i had ADCO cut and thread my barrel after i found out the specs for the OE barrel. youll find that TROS and HDCP and even Hi Dessert Dog have fairly long turnaround times. ADCO will get the barrel back to you in a week, and youll have an OEM barrel in your gun when youre done, with properly cut threads.
i actually had a hell of a time finding out what the exact specs were. on FNForum.net, their resident NFA retard went so far as to give me the improper information, then pitched a fit because a "newbie" like me called him out on it. frankly, hes an ignorant asshole whos going to cost alot of people alot of money. the mods were ok about it, but hes got a little group of followers over there that believe everything he says, even when its verifiably wrong. when you get on their bad side, no technical post will go unmolested. its silly, actually.
not that im mad or anything.
anyway, i can provide the barrel specifications to you if you need them. this goes for anyone who needs them.
i had my receiver engraved by Ident Marking, who did a great job and has wonderful communication. i used the Gemtech flash hider, because i have a G5. otherwise id use the TROS adapter.
as for accuracy and function, dont worry about it. its a blowback gun, the barrel length wont effect function at all, and ADCO, TROS and HDCP are all competent machine shops. accuracy wont drop off either.