20mm TP-T, that pic is a prototype 20mm AMR built on a Mk82 receiver for the Navy, I think.
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.