From Military.com:
The XM-109 is essentially a reconfigured M-107 .50 Caliber semi-automatic rifle -- if you can imagine a .50 caliber rifle being mated with a 25 mm receiver.
Thanks to the increased power, the XM-109 rifle is designated as a "payload" rifle, designed to destroy light armor, and light enough to be carried by a single sniper. Essentially, the 25mm upper receiver attaches directly to the lower receiver of the M-107 (in effect, swapping out the .50 caliber components for 25mm ones).
In the process, the rifle's weight actually remains unchanged at 33 pounds, but its length has been shortened considerably, with the XM-109 (at 46 inches) being 11 inches shorter than the M-107.
Two field reps for the General Dynamics Corp. in Burlington, VT showed it off for him. They claimed a barrel change of "less than two minutes" so the host said "OK I'll time you." They did it without breaking a sweat in one minute 47 seconds.
The really cool thing about those rounds is that they can be set to burst at a specific range. They are essentially shrapnel grenades. I mean very specific too. They set up a false frame wall with a window about 200 yds downrange with three dummies just inside the window. Whatisname shot a round right through the window and it detonated about two foot or less inside the wall. They showed that in slow-mo from a side camera angle.