BryanP,
Some more info on the MPX:
I was at the Sig Pro Shop last week looking at the carbine-length MPX. I was thinking about buying one to use as a PCC, but based on what I learned, I will get an AR-based PCC.
The MPX carbine uses a 14.5 inch barrel with a pinned and welded Sig flash hider to make the barrel 16.5 inches OAL. The flash hider is not compatible with any suppressor currently on the market and there is no way to mount a suppressor on the FH anyway. So, if you want to use a can, you are SOL unless you buy a new barrel or FH. A competent gunsmith should be able to remove the Sig FH, but may damage the barrel threads in the process. Or you could buy a new barrel from Sig, if they ever make the parts. Sig is notorious for not making parts. Also, according to the Sig customer service guy at the factory, Sig has no plans to make a true 16 inch barrel.
The good news is that the MPX pistol comes with a standard A2 birdcage flash hider. The bad news is that it is a pistol, not a rifle. You could turn it into a SBR if you want, but IDPA rules don't allow SBRs in PCC. Your local club might, though. Want to put a carbine barrel on the pistol? You can't. Sig says they aren't compatible. I have no idea why, but that is what they said.
Finally, Sig is producing the Gen 2 MPX now. If you happen to own a Gen 1, you'll probably never get new barrels or handguards since the Gen 2 parts won't fit. The good news for Gen 1 owners, like me, is that both Gen 1 and Gen 2 magazines work in both generations.
