Alan,
Since you have a rifled barrel, you'll likely be better off using sabot slugs through it. That means you will have to spend a bit more on ammunition, as the sabotted type are somewhat more expensive. Nothing says you absolutely can't shoot regular Foster type rifled slugs or standard Brennekes, but they likely won't perform as well as sabots and will likely lead up your barrel.
Best bet is to buy a 5-pack each of several different brands, and get ready for some slug group shooting sessions, just as you'd shoot groups with a rifle. Sometimes shotguns show a decided preference for one brand over another- it can help to let your gun decide what it likes.
I prefer to shoot groups with slugs from a standing supported position- benching a shotgun with slugs is not high on my list of fun activities. If you do decide to shoot it sitting at a bench, the more upright a sitting position you can manage the less you are likely to get beaten up. You won't be able to tell what brand is shooting better if you flinch off shots all over the paper. Don't make a marathon session out of it, shoot a few and quit for the day.
Before you go out to the range, tighten up your magazine cap as tight as you can get it with your fingers with the slug barrel in place, and then tighten it one more click (just ONE more click) with a strap wrench or a pair of well padded slip-joint pliers so as not to mar the finish. Having the barrel on tight is a necessity to getting the gun to group. But remember how thin the steel is in the magazine tube- don't overdo it.
Stay safe,
lpl/nc
