Kind of lost, since if you are just installing a standard FS with crush washer, then all you need to do is spin the FS on by hand until its snug, then take is less that one wind to index the slots isntead.  Hence you really don't want to take a crush washer more than just one wind, or just slightly more that that one wind to index the FS.    Past this point, the crush washer has crushed too much, and your just over torquing the FS to the barrel instead (read screwing up the last inch of barrel).

So with that in mind, thin wrench to work the flats via the top of the barrel should not be a problem.

As for a 10.3" barrel with the gas port so close to the muzzle, going to be ammo sensitive as fuck, and can not see the reason for such.  
Hence the Xm-177 had a 10.5" barrel and the huge mod on the xm177E2, was to install a 11.5" barrel on it to make is less ammo sensitive isntead (even with both rigs wearing sound moderators that helped to add barrel bore back pressure).