You mentioned in your initial post that your goal is to create enough gas to work a comp, but I've had way better results (in the AR platform, at least) from a fast powder like TiteGroup with a lighter spring and less reciprocating mass than I ever did from trying to work the comp on the end of a 16" barrel.
Just my $.02
I noticed the same thing when I was developing a load for my MPX. Lower charge weights of CFE around 130 PF were leaving unburnt powder in the upper. Loading it a few tenths of a grain hotter (closer to 138-140 PF) resulted in a more complete burn.
147 gr. Xtreme Plated HP
3.8 gr. Autocomp
1.13" OAL
Averages 901 FPS (132 PF) out of my Gen 2 MPX Carbine
I've put thousands of rounds of that recipe through my gun with zero issues.
Getting permission isn't all that difficult. It's a simple notification form sent to BATFE.
The real issue is that SBRs are illegal at the state/local level in certain areas.
I'm just gonna throw this out there, but PCC-only matches are super fun.
Last year's AZ PCC Championship had several stages with 100+ yard shots, tiny high value targets, and a few huge penalty no-shoots.
I'd shoot that match again in a heartbeat.