Honestly? Because you can.

The $200 isn't going to break anyone's bank, and it is fun. The gun is a little lighter and points a little better.

If you want to do things like put a real suppressor on it then you don't have 16" +4" of suppressor.

If you've got a .223 suppressor for it you can use it on other rifles, or if you get a .22 suppressor you can use it on other .22's.

EVERYONE who has a M&P 15/22 has a 16" one. If you make it shorter you can make it personal.

My personal reason is I use my 15/22 in 3 gun matches. It's easier and quicker to navigate corners and doorways with the shorter rifle than the longer one. If I had a .223 AR I'd also chop it.

You also don't lose THAT much velocity with the shorter barrels. You are generally going to want to keep it inside of 100 yards anyway, something you can do quite well with a .22 pistol with 4" barrels. Leaving it at 10"+ means you can easily do that.

Someone posted a chart a while ago about the velocities of .22 from different barrel lengths and the 10"+ were all in the same ballpark anyway.

KBK