It's been shortened to 22" which makes it cylinder bore. The theory is that Remington makes the largest diameter slugs of the manufacturers, and with no choke to disrupt the path, the slug flies true. Works for me. I did alot of reading here:
http://mcb-homis.com/slug_410/index.htmand agree, Remington Sluggers are consistently more accurate than Winchester and Federal in this gun. The new Brenneke Close Encounter 2 1/2" slug shot very well, but cost too much for what I'm using them for, the 3" was wildly innaccurate out of this gun. So, bulk-pack Remmies from Wally world for me....

...Scott B.