Just a note, an importer of an alternative manual action firearm confirmed with the CFP that muzzle devices count towards overall length (OAL) of firearms. This allowed them to add a longer muzzle device to a 16" barrel length variant with a folding stock to bring the length over 660 mm folded and keep the firearm non-restricted. This confirms what was said by Spectre and others up-thread. So if you put a truly ginormous muzzle device on an LPC, like a reverse fake can/suppressor, you could get away with using an even shorter barrel with the LPC and a folding stock. Conversely, if someone had an extremely short barrel, they could use it on the LPC with a large muzzle device and a longer fixed stock. Don't know if Spectre has the parts, but it'd be cool to see how the LPC functions with a shorty .300 Blackout, a big brake like Spectre Aegis 30, and fixed-pull stock.
If my measurements are correct, I found the shortest you can go with a practical muzzle break (e.g. not adding another half pound to you gun through a fake suppressor) is 16" if you want to use a folding stock. If you don't, and just use a standard 6 position folding stock, you can go 12". I'm hoping to build a 12" 6.5 Grendel rig...
Bóg, Honor, Ojczyzna
According to these former operators interviewed on Garand thumb for regular .556 you want to keep the barrel length at least 11.5" to get some realistic useful effective distance, so hopefully your 12" Grendel will work well.
^
For some reason the link won't take you to the correct time index. Go to 29 minutes in to hear them talk about optimal barrel length.
Just an update for you guys, we test fired the rifle we built using parts from an M&P 15. It worked great. Zero extraction issues with steel case and brass case ammo. We are going to fire a few hundred more though it tomorrow.
Firearms reviewer Adriel the Hunting Gear Guy posted a review of serial #1 LPC. Other then seeing a bit depressed for the reason for its existence, Adriel seemed impressed with the firearm.
He did note that polymer mags didn't want to drop free, but he suggested on later firearm iterations this may have been addressed; perhaps Spectre can give feedback on this.
Can't remember if it was here or on Reddit, but for those who were concerned the BCG would slide back if the rifle was held vertical, Adriel showed it would remain in place with no mag in even when jiggled; mortaring the rifle to the ground did knock it loose, but the same would occur with a semi-automatic rifle.
Overall, good review of a promising Canadian firearm.
I noticed in this video that with the upper alone (no lower), held in a vertical position, he waves it around a bit and the bolt stays locked. When he disassembles it, you can see how small and light the carrier is.