like I was.  What I'm trying to figure out is the tolereance stacking of the components.  The reason I want to know what shaped bullet, and who's barrel, is not to say it's a bad bullet or barrel, but to see if that otherwise fine barrel, with that otherwise fine bullet, and that otherwise fine BCG in that otherwise fine upper, that will run for 20,000 rounds just fine; have a tolernace stacking that the shape of that particular bullet, loaded to that particular OAL, engages the feed ramps and is positioned to enter the barrel in a way that comes out of that particular magazine with the way that causes a binding issue.  Either from the bullet striking the face of a non-radiused barrel (a compliant I've seen with 6.5 CM), or from the shoulder maybe doing same.  Is what I'm trying to figure out.

So I'm not asking who made your barrel so as to say it's a bad barrel.  I want to know who it is because I want to know so as to keep an eye on who's dimensions maybe aren't compatbile with others dimenisons.
For example, I'm wondering if the long ogave of the Hornady bullet vs a more blunt ogave of say an S&B bullet, is a factor in that magazine/Barrel combination.  

None of this was ever ment to imply your ammo itself was defective.  Indeed, from your commentary, the ammo itself seems to have been properly made, and certainly doesn't appaer to have any sloppy aspects to it.

Right now, I'm hesitant to advise rebuilding your rifle to exactly what it was with exactly the same parts, and to use the same external dimensioned ammo.  If it Kaboomed because you made bad ammo, that would be easy - don't do that, and you're good to go. But in this case, the bad news is, I think you made good ammo.

That's an interesting thought, and I've not considered whether the bullet type had an effect.  I really doubt it.  The way my failures to feed were remedied that day screamed of either BCG under lubrication or under gassing, and not wanting to adjust gas in the middle of a workup, I just pressed on.

Also, you hadn't made any comments about my ammo yet other than asking what I used, so no foul.  I was trying to preempt any assumptions about my handloading.  A lot of folks who do not reload have been quick to say that's what caused it, and I'm a bit tired of explaining it all.  So you are absolutely fine and I appreciate your thoughts about tolerance stacking.

Barrel is a fluted 22" extra length gas Rainier Ultramatch and I love it.  If the xray images show it to be clear of fracture I have every intention of building this gun back and keeping it clean as hell, wet as hell, and monitoring the hell out of the receiver wear.

If the bolt was closed outside the chamber, as suggested. It is unlikely the barrel was damaged.  The reward thrust on the unlocked BCG might have caused some damage there to those components.  You should also inspect you buffer and buffer tube as they may have experienced a lot more force than typical. As discussed. The bare upper is shot.  I dont think the charging handle has a roll in this, I wouldn't think.


Please show a photo of the breach end of the barrel, and feed ramps

FWIW, here's what the relevant parts look like from a PSA AR10 in 6.5 CM, with a 20" SS barrel, after over 2,000 rounds.  

NOT OP's GUN
Basically, there's no wear at all in the area of interest.  A little wear on the side between the cavity and the barrel.  Not sure why that would be, but it's not much and isn't relevant anyway.


NOT OP's GUN
Here's what the barrel breach looks like.  Note how extensively radiused it is, to make it forgiving about feeding rounds not perfectly pointed down the chamber.

Thanks for those photos.  My barrel looks similarly radiused.  Lazyengineer there is a photo of my barrel extension and breech in a photo below the picture of my bolt lug damages.

Yep, I see it now.

Hmm, something caused the bolt lugs to not be aligned with the barrel extension lugs correctly, and I'm wondering what.

Upper and BCG are same manufacturer (Aero).  Which is good, as mixed parts in AR10 is something I keep seeing people warn against.  Lower as well?

Who's charging handle? Same as BCG and upper?  Not sure of its it, just checking.  The gas key riding in the channel of the charging handle is what keeps the BCG aligned, so that it engages the barrel extension lug clearance correctly.  

I can't see how the barrel extension might be misaligned- I doubt it. But all the same, any reason to suspect it's not lined up right on the barrel, or the barrel mount into the upper is mis-timed?  

Since the barrel is a different manufacturer that the upper, any reason to suspect the relative depth of the barrel in the upper is not correct?  Again, I dont think this seems likely, but something went amiss, so I'm just taking some thoughts.  The stripping rounds out of mag during feeding isnt something I see people reporting a partial bolt rotation problem about.  

