I've never cast a chamber because I don't shoot or collect antiques that need that sort of attention. Measuring the leade with a COL gauge, pin gauging the neck, headspace gauging, bore scoping the throat, and inspecting the crown pretty much covers all you need to know to load for and shoot the barrel. If the chamber is somehow out of print, neck size only on the brass will negate any issues without resorting to attempting to measure a cast slug by hand (it's a comparator job). The neck on factory barrels is usually at the max, and box ammo is usually near the min, so that has an effect on how tight you can group, but won't be the sole cause of shotgun pattern groups. If it doesn't shoot a 75 grain bullet with the COL within -.015/+.005 of the lands, and you know the crown is good, then do the rebarrel. There isn't much more to it in my opinion. Bottom line simplified is there is a mechanical problem (action screws, bedding, scope assy) going on, the barrel is shot out (throat), or there is a pretty big ammo mismatch. If it doesn't have loose screws or a smooth throat as acquired, my money is on the ammo. Factory barrels aren't premium match barrels, but they're not total junk and can shoot well when set up right, usually. Ok, let me qualify that- I feel pride and satisfaction when bringing a factory barrel in consistently under a minute of angle, maybe even content at .75 (5-shot). But I feel giddy as a schoolgirl and run around with a stupid grin just breaking in a new Krieger. So it's a spectrum.