Cantilever scope mount is the only type of scope mount I would even consider for a shotty. I'll second the 20ga option too, 12's are ridiculous on the recoil end. A decent mossberg combo gun will run around 300 I think. I still have the POS scope that came with my mossy 500 about a dozen years ago and it shoots very well.
The comment about the hastings barrel not grouping - have you tried the hastings slugs with it John? I'm only asking b/c i spoke in depth with the guy who is running Hastings ammo division and he stated that his slugs were designed around that barrel so i'd imagine it should shoot very well., Just my .02
In closing -
- get a pump action (or a lever...there are a couple nice Puma lever shotguns out there)
- try to avoid 12ga unless you are very tolerant of heavy recoil. A 20ga is more than sufficient and has about 25%less recoil IIRC
- get a rifled barrel if you desire sub 3" accuracy at 100 yards out of your gun, and then commit to getting that accuracy by trying every brand/load available until you find one that shoots well. There are many threads on this forum on this very topic
- if 5" accuracy at 50 yards is sufficient for you, a smoothbore barrel with iron sights would work, be a little cheaper both to buy, and to feed. Foster slugs will run about 3 bucks a box or so. Sabot slugs (required for rifled barrels) run about 3 bucks per SLUG.
- replace your recoil pad with a Limbsaver or similar. You will not fully appreciate this until you shoot the gun without one...so do that first
- there's alot to be said about carrying around a lightweight single shot smoothbore with iron sites and punkin balls...sometimes I wish I didn't have my pump gun.
- I would NEVER exchange an inline for a slug gun - too long to reload and with the accuracy of smoothbore guns, a follow up shot will be on order sometime and I'd rather be able to follow up quickly but that is just my preference.
Welcome to the darkside...slug hunting just has a different feel about it.