I came to own my M4 by trial and error...
The club where I was shooting IDPA (pistol only) started to hold 3-gun events. I did not own a SG, so borrowed a 20 ga pump from a friend. After that first match, I jumped onto the VA Gun Trader sight and picked up a 12 ga pump (FNH Tactical Police). My shoulder would be sore after each match, so I picked up a 20 ga Saiga at a gun show. The Saiga did not beat up my shoulder but is was a pain to load (mags would not go in w/o the chamber being held open) and I would come home with cuts on my hands (could never figure out what part was sharp). While cursing the Saiga, the SO let me try his M4. Need I say more?