The BCG going over and around the end of the gas tube is part of the cycling.  Any reason to think the alignment and sizing of the none Areo extra-long gas tube isnt right?  Really doubt that's it, but I have seen loose or misaligned tubes cause problems for someone (during competition of course, never happens at home!) - just one more thing to look at.

@bfoosh06

I have a brand new 308 bcg from aero (same as previous) and the cam cut in the carrier looks the same, yes.

Gas tube came with the rainier arms barrel, as it's an obscure gas length (rifle +2").  The tube seems to portrude about the same into my receiver as the PSA setup posted above.  Also, with the odinworks adj. gas block, I've worked the cycling pressure down to where it's a damn smooth shooter.  I was actually considering increasing the gas for these handloads.

I have an Aero m5 built with a Faxon big gunner 6.5 16" barrel (armalite pattern gas port location, DPMS everything else).  I see the beginnings of wear at that point in the upper receiver after <1k rounds, though it's not nearly as severe as what you're seeing (barely into the anodizing), and it goes further back.

You seem to have a correctly-sized gas tube, which makes sense that it came with the barrel.  @lazyengineer above seems to be running an armalite-pattern barrel with a standard-length gas tube, presumably as-delivered by PSA.  I missed that when originally building mine, so I ended up chasing short strokes for a while when using a normal mid-length gas tube.  Before I figured that out, I was running a sprinco orange spring, heavy buffer, and gas block pretty much wide open, which seemed to work with hot ammo.  Now that I have the correct gas tube, the gas block is in a reasonable range, and the normal weight buffer and spring are back in.  No more malfunctions across a wide variety of factory ammo, with and without a suppressor.

To start - I think we're all clear what happened here.  Once there was enough wear on the upper, the bolt was able to fully rotate when it contacted the back of the barrel lugs (previously rotated by the force of feeding - where that groove starts, the round has long since been stripped), allowing the firing pin to make contact with the primer with the bolt lugs behind the barrel extension lugs.  This would require the cam pin head to sit between the upper receiver bore and the bolt, as the worn groove in the upper is only 1/8" or so deep, not the ~.4" i'm measuring (cam pin tangential motion) for the bolt to be fully retracted and rotated (and able to ignite a primer).

*to be continued - breaking this up because I don't have enough posts for the site to let me submit a long one.  Also, mods, mind approving the pics?*

I don't see how wear to that spot would happen upon extraction - even with massively excessive headspace, you'd only wear the pocket back until the barrel/bolt lugs would properly regain contact.  That wear had to happen on feeding, and in the last little bit of feeding - the worn groove is only .75" long or so.  Just to check, I pulled my bolt out of the carrier, inserted the cam pin, and pushed it against the barrel extension lugs.  The location of the cam pin head exactly matches your groove, which makes me believe that was caused by the bolt repeatedly hitting the barrel extension while already rotated and wearing (bludgeoning?) away that portion of the upper.

Now that I look at the magazine carnage picture (reproduced below), you can see a groove in the top of the receiver bore much further back.  That's where it was stripping a round or against the bolt catch.  At that point, the cam pin was riding in the bore intended for the bolt carrier body.  Granted, this could have been when the kaboom happened, but the wear pattern makes me think it came from the other direction (rear, moving forward), and repeatedly.  Again, placing a bolt in the upper with the cam pin inserted, the cam pin sits exactly where that groove starts when the bolt face is against the bolt catch.

So my questions - Did you have bouts of the bolt getting ramdomly stuck? Or did you have rounds that just wouldn't feed for some reason and have to mortar the thing open?  Did it ever get stuck with the bolt against the bolt catch?  How gentle are you with mechanical things in general?  Have you measured the bore of the upper receiver?  What's the spring/buffer setup?

*edit* And what condition was the thing in immediately after the kaboom?  Did you separate upper and lower halves, then have to hammer the bolt carrier forward to get it to move out of the upper?  I'm trying to rule out that damage being caused after the kaboom.

My guess - you lucked out with an oversized bore in your upper, cam pin head that's too short, and maybe a carrier with a short radius where the cam pin rides.  This allowed the cam pin to ride between the carrier and bore in the upper.  You can see this in the groove further back in the picture I reproduced above.

The rifle definitely seemed to be short stroking, but thinking through it now, those failures to fire were likely the pin going into that groove but it wasn't eroded enough for firing pin contact yet.  There were about 40 rounds between the first F-T-Fire  and kaboom, and that bolt never got stuck 9r jammed forward or rearward.

Immediately after kaboom the halves separated like normal.  Someone asked earlier if there's any damage to lower.  There is none, and both receivers are Aero.

