​9-inch tube with a captive brass piston, a spring running the interior length of the tube, and a stud on the other end with two flats.

I got this item as part of a lot of firearm parts and gunsmithing tools at an estate sale. The owner was a machinist and also a gunsmith. He had barrels for shotguns, M1903A3s, an M1 Garand, a Ruger 10/22, and a Colt M11911A1. He was also a reloader. The tube is exactly 9 inches long; the small diameter is approx. 9/16″ and the flared ends are about 11/16″. The internal diameter of the brass cup is just under 7/16″. I cannot find any manufacturer stamps or proof marks. The holes go all the way through. The piston/plunger is spring-loaded with a spring that travels the length of the tube. The spring tension is weak. The “rear” stud is about 1 1/16″ and has two flats on the tip. Please, what is this thing?