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
I have a .223 sbr already, and a .223 can. That's why I find it tough to swallow doing it to a 15-22. Where to they allow you to run a .22 in 3 gun matches? That's pretty cool. In the 2/3 gun competitions I compete in they do not allow rimfire, however they have made an exception for my son to run his 15-22 due to his age.
We had a course of fire at our last match that had steel tgts at 100, 200 and 350 yrds. My son struggled to find the holdover on the 200yrd steel and my concern is chopping the barrel down would hurt him in a long range COF.
You are right though, it would be pretty cool if I sbr'd his 15-22.
