Wzrd

So I couldn’t leave well enough alone & have new problems & questions. I “upgraded” my BCG to a Lantac 308 Heavy E-BCG. The idea was to use my original KAK BCG in a new 80% 308 build & use the Lantac in this build which I plan to shoot suppressed a lot. (The Lantac supposedly being designed with suppressed shooting in mind). I’ll try as best I can to describe the problem I’m experiencing: I haven’t shot the gun with the Lantac bc when I cycle it by hand it is binding inside the upper &/or buffer tube & actually gets stuck. A slap on the side of the upper is enough to unstick it & send the BCG into battery. The odd thing is if I cycle it with the takedown pin out it doesn’t get stuck but it’s forcing the receivers apart by approximately 1/32-1/64”. This never happened with the KAK. Perhaps the Lantac isn’t compatible with my receivers? I have a PWS anti-tilt buffer tube despite using a DI system. I’m wondering if that might be causing a problem but I don’t see why it would. I will contact Spike’s & Lantac to see if they have any insights. I’m including some photos showing measurements of the rear end & widest section of my two BCGs (the black is KAK, silver Lantac). I’ve also noticed inside my upper there is a small ridge. It seems almost like the machining stopped halfway through its last pass & didn’t finish the rear of the upper. You can see in the photo where it’s being worn. I can see on my KAK where it was rubbing on this spot but no evidence thus far on the Lantac. However this is my only 308 build so I’m not familiar with all the nuances of the pattern(s). I just know none of my ar15s have ridges like that inside their uppers. Any help is greatly appreciated. Ultimately I may just have to go back to the KAK. I’ll update with any info from Spike's or Lantac.

I just noticed in my photos that my muzzle brake came loose & must've twisted while I was shooting. I have it installed with a jam nut so it's easy to index & easily removed/reinstalled for when I eventually get my suppressor out of purgatory. Does anyone have any suggestions for how to keep it from coming loose while still allowing it to be easily removed when necessary? Did I just not tighten the jam nut sufficiently? Should I post this elsewhere or read an existing thread?

Sorry it took me so long to get back here. Anyway my buddy's BCG has same relief cut as mine, so fit was the same. I don't know anyone else with a 308 AR. But I recently took it out to test fire for the first time. After some tuning of the adjustable gas block it is all working fine. No problems with anything through a couple hundred rounds. Feeding is good in spite of the feed ramps' presence ;) The alteration to the SI dust cover allows it to close enough to stay closed when not in use, & I don't feel I've compromised its integrity. So I'm letting it go. It would be nice to find out if anyone else has had this problem, but it doesn't affect functionality any so no big deal. I'm happy with the build. Thanks to all who replied.

I know one guy who has a 308 AR. I'll see if I can use his BCG. I haven't checked headspacing. Don't have any gauges. I can order some if necessary. I don't know if the BCG is cut wrong, or if it just isn't fully compatible with my spike's Livewire upper (the dust cover seems to be only problem). I contacted KAK. All they said was they've had no reports of this problem. Same with Faxon & Spike's when I originally contacted them. The first post of this thread was a followup email I sent to all 3 & I also sent them a link to this thread. None of them responded with anything more. One of my earlier posts shows multiple 308 BCGs & there seems to be many different relief cut shapes. The KAK & Aero look the same. Others have wider cuts. Not sure if the depths vary. I'll test another brand & see what happens.

So I ground on the dust cover detent a little at a time, testing the fit as I went. Eventually I got it to fit, however I had worn through to the spring by that point. The exposed area You see wasn't ground completely off, but the metal there was so thin that it just peeled away. I got into the spring a little too. Because of this I don't really trust using it. I'm worried the spring will break & I'll get bits of metal in the chamber, bolt, etc. at the worst time. BTW the white powdery stuff is just residue left from the flashing of electronics cleaner. I got one of the strike industries UDCs figuring it would be easier to alter, more forgiving & flexible, & I could complete the build & still remove/install it with the barrel nut in place. It too wouldn't close at first so using a razor blade I shaved off plastic, again testing fit as I went. In the photo you can see reflection off the detent ball as it isn't actually locked in behind the lip of the ejection port. If I shave off any more I think I'll compromise its integrity. At this point it is wedging closed tight enough for my purposes. This whole thing has been so frustrating. I know the dust cover has absolutely no effect on the functionality of the rifle, but I like to keep the covers closed when not in use. I'm a little OCD sometimes. I don't like the current state of it, but I can live with it. I just don't know how there's no info on this happening to anyone else. I'm thinking maybe my problem is the receiver/BCG combo. The relief cut not being deep or wide as some, & the receiver maybe having the ejection port just a tad tighter to the BCG than most. Does that make sense? Anyway, thanks again to the responders. Feel free to comment as long as you like, I will see it.

