The velocity loss with an 11.5" barrel is significant.  How (much) this affects performance and useful range is going to depend on the load used, as each bullet type will have its own velocity threshold below which it will not give you the necessary performance.

Here's a chart that will give you an idea of the velocities you can expect from chrome-lined barrels of various lengths using true M193 or M855 ammo:

[img]www.bushmaster.com/images/faqchart.gif[/img]

As you can see, with a MV of about 2870 fps for an 11.5" barrel, you're not going to have much range (20-25m) before the bullet drops below the 2700 fps threshold.  It's even worse with M855, which is barely above the threshold at contact range from this barrel.

Our Spec-Ops guys who have a mission requirement to use short barrels are using Mk262 ammo (i.e., hot 77gr match loads).  Muzzle velocity is even lower than M855, but the different bullet construction used for these bullets gives them a lower threshold velocity for fragmentation; down to about 2200 fps.  This will give you 30-40m fragmentation range from a 10.5" barrel, and around 50-60m from an 11.5" barrel.  Of course, you must have a 1:7 twist barrel, and this ammo isn't cheap or (what I would consider) "readily available."

Another possibility with such a short barrel (and an option that our military doesn't have) would be to use a heavy soft-point bullet such as the Winchester 64gr PowerPoint.  These bullets expand instead of fragment, and will expand to some degree down to 1800 fps, so usable range is increased.  The downside is that the wound damage at any velocity isn't as large as a fragmenting bullet would cause.  Also, there is some concern about soft-point bullets feeding reliably, especially with the shorter-barreled carbines where the gas system is under more pressure and cyclic rates are faster.

IMO, unless you have a very specific NEED for such a short barrel, I would recommend at least a 14.5" barrel.  You give up way too much for those extra inches.  Unless you were a police officer with lots of backup (carrying rifles with longer barrels) on the outside of the building you're performing the "CQB" in, there will be times when the fighting will spill outside and ranges will increase.  This happened in Somalia (Black Hawk Down) and Afghanistan, both places where the battle was primarily infantry, unlike Iraq where the fighting was mostly done from vehicles.

-Troy