I'm not sure there's anything to a theory of the bore being oversized, since my lapping tool is still snug in the upper bore, and for the cam pin to ride outside it's groove that far back in cycling would require the gas key to do the same and that didn't happen.

I genuinely appreciate your interest in this incident.  Thanks for the photos and investigation!

Took a photo of the grooves for your viewing pleasure, and now that it's clean, you can see that other groove you're talking about.  For anyone peering in without reading, THIS UPPER IS SCRAPPED.  NEW UPPER ENROUTE.

Edit- I'm an idiot.  I see which groove you're talking about now, only after looking at that photo huge as hell.  The groove isn't in the round circumference of the bore, but in the side of the cam pin's normal groove right at round-pick-up.  It wore in pretty good there and I haven't noticed.  Nice catch!  Now how do I prevent that.  New Different mags?

I don't know.   I don't know why your build is doing it,  yet this isn't normal for everyone else.   That's what's bugging me.  

Since it's all the same parts or same replacement parts, there's little reason to be confident this won't recur.  

As a suggestion, on your next build, is to take your smart phone,  and film the cycle of dropping your bolt, and see if you can tell if it's hanging up and connecting anywhere.

I wish the cam pin was not so square.  I may either look into rounding off the edges, leaving as much flat as possible, or getting a roller cam pin.  I have heard of them but not seen them.  Anyone have experience?

Let's see if that goes through....

Front purple highlight is obvious - That's where the bolt hit the extension lugs and forced the cam pin into the upper, resulting in kaboom.

Blue is the one that concerns me.  That looks like a cam pin.  Quite weird, though.

Red is from stripping a round out of the mag.

Rear purple (how the hell did I do that?) is concerning to me.  Something rotated up and over.  You mentioned a lapping tool?  Perhaps it was from that.

Better view here when it was dirty - something got dragged from the rear across the inner bore right where the cam pin would be when the bolt is in its closed position.




You using an Aero Bolt Carrier?  Standard spring and buffer?

I'm not sure if I'm seeing that.   The bolt carrier has rails that contact and ride along that area, and you can see that in the PSA gun photo as well; so to a degree, that wear is normal, and generally doesn't include any actual metal removal.  This is where almost any Bolt Carrier has it's own finish worn off on an AR15 or AR10; up there in front of the cam-pin area.  So to a degree, that part is common to all AR systems.   Or did I misumderstand and maybe that's not what you were trying to call out?

As to the other wear marks - that's just it, my gun isn't doing that.  My PSA gun with P-Mags isn't exerting enough force from stripping a round to give any sort of wear marks there at the round-stripping-engagement spot.  Nor have I ever seen anything to indicate this is common to the design.  So what's different with OP's gun?  I think you're right though, there was a enough rear movement and rotation of the bolt during round engagement; and enough slop in the clearance of that bolt lug pin and the reciever wall, to let it ride forward enough partially rotated, to just engage the barrel extension lugs, and close the rest of the way.  And with enough battering of that, it eventially wore enough of a hole that it could close enough to allow the kaboom.  AR10 BCG's are massive, the slam force of one of those things is quite a bit more than an AR15 system.

I'd be curious if the tolereance of the lugs of the Brand X barrel extension dimension and that Brand Y bolt's lugs.  And are they typical of other manufacturers?  Are the lugs of that barrel extra-strong (i.e. extra tight), and making that bolt in the tolerances of that gun spotty?

OP mentioned that he feels like his cam-pin is excessively square.  I'm not sure what that means, but if it has a sharp point on it, maybe it bites into the reciever side-wall there, making the engagement worse?  I'm going to post a photo of my 2000 round PSA bolt carrier, as a comparison at least.

It is a normal part of the design for the bolt to hit the back of the round and strip it out of the mag going forward.  So what's unique about OP's gun that caused that to fail like this?  I don't know much about Aero, but if it's an Aero Upper, lower, and BCG, There shouldn't be any sort of incompatability there.  It is a different barrel, but I don't know if the timing of that back wear mark is where the round is first touched, where the round nose is trying to clear the mag, or where the round is starting to try and enter the barrel.  The barrel is radiused, so it's not super-obviously made wrong that I can tell.

I can't tell, but I'll say it again - until this is figured out, rebuilding the same gun from the same mix of parts; is a discomforting prospect.

