Will just start by saying that I haven’t looked at the SCAR and FNC fire control groups beyond the casual observation that they are quite similar.  I have seen discussions over the years about the desire to use NFA FNC sears in the SCAR and the legal issues with that.  But I’m vaguely curious how backwards compatible SCAR bits are.  Anybody tried to fit a Super SCAR trigger group into an FNC?  In obvious big roadblocks?

If you had a registered receiver FNC, it "might" be legal to do (try) that. If it's a semi-auto only FNC or a registered sear FNC, it would not be. I Have neither so I wouldn't know.

This is not correct. Why wouldn’t it be legal to install a semiauto SCAR FCG into a semiauto FNC? It’s a matter of whether it works mechanically.

OP, I have a registered S&H sear fnc and a scar. I won’t be able to compare side by side until the end of the month. IIRC, the hammer and trigger pins of the SCAR are a different diameter than the FNC. This is something I would attempt with one of the old ORF FNC lowers, not a factory Belgian lower.

Yeah, it’s all semi and I’m not interested in getting into the NFA realm with any of this.  Just curious about making the best possible FNC.

I certainly wouldn’t want to make any alterations to an original lower.  Depending on the size difference it could be pretty easy to make up some custom pins or maybe sleeve the original pins to adapt them.

Do the ORF lowers work?  I had read some fairly negative reviews but sometimes it’s hard to figure out truth from internet BS.