if you're going hpa on the m4 try to go for a closed bolt system, and bear in mind that you may wish to swap it out to a different gun at some point in the future. you could always hpa the arp and become "that guy"
Mate you're asking for trouble when you post stuff like this,you know how much we all enjoy spending other peoples money ?
you could always hpa the arp and become "that guy"