The only good news of all of this, is that discharge is so OOB, that a significant portion of the casing is fully unsupported.  Which means unlike an almost-closed OOB, this one probably has a whole lot less pressure build-up before Kaboom; and so the only part that probably gets damaged from the kaboom, was probably the mag.  The rest of the parts very likely are mechanically fine.   Well, aside from potentially being defective enough to have contributed to the root-cause of the whole failure that is.

On the note of differences, OP did indicate that he thought his gun was pretty dry.  In my AR's, I tend to run them wet.  Bearing surfaces such as
-bolt lugs (on all 3 faces of each lug),
-cam pin (and it's channel),
-gas rings,
-bolt tail, and
-any wear-spot on the bolt carrier where the finish is worn off,

all get a touch of lubriplate grease, and then the whole rig gets oiled, including gun-oil on top of the grease.  I don't use CLP.  

I don't know if that contributed to OP's issue though.  If dry AR10's Kaboom;ed, we'd see a lot more Kaboom reports.

Thanks for making those edits.  I agree with all your conclusions and yes my lapping tool definitely caused that marring in the receiver.

Yes, it is an Aero black nitride bolt, TUBBS AR10/SR25 buffer spring from Larue, and a fancy AKTIVE buffer from CTS engineering.  It has a nipple on the end to hold the carrier up and keep it from dragging.  Makes disassembly a little trickier but I do recommend it.

In other news, I WILL be using this roller-style cam pin, if only to prevent the damage circled in red above.

https://www.opticsplanet.com/pof-usa-308-roller-cam-pin-assembly-rcpa308.html

I'm starting to think that the lapping tool had a bit to do with it.  If he had lapping compound get down to just behind the slot and got through the anodizing, this could happen.  I think that's what I'm seeing at the rear, behind the slot worn form stripping rounds from the mag, and if it's anything like my brownells ar15 lapping tool, there's a shoulder on the tool right in the vicinity of the cam pin slot, and if you have the receiver pointing up, lapping compound will end up there.

The wear from stripping rounds?  That's why I'm asking about spring and buffer - if it's wildly over-gassed and bolt is flying at that point, I could see that happening.  Or if he ran in the rifle with lapping compound on bearing surfaces to smooth it out.  Another thought is that bolt speed fast enough to outrun the magazine spring, nose-diving rounds into the extension, would put a ton more force on the cam pin in this area.  I've only seen this with out-of-spec AR-15 mags.

I still think it looks like the cam pin ran outside the slot (second pic above), but that definitely could have been during the kaboom, which could have distorted the receiver.

*edit - wrote this as if I was responding to @lazyengineer*

And I can buy that being normal contact wear from the bolt carrier, but it still looks obnoxiously deep for the number of rounds he had through it.

Another edit - Once you get through the anodizing, all bets are off.  Steel on aluminum coeff of friction is massive, like greater than 1.  The nitriding on the cam pin head is even worn off.

Lazyengineer, I missed your responses, and thanks for your contributions too!  I think you're right on the money about the carrier rails causing that streak.  I think I didn't break this action in quite right and a little wear in the pin channel turned into a lot of wear.  Maybe something wrong with the upper forging allowed the wear to progress.  If Aero wants this upper back to formally assess it they're welcome to it.  If this had been a warranty-able situation they would have it back anyway.  I genuinely believe that the addition of that roller cam pin will rule out the possibility of wear in either of the locations we see it in the old upper.  Here's a nice little photo of my old cam pin.  Decent shape considering!!

Don't think so.  I never even got any lapping compound past the face of the receiver, much less to the back of the ejection port.  I lapped it horizonally, and slowly with a drill.  I think it just rode there the most from the forces I put on it while lapping.  If any compound had gotten to that spot I'd have lost the cerakote all the way around the inside circumference.  That marring is superficial, but I can still appreciate you considering that as a factor.  

I'd be very careful about lapping.  With the new receiver, take a look at the points that failed on this one after you're finished lapping.  Those wear patterns suggest you had a bunch of side load on it when lapping.  You might also take a look at how much you're taking off of the front of the receiver - Sliding the barrel extension far back in the receiver is going to cause lots of bad things to happen.  Looking at your pics, I can still see the machined surface ahead of the gas tube cutout behind the barrel extension, so I don't think you went nuts there.  I'd highly encourage not using lapping compound to work in the bearing surfaces and watching the new one as you break it in.  Those gouges took a long time (or a ton of lapping or a serious amount of force) to develop.

Something I hadn't considered before - Lazyengineer and I both have anodized receivers.  This one is cerakote.  I don't know how hard the cerakote is, but I'd have a hard time believing it's anywhere near as resilient as aluminum oxide.

