Just say no to a P90... If you want interchangeability get a Stanag rig. It doesn't have to be M4's how about Some Scar-l's? Tavors, L-86's, Famas's they all take the same mags and you can all still be gucci and have your own unique set up if want.
“We sleep safe in our beds because rough men stand ready in the night to visit violence on those who would do us harm.”
To those Rough men... Thank You.