With some experience in CQB and talking to some marines who came to Israel to train in CQB using their M16A4s, I get the feeling that weapon collision isn't significant enough using the long weapons (m16, sig, psg, rc50). It's too easy to move around corners and rooms using these. Compared to bullpup famas and SMGs, not to mention pistols, it's rather easy to handle the weapon collision with longer weapons (I mostly talk about M16 because I use it the most). M4A1, especially with buttstock extracted to 1/2 (1 click longer than fully retracted, exactly between fully extracted and fully retracted), not to mention SMGs and pistols, would have the advantages they should have in CQB - easier handling and less collision, and overall being more comfortable weapons.