Any welding to our firearms should be only done in TIG. I would not use a mig welder to put on a muzzle brake, compensator, etc.
Instead of shipping and waiting a week or so and paying 45 bucks, local welder could do it in 5 minutes for min. charge.
I'm 99% sure it is 100% legal if you brought the entire upper receiver assembly to him and did the work. The lower receiver is what counts as far as becoming a firearm, if I recall.
This is what mine looks like
I'm building my own upper in a 14.5 barrel with a 2.0 comp. making it legal and not a SBR