With the BRS47 (7.62x39) receivers, the upper and lower are not compatible with standard AR uppers, and BRS says that their BCG is not either. What is modified on their BCG which makes it different? Is it just the carrier ramping, or is their bolt modified as well? I'd like to run an LMT 7.62x39 bolt if I end up going down this rabbit hole.

I have looked at their stuff but never tried them.  I've been playing with full auto 7.62x39 setups for a long time.  
I run the LMT 7.62x39 bolts also..
Below is a picture I took a long time ago:


All the carriers below are ones I had clearanced while the top one is from PSA and works fine.  I think they are a great value and work well.   Out of stock frequently so you have to be ready to buy when they come back in stock.  Any reason you don't want to get the PSA stuff?


BTW, I have an old AR47 lower but primarily use an MGI modular lower and then use various uppers including the PSA KS-47 uppers.  They work fine with my old AR47 lower or MGI modular lower.
More details on my site: http://www.c3junkie.com/?page_id=185

I'm in Connecticut, and PSA basically told me to get stuffed despite it being perfectly legal to get some of their offerings. I appreciate the information.

So I actually have a BRS47 set. It's a standard AR 7.62x39 bolt but the only thing different with the carrier is the bottom where the skis are. It's milled slightly deeper to accom AK47 mags. I recommend getting a BRA Enhanced Firing pin as well for hard primers. Keep in mind the BRS lower has some weird milling so you will either have to modify the lower or the drop in trigger kit if you use one.

I got this milled out by a reputable shop I found here. It's a Superlative Arms piston carrier.

I appreciate it - I have a stack of BCGs and a spare LMT bolt, so I'll probably just send the carrier off for the work instead of plunking down for a new one.

Now for them to come back in stock...

Paladin Machining does a similar cut on a standard BCG for the BRS47 (They also machine the upper receiver to allow the AK mags to extend higher up).
The BCG is backwards compatible with a standard AR.

They do both. This is where I got the above picture's work done.