Hi Guys,
So I have a Sabre Defence M4 carbine, I bought complete a few years back.
This rifle has a 14.5" barrel with the muzzle device somehow fixed so that it cannot be unscrewed off.

Anyway, I took it to a gunsmith to remove the muzzle device and the gas block because I want to mount one of these sights/block
http://www.brownells.com/.aspx/pid=2...IGHT-GAS-BLOCK

But he said he couldn't do it, because I don't have an SBR stamp!
I don't get it, he can work on anything right? when he gives it back to me, he'll have the hider fixed back on so I'm always in possession of a legal length barrel.

am I assuming right? do I need to find someone else to work on this?
Thanks

Thanks