It's a PWS enhanced tube. I had it screwed in as far as it would go while still allowing the detent to retain the buffer. Obviously that's not the way to do it. I don't think there's any chance of it coming out. The tube is holding it down on left & right sides as well as the rear. I'll try to grind the door. I think it will work. It just seems really odd that no one has heard of this problem. From the photos the Aero BCG, the cutout looks same as KAK, & I know Aero is popular. I'd think people have used those in all types of receivers. Maybe my door detent sticks out too far. Thanks all who responded. If you have anymore advice or suggestions I'd be happy to hear it. I'll report back when I'm done grinding. Now I just have to figure out how to remove the door. Don't see any c-clip or set screw. I'll ask Spike's. hopefully I'll have a finished, functioning rifle soon.

What do you mean standard feed ramps on the extension? I'm familiar with the whole m4 vs rifle thing, but my extension looks like it has m4 style ramps to me. Or at least they seem to line up with the ramps on the receiver. Maybe wider than m4, for the larger cartridge? The last photo of extension is one of my ar15s. Seems the same to me. It doesn't look like the classic comparison photos I included. Am I wrong?

It is 2-1/8" from front of receiver to front edge of detent. Detent assembly itself is 5/16" wide. It's just a hair over 3/16" deep Heres a few pics of other 308 BCGs showing the various shapes & sizes of detent relief cutouts. In order: Rainier, EP, Black Rain, Aero, Anderson, Young, Lantac, Rubber City. Why so much variation?

Ok so here's the update. I checked the buffer tube & saw the top edge was being worn. It wasn't actually protruding into the receiver, but was close enough to the edge that the BCG was catching it when closing. Backed it out one turn & its closing fine with no interference now. The photo showing the wear has the buffer tube in its current, correct position. So that issue is resolved & was all my fault. That still leaves the problem of the dust cover not closing. These photos show the carrier only. It is in contact with the barrel extension. As you can see it made no difference. The relief cut still doesn't line up with the detent. But am I mistaken that the photos of the extension in receiver look normal? So if that's normal, & the bolt isn't causing any issue, then what's the explanation? Detent is in slightly different position than most in relation to BCG? Or it's a little larger/wider than normal? Wtf? If I grind down the forward bottom corner of detent it might close. As I said before I could live with that. I'm just worried it might be an indication of something being wrong. Any thoughts?

I understand that. There's no problem with the buffer or its retaining detent. I'll take photos later when I'm at home. The wear is at the back but as this is a brand new build it's very minimal- just from me opening & closing the receivers. I think I'll try to back the buffer tube out one turn to see if the BCG is actually hitting it (instead of the RE "arch") when closing the receivers. But that still won't have any effect on the dust cover not closing. I looked up images of other 308 BCGs & there seems to be many different shapes & widths of relief cutouts for the dust cover detent. Some that look like they might allow mine to close. I just don't understand this if all my parts are supposedly dpms pattern. KAK said they've had no reports of the dust cover being obstructed by their BCGs. But that's all they said. Haven't heard from Spike's or Faxon yet since that email. Ultimately I could deal with these problems if I know the gun is safe. I might be able to grind off some of the dust cover detent assembly enough that it'll close on the BCG. But I'd rather not have to resort to things like that.

The bolt is still in carrier in the photos. Will try removing it like suggested when I'm home later. Sorry the picture from under BCG isn't so clear.

I'm building my first 308 AR & having some fitment issues. I've sent an email to manufacturers of my parts. I've copy/pasted it here. Looking for some advice from experienced builders. Any & all advice welcome. I think I have it mostly figured out. The receivers are getting tight when closing because the top edge on rear of BCG is hitting the forward face of receiver extension "arch". I can see wear on this part of the BCG. I removed the buffer & spring so those are causing no interference.I removed the gas tube but it made no difference, so it was not bottoming out in the gas key. Also removed CH & again no difference. As far as I can tell, the barrel is in proper position & the bolt is fully rotated & locked into battery, but the BCG is still too far back. The relief cut is not lining up with the dust cover detent assembly, so it won't close. This also would account for the BCG sticking out the back slightly too far & hitting the arch. So I know what my problem is, but not sure what the cause is. I know there are no real set specs for the 308 AR platform, but I thought all of my parts are considered DPMS compatible. I guess I was wrong to assume they'd be compatible? These are the parts:-Spike's Livewire 308 billet receiver set -KAK LR308 BCG-Faxon 16" medium taper, .308 Win., mid-length, 4150 QPQ (10A810M16NMQ)So I'm not sure which of my parts are the problem. I'm thinking the BCG doesn't fit with the receiver/barrel combo. But I think it could be the barrel that's the problem. Or maybe the barrel/BCG combo are fine but the upper isn't. Regardless of which part(s) are incompatible I'm out a few hundred dollars if I have to replace them. Neither the BCG or barrel could be returned & resold as new at this point. Does anybody have any suggestions or advice? Does Spike's have any BCGs they suggest & know will work with these receivers?Is KAK aware of their BCG being use successfully with these receivers or this barrel?Has Faxon ever heard of any problems like this? Does this sound like a headspacing issue?Again, I'm a complete novice to the 308 AR world, so I'm all ears. I'll post on some forums to see if I can get any advice. I don't have anyone locally I can go to. End email. I do not have go/nogo/field gauges so I have not tested that. Can get some if needed. Have not chambered a round yet either.