Hello and welcome! You are really going to enjoy this gun board. It's a terrific forum to get and share info and advice about Mossbergs. Have you checked out the Product Reviews Section that we have available? GLG2 moderates it and it's a vastly deep pool of good information regarding modifications and add-ons.
So WE are glad you are on board! New guy buys the next 3-day training/shooting seminar for everyone!!!!!! Thanks.
Now to a comment about your shotgun. I may be mistaken, but pinning on a brake or extension doesn't make it legal. Welding does.
If it can be removed, its full of suck as far as the BATFE is concerned. Do you know that a tape isn't the determining method of arriving at legal length? A measured and calibrated rod is dropped down the barrel until it hits the bolt face while it's in the locked breech position, and if any portion of it is beyond the barrel crown it is too short.
I'm not trying to be discouraging, please believe that. I just want you to be sure that you are compliant because the possible repercussions could be rather unpleasant.
I think an optic is a modification left for later after you first add a fixed or adjustable shoulder stock and shoot it for a while. But that's just me. I'm often criticized (and rightly so) for keeping things painfully simple.
Again,
Welcome to a truly great forum and we're not just here to help... we also want you to help us as well!