List it as an 8". By the time you get it together, it'll be at or slightly over that.
Removable mounts don't count toward OAL, either. stand alone booster housings, Dead Air Keymo adapters, etc. will all increase the actual OAL over, say, direct thread, but unless you make them a permanent part of the can, they're not considered to alter the OAL.
And in truth, this stuff is mostly academic. I've never heard of anyone, anywhere, ever having had an ATF agent put a ruler or caliper to their can and compare the reading to the F1 or F4 paperwork. For that matter, having it checked at all would be an anomaly. I go to all kinds of public and private ranges with cases full of silencers, screw them onto machine guns and invite people to come try them out. Not once have I had any kind of LEO or agent ask to see papers.
I've been told that it's best to keep the total length of the suppressor slightly short of the listed length on the form 1, but don't know for sure. I would try to keep the length as close as possible to what is listed.
Other way around. It's permissible to shorten them a small amount for repair purposes, but never to lengthen.
As far as the 1/2 inch hole in the .223 can, you might have a hard time explaining that if ever questioned, but I've never found anything stating how much larger the bore can be over bullet diameter.
There's no edict on minimum or maximum bore size for a given caliber. I routinely overbore cans for reduced backpressure on semi autos, or concerns over baffle strikes with firearms like the AK that have crappy barrel thread alignment and whippy barrels.
That said, it needs to be able to pass a projectile at least as large as what's listed for caliber.
