The weapon system should be chosen for the exact application or mission. I have been through plenty of doors with a MP5/40, AR15/M16, as well as a 12ga with bean bags, and sometimes only a handgun, and recently a M26 taser. My preference depends on the particular situation and what my partners are employing as well.
If I had to choose an overall one weapon system, then I would opt for an AR15 Tactical 16 (16" barrel with full length sites), with surefire millenium lite, night sites, giles sling, dual stacked mags, and an A1 or shortened tactical stock. The AR15/M16 is very capable of surgical shot placement, and the ballistic capabilities give excellent results while also reducing chances of overpenetration versus handgun and shotgun rounds.
My shotgun choice would be an entry shotgun such as the Benelli M1-90 Entry with ghost ring sites, surefire lite, night sites, giles sling, extended charging handle and safety, and side saddle. Or the Remington 870 setup for entry as well. The shotgun has great versatility for bean bagging, duty buck, slugs, door breachers, muzzle dispersion, chemical, etc. The shotgun is not my weapon choice for surgical shot placement, plus if using buck rounds then one has to account for multiple projectiles per pull of the trigger. Shotgun rounds have a tendency to penetrate more than .223 so again it needs to be chosen mission specific.
For a handgun I prefer my Glock 21 with Heinie Slant Pro sites, and Tac3M light mount, or an M6 lite for shield work. Given the choice I would always opt for a shoulder weapon because it is easier to shoot due to being able to stabilize in the shoulder, and more accurate due it a longer site radius. Handguns are not my first choice for entry work unless you are employing a shield.
Just my $.02
Sully
DPMS Master Instructor
www.defensive-edge.net
www.dpmsinc.com