That compensator should be blown off in about 1000 rounds or so ( like, exploding off. No harm to the gun tho ).
That only happened to a few of the older 6400c carbines that used hollow roll pins to hold on the brake. Newer examples have solid roll pins that aren't going anywhere. I'm up to 2400 rounds of LC M193 through my 6400c and the brake has stayed put. It's not going to come off unless I remove it.
To answer the original posters question, the LOP of the fixed telestock on the 6400c is 5/8" to 3/4" less than a standard A2 buttstock. Here's a pic of the fixed telestock from my 6400c placed on top of the A2 stock on a preban 6601 HBAR for comparison:
Tweak has a pic somewhere here that shows what the barrel looks like with the brake removed. You might want to consider leaving it on though. It isn't nearly
as obnoxious as a Mini-Y comp on a Bushmaster. After the sunset next year I'm simply going to thread my barrel and install a proper A2 hider. In the meantime I'm leaving it alone.