jrkrk - The chamber is 2-3/4 inch. When I had the barrel put together I think that virtually all sabot 20-gauge slugs were 2-3/4 inch. The brass cases are from Rocky Mountain Cartridge Company and were based on a chamber casting so they fit very precisely. I don't even have to resize them after shooting. The neck area has enough tension to hold a new bullet. They use shotgun primers. I have a NEI Handtool mold for the bullets. It is a for 720 grains and is a cutdown design of their 900 grain bullet for a .600 Nitro Express. I have hollow pointed some with a 3/8-inch drill to reduce the weight to 550 grains. I load AA5744 powder (the black powder substitute) for 1,000 fps with the 720 grain bullets or 1,300 fps with the 550 grain bullets. The 720 grain bullets are fun for plinking and very accurate at 50 yards. I've got one deer with each of the big lead slugs but lots with Winchester and Hornaday sabot slugs.
358-Thumper - The top one is a magnum action and the bottom one is a standard action. That is not the original stock on the top one, but is from an Aristocrat. Thompson Center put it on the gun after the original stock broke. Guess they did not have one to match the original. Not sure, but I think the Aristocrat model featured a double-set-trigger, a buttstock with a cheek piece, and a forearm with the release lever rather than a release screw. I don't know if they were made in magnum, but I know they could be converted. The Thompson Center custom shop converted one of my regular actions to magnum and then stamped it as a magnum. I also have a double-set-trigger action which Virgin Valley converted to magnum. Someone at the Thompson Center custom shop told me the standard actions are as strong as the magnum actions. The difference is that the locking pin in the regular actions can move under recoil and result in the action opening. On the magnum actions that pin is locked. I had a gunsmith pull 12-gauge smoothbore slug barrels out of the monoblocks and replace them with fully rifled 20-gauge barrels. The short barrel was from an ER Shaw barrel blank and the other from a french-made barrel blank sold by Hastings. After the gunsmith press-fit and welded in the new barrels, I had them Cryo-stress-relieved. The outside contour is the same as the factory 10-gauge barrels. Even then they are not heavy. The one with the longer barrel probably weighs 8-1/2 pounds with scope. With the factory 12-gauge slug barrels they are so light that recoil is really bad. These are very pleasant to shoot with the sabot slugs and the 720 grain bullets at 1,000 fps. The 550 grain bullets at 1,300 fps feel about like a a regular 12-gauge field load.
