I have a Mk18 as my house gun, which is of course registered, as is the silencer on it.

I've heard two different reasons not to use an NFA weapon. One, they don't want to lose the weapon. Two, they think it's somehow going to work against you if you find yourself in court.

In my opinion, they're just lame excuses from people too cheap or lazy to get an NFA weapon. If you find yourself in the unfortunate position of having to shoot someone, losing the gun to the evidence locker is the absolute last or your worries. That is also extremely unlikely, and in that extremely unlikely event I want the best weapon for the task that money can buy.

As for it working against you in court, I think not. If you find yourself in court there are already problems with your case to begin with, and the fact that you complied with the law by registering your firearm and undergoing a background check isn't likely to be a good angle for character assassination. I think an unregistered gun would give prosecutors more to angle against you. Even if not illegal, it suggests to the average person that you were planning on using it and therefore went to the extra trouble of buying or building it in such a way as to keep it off the books. Not saying that's a good argument or even that a prosecutor would try and go down that road. Just saying that out of all the possible weapons you might have, a registered NFA weapon seems like it's probably less likely to be the source of some kind of character assassination.