Masking tape? clay?
If rattle canning it with crap paint I would just skip the whole process and leave it black, I've tried Krylon and it is not a durable finish, you will go through a lot of work making it look nice then every time it bumps something it will chip or mark it in some way and you'll have wasted a lot of time.
If on the other hand you are Cerakoting it then you can paint the inside and outside, my PWS AR came factory cerakoted and it was inside and out and even where the BCG traveled it looked great after 5000 rounds.
If you want a quality job that will last you have to spend the money and use a good coating, so far my favorites are Cerakote and Arma-Coat. Both have help up well for me so far.
Coatings like these don't require 4-6 coats to get coverage so the thickness of a layer of paint doesn't interfere with function.