So, just an update and a word of warning to anyone headed down this path. I ended up getting a Saker ASR 556k from Silencerco. I went this route because the Saker is designed for short barrel applications and is nearly indestructible. However, the Saker Charlie mount requires a 1/2 28 thread pitch rather than the 5/8 24 pitch that the JMAC extender provides.
So, I bought a 1/2 28 to 1/2 28 barrel extender from KAW Tactical. After 16 rounds, the KAW Tactical extender ruptured catastrophically. Thankfully the Saker and the Rifle are both ok. But rather than going down the barrel extender route again, I have ordered a 10.5 barrel from Faxon Firearms to get out past the handguard. I'm not thrilled about going from 7.5 to 10.5, but considering the 1" extender, it's really only a 2" increase.
Here's a shot before everything went south:
View attachment 56395
And the ruptured extender: