No 5.56mm is going to suppress like a 9mm, because the 9mm is subsonic.  A 9mm also can get by with a much smaller suppressor, or with an integral suppressor that reduces overall length (think MP5-SD or Gem-Tech Talon).  These will still be used as needed, especially when a silenced capability is needed.

11.5" is the absolute minimum barrel length that is really effective beyond contact length for 5.56mm, and even that is pretty limited in effective range (meaning: fragmentation range) even with high-end ammo.  Look at the problems we had with the 14.5" M4 in Somalia and Afghanistan (using M855).  That tells you how important velocity, and therefore barrel length, is.  Of course, had they been using 77gr Match ammo, they'd have quadrupled their effective range from the M4s, and there would have been no problem.

Every weapons system is a compromise, and reducing barrel length will always mean reduced velocity & range and increased blast & flash.  In a caliber that is utterly dependant on high velocity for terminal performance like 5.56, the price is quite high when you start chopping the barrel.

Also, you really need to realize that many people choose the shorter weapons without any real understanding of the terminal ballistics penalties the short barrel gives them.  Don't assume that everyone understands what you know about 5.56 ballistics.  They don't.

-Troy

A GREASEGUN?  You've gotta be kidding me.  Those things were POSs from the beginning, and were mainly intended for sticking out the turret of a tank to hose the enemy infantry (i.e., "crunchies") off of your tank.  To compare this gun to an MP5 clearly shows a lack of experience with either.

Fragmentation range from a 10" barreled AR with 77gr ammo is about 10-15 yards.  That's acceptable for a few limited types of work, such as VIP escort or in-building CQB while the OUTSIDE of the building is surrounded by guys with longer-barreled guns, but, like the MP5, it's much more of a specialty weapon than a general-purpose one.

And, when considering YOUR gun, remember that you probably don't have a company of other trained soldiers to watch your back, so you should be focusing on more general purpose items than on limited, special-purpose ones, at least for your primary weapon.  For range toys, heck, get a 7" AR if you like!  [:D]  Just don't forget your ear protection!

-Troy

The SD is used in covert ops, when we don't want to advertise our presence.  These things are VERY quiet.

Obviously, if we had intelligence that suggested the (multiple) bad guys would be wearing body armor, than a short M16 would be a better choice, and you probably care less about noise anyway.  But body armor isn't as common as you'd think in most of the countries where they'd be working.

-Troy