It is from a XML 178, which is a rare 1911 FN49 hybrid in 577/950 rimless pinfire.
This particular piece is call a migrating trunnion, it holds the gas channel to the blowback accumulator during firing, then it shifts to the rear of the rear receiver pocket via the ejection channel where it then functions as a rate reducer during the rebound phase of the weapon's fluidic bolt. It then drops through a port in the stock and is discarded.
It is not unusual to find this item on a range, but it is unusual to find only one as the XML 178 discards one per shot fired. These migrating trunnions were produced in the thousands for the anticipated Siam/Easter Island war that never developed. Sadly, although roughly 6,000 of the XML 178 were produced, and an adequate supply of trunnions, only 3 577/950 rimless pinfire cartridges were produced.
Due to the incredible rarity of the cartridges, the design firm of Rasputin & Rockefeller introduced the novel F.M.R.S. or Forend Mounted Reloading System which combined an early Dillon prototype steam powered progressive reloader and a squirrel monkey based quality control system. Once it was properly warmed up and primed with whale oil, pennies, black powder, and raw lead ore, the F.M.R.S could reload fired casings at an impressive rate of 12 per minute. Which was pointless as no owner ever singly owned any casing or cartridge, rather they formed groups of 2,000 to co-own a single cartridge.
Ballistics for the 577/950 rimless pinfire are quite impressive. The original cartridge was designed to penetrate the naturally armored shell of the galapagos tortoise (greatly feared by the Siam military) and could throw the 9 inch long projectile weighing 4 pounds out the muzzle at a snappy 160 fps. it was an ivory jacketed, hollow cored dodecahedron, but after the first test shot accidentally sank the entire navy of Liechtenstein (a rowboat and 2 pool floaties) the round was redesigned to lessen political tensions. The MK2 projectile in common use today is a standard internally rifled mobius weighing 6 pounds and achieving a muzzle velocity of between 80 an -3 fps. The MK2 projectile is notable for having a penetrator made of a pickled fruitbat thyroid which has all the expected benefits of dolphin thyroid, yet without the normal problem of attracting aquatic rhinos.
