Berrien,
I have shot 16ga low pressure 16's in my Mullin. The heads swell pretty good, but I had no issues. The diameter difference is something like .014" per side on mine. Rim engagement isn't as good as with the 14ga brass, but they extract ok and the convenience is quite nice.
I tried to fire form 16ga magtechs by wrapping tape around them and it didn't work out very well. They tended to split.
There are several way to skin this cat, I've found, but if I were starting fresh and trying to make a low volume shooter I'd buy RMC.