is it hard to remove the fn flash hider off the 13'' barrels ?

It appears to be hit or miss.  Some apparently loosen right up with normal resistance and others apparently are on there like a MF'er.  The horror stories are what have prevented me from attempting to swap out the hider on my 13" assembly.  There's also the problem of not having enough exposed barrel available to be able to use barrel vise blocks.  You don't want to be clamping the gas block by itself as the barrel will be trying to spin within the gas block as you torque the FH on and off.

Tango Down has a barrel clamping assembly among its armorers block kit.  However, I have yet to see anyone reporting a thumbs up or thumbs down on the TD barrel clamping assembly for FH torquing on a SCAR CQB.

Thanks for posting the question.  Hopefully, some guys will share their experiences.

i want to run the AAC Miter and can. it will be cool if its not murder to get the fn flash hider off.
thanks

It's the 10" barrel you can't get barrel blocks on and need the TangoDown blocks, the 13" you could cut some blocks out of wood to fit.

Some heat wouldn't hurt either.

How are they on there so tight ?

Mine was a pain, but it eventually came off with some hammer hits on the wrench after heat. Has some marring on it but whatever, it is doable to get off. This was on a 10"

Bummer, but glad it ultimately worked out for you.  Did you use the TangoDown accessory to clamp the barrel assembly in place?  If not, can you share with us what you used?  Thanks in advance.

Talked to Marvin Pitts last night, he was concerned about unthreading my factory 14" barrel from the trunion it was torqued on so tight. First question was "is this thing silver soldered on?" Eventually it came off with what he estimated to be almost 80ft lbs of force, and he did confirm the presence of thread locking compound on the flash hider. This was my third SBR barrel assembly and every one has been a massive pain to remove the muzzle device.

I used the Tango Down barrel block on a 16s 10" barrel.  Found the blocks at Brownells.  I used a crescent wrench and the flash hider torqued off ... not easily but wasn't that hard compared with other hiders I torqued off.  I don't think I could have removed the flash hider w/o the barrel block.

The factory 10" 16s barrel threads were clean as was the threads on the flash hider...no thread locker used from the factory.