Civi= Civilian.

That's a matter of opinion really, on what's allowed.
Some MilSim Events want ya to have matching gear, guns, and and totally period correct. If you bring a Tommy gun to something more Modern, I doubt that would make much difference, especially if your squad all had Tommy guns.
Some don't matter so much.
I think when ya get into obscure things, such as a Pulse Rifle, or one of the "Fish guns" JG made...,
Those probably would get alot of odd looks in MilSim... unless the MilSim was already set "futuristic".
Kinda like... the helmets in Starship Troopers, I've been searching for one high and low ( Authentic version.). I'd actually use that thing when I could. Might not be appropriate for every Event though.