Yea.... so, that part and Aero's response on all that isn't lost on me right now...

As to lapping, unless there's something really amiss, I think the process is pointless.  In my tests, it didn't do anything for me in the guns I've tried it on.  And has the potential for creating problems if not done perfectly right.  Also, if done horizontally, then gravity is going to try and make it lay with preferences - everything I read said don't do it horizontally.  I say, don't do it all.

This receiver had to be lapped a lot so I didn't have to jack with the shims provided with the barrel nut.  Long story but the barrel extension was still not even flush with the feed ramps when I was done with it = safe.  There's a photo at the bottom of this post of the feed ramps where you can see.  Also there are no gouges in the receiver bore due to lapping. The marring is normal enough on a tan cerakote.  You'd be happier if this thing was black - we wouldn't see it at all.  And yeah I do bet the cerakote is softer than aluminum oxide.  Thicker too!



Okay...  thanks.  If you don't like lapping don't do it.  It's not hard and some receivers do need it.  The next one won't be lapped since I went with the enhanced receiver.  Hope it's a tight fit at least.



I never said I had that many photos lol.  Happy to try though. Photo of lugs at bottom.

1) No cracks on the carrier, or signs of being anything other than banged up from OOB

2) Firing pin is fine.

3)  Yes!  I have definitely tried to replicate this condition and it's not hard.  It definitely turns into the gouge early and I can get the firing pin to float out of it's hole in this condition.  My only question is whether the hammer can make contact with the back of the firing pin with the carrier that far back.  Might just put it together and load up a resized empty case with a primer and see if she'll pop.  That might take me till tomorrow, or next time I'm bored.  It's not often.

Thanks guys, for your continued enthusiasm regarding my misforune, or negligence - whatever your theory may be.  Thanks regardless.

Nah you weren't badgering.  And thanks for that offer.  I have the full-bore ar10 lapper from pacific tool and gauge.  Are you saying you have the m5E1 ENHANCED lapper?  The one with the smaller bore to fit within the female threads on the receiver?  If so I may be interested.  We could trade my broken junk for your tool for a day.  Just wish I could offer something you actually could use.  Anyway, thanks, I'll PM you about that.

The firing pin retainer isn't bent.  On the Aero BCG it's pretty damn stout, and doesn't pull all the way out. It's kinda captured, but I can tell it's not bent, and the firing pin itself looks fine.

Alignment pin on the barrel extension is also 100%.

I bet we could probably wonder what went wrong forever.  But my plan is to use the roller cam pin (en route), religious oiling and monitoring of any wear progression, and I think I can at least anticipate another failure this time.

UPDATE:

I was able to make the rifle fire a test round with live primer with the bolt lugs inside the barrel extension, but not locked.  This pretty much explains the kaboom.  I performed this test with all the old parts assembled.  I now believe that the force of the explosion against the unsupported bolt forced the bolt into the carrier during back-travel, rotating the cam pin once the bolt and barrel lugs were clear of each other, causing the cam pin to gouge the receiver.  I do not see at this point why this won't happen again.  My money is on the failure here being within the BCG.  I'm afraid to try with the new BCG but I am definitely going to.

First photo is the position I was able to make the test round fire in (on only the second try - i did not check the primer between attempts.  I may try that again).  Second photo is the bolt in the full forward and locked position.

I got it to happen again and immediately got the same result.  Click on attempt 1 and pop on attempt 2.

First photo is after the fist hammer drop, second photo after firing.  Same exact bolt position as photographed above.

Thank you so much for the update! And yes, if an unlock bolt allows the firing pin to light off a primer, that's an OOB kaboom waiting to happen.  Especially in a precision build where tight chambers and tight brass are going to be typical.  An AR15 pretty much would never let this happen, but AR10's are different animals than AR15's.

To ask the obviously dumb question, is this an Aero Firing pin in that AeroBCG?

Yup, so far I'm trying to get the same to happen with the new BCG (same exact black nitride BCG from Aero) and so far after 5 or so tries it has not happened.  I'm going to try some different combos such as new carrier and bolt with old firing pin, old carrier and bolt with new firing pin, etc.  I MIGHT just be able to narrow it down to one component.  I'll keep updating.

Thanks.  It's possible the normal erosion in the bolt, BCG, and Control pin are allowing enough slop for it to happen then?  Regardless, it still shouldn't be happening.

Please keep us informed if Aero continues to crawfish away from aiding you in this manner as well.