Hey all!

So I just got into the mR556 game and I'm slowly learning how everything works. Please be patient with me. So my main question is can I run just any piston BCG in my MR556? I ask because I want to run something other than the HK MR556 BCG due to the firing pin safety lever not allowing me to use my Geissle SSA-X trigger. I swap different uppers around on my lowers depending on what I am doing and what the same trigger in all my lowers, that is more import to me than running an HK branded MR556 BCG.

One option I am thinking about doing is running the brownells BRN-4 BCG. Will this work for what I am intending and is there any other options you all recommend? Thanks in advance!

No. Generally piston bcg's are non-compatible.

You can either remove the firing pin safety(i have done this a few times for myself and others) or purchase a dedicated HK geissele trigger, which, when you can find them, go for about $500 each.

I think the BRN-4 bolt groups don't utilize the HK-style firing pin safety.

https://www.brownells.com/gun-parts/rifle-parts/rifle-bolts-bolt-carrier-groups/brn-4-416mr556-complete-bolt-carrier-group/

ETA - Derp, you directly mentioned that.    I'd just get the BRN-4 BCG and leave the OEM BCG as-is.  Switch the bolt over if you'd like to keep a little of the Teutonic magic.