I prefer the M-1 Garand (.30-O6) or the M-1A - also known as M-14 (.308/7.62mm) with a match grade barrel. Ammunition is heavy to carry in the field, but I think they did the job better. General Patton called the Garand the greatest weapon of war ever invented for a reason.
Also, due to complaints, Ranger medics were at one time being issued HK MP5s instead of that horrible M-16 carbine. I loved that little beauty, it was so easy to carry on a sling with the heavy medic pack, gave me great close quarters firepower and I could have it right there slung over my shoulder at the ready when treating a patient.
GAU5A/AA & XM-177(Pre-M4 carbine)
M14/M1A
1911A1
Browning Highpower
M82A1 Barrett
M79
M60
HK21
MP5 Series
M148/M203
M12 or M870 Shotgun
Swedish K & M76 S&W
I was lucky enough to be in a job that allowed "me" to make sure my weapons and ammo were up to the task to be performed......prior to deployment or hostilities. I understand small team logistics are better suited to such selection and use but MOUT wasn't something that popped up yesterday. Rangers have used the short barreled pistol grip equipped shotgun for decades to breach reinforced doors . The lessons learned during DS/DS 1 regarding lubrication of firearms in desert conditions seems to have been ignored.
As to the M9 Beretta I was never issued that weapon, albeit I trained with it a bunch, it worked fine for me. We kept our 1911A1's and were allowed to carry a personal P-35 Browning Highpower if the mission needs allowed.
Were I to be tasked today to issue a list of standard weapons for MOUT needs I am of the opinion that a M4 carbine with a M203, A M249 SAW, and a Designated Marksman with either a SR25 or M1A (prefer the SR25 as the rifelman wouldn't "appear" to be anything special to a enemy sniper), a pistol grip sawed off 12 gauge pump like a Serbu 12 gauge albeit I'd like to have a 6 round magazine tube that would dictate a longer barrel.....(est 3 hinges hit twice as a minimum need) .......and the 1911A1 in 45ACP.
All these things being brought up have been known for years.....the small indentation on the primers, the CLP use in desert conditions like Kuwait, Saudi and Ft Irwin !! But hey if the DOD's current leadership will listen now then keep shouting and bitching loud and long ! Hopefully someday they'll listen.............
Just my opinions ............Stay Safe !