I carry a 16" Bushmaster, A2, semi-only, with Smith compensator, hi-low sling (I really like it) with mag-cinch.  Id like to try a shorter AR, but I'm not interested in doing the tax stamp deal.  That’s about the only thing I'd change.
    We supply our own quality .223 semi-auto (fairly broad policy) or get issued a mini-14 if we want.  I just got assigned a new (to me) unit this week and took the last of the 12 gauge ammo out of it about 20 min ago.  Oh what fun it is to work the holidays!
Art in KY

There are many advantages to a carbine as well as a shotgun in police patrol work.  The shotgun offers great stopping power potential of putting multiple projectiles on the intended target at once.  The shotgun also offers the versatility a many types of ammo resulting in a multitude of uses such as breaching, precision and maximum penetration of slugs, birdshot, gas and oc type ferret rounds, gas and oc muzzle blast dispersion rounds, etc.  The shotgun should not be replaced, imho, it is becoming more of a specialty weapon in the police arsenal.

    The .223 carbine offers the great advantages of surgical shot placement, extreme accuracy at longer distances than pistol caliber weapons, lack of overpenetration as well as penetration capabilities, etc.
It tends to be a better choice, imho, for patrol due to its supreme accuracy as well as less liablity concerns of overpenetration.  
It is lacking in recoil compared to the shotgun which makes it very user friendly.  The ability to have a 20/30 round magazine is a major plus as well.

    Many agencies are opting for the M4 due to its short barrel and collapsable stock.
Our agency guns are in our cars mounted vertically between the bucket seats.  Because of mounting space the 20" barrels are too long to be mounted between the seats or overhead.  The M4 and other types of collapsable stocks fit nicely between the seats and overhead but have several downsides.  First the collapsable stock riding in a vertical rack takes a pounding.  Next the collapsable stock if carried in its collapsed position must be extended as it is brought out of the patrol car which only adds time on rapid deployment or felony carstops.  The collapsable stocks are catch mustache hairs which hurt and have a tendancy to collapse while shooting them if the sling is catching on it and pulled tight.  Also a collapsable stock doesn't last long if one needs to do many butt strokes to clear a jambed round/brass.  The collapsable stocks also offer only a low cheek weld.  As you can tell I prefer for patrol applications a fixed stock.  The M4 also has a shorty site radius which is accurate enough but the longer site radius of the 20" or Tactical 16 (dissipator/eliminator) is more accurate and user friendly(imho).

   My ideal set up is the A1 upper receiver because of the fixed site system, on the A2's upper receivers officers get bored in their car and have a tendency to twist the sites which now effects the zero.  A DPMS Tactical 16" barrel (dissipator/eliminator).  My recommendation is to stay with iron sites and not optics.  Iron sites work in all light conditions and don't fog up or run out of battery power.  I highly recommend tritium iron sites though.  A surefire millenium light, ambidextrous safety selector switch, and giles sling.  Two magazines clamped together as well, the buttstock carriers are fine but are hard to get a mag out of if the weapon is cheeked at the time.  We have one of these setups in every patrol car.  I also have several personal ones.  When I take this configuration to the courses that we teach the attendees can try them and find that it is a very good set up for patrol.  Other configurations are nice for different applications.  Find something that you like, shoot well, are comfortable with, and stick with it and get to know its full potential as well as limitations.

Sully
DPMS Master Instructor
www.defensive-edge.net
www.dpmsinc.com