Each shotgun is a law unto itself about this. I do better in one shotgun with a rifled tube, but my HD 870 is chokeless, and shoots about a half inch larger group at 50 yards with its slug of choice. I'm inclined to believe that slug testing will do you more good than getting a new bbl,which should then be tested with slugs anyway.
A comparison seen here before....
HD 870- chokeless 18" bbl. Groups about 2 1/2" at 50 yards from the bench with KO Brennekes, very slightly larger with Winchester Forster type slugs.
Deer 870- Remchoked 20" bbl with rifled tube. Groups close to 2" with KOs, and similiar with that Winchester slug.
Both are set up identically, good triggers and peep sights, mag extensions clamped to the bbls to stiffen them up.BTW, the deer 870 took another doe this year, hit POA at around 50 yards in the field.A short blood trail Ray Charles could follow....
Do NOT shoot a shotgun with tubes, without a tube. You probably wouldn't get hurt, but the thin steel of the threaded portion would probably expand, and maybe tear.It'll definitely ruin your bbl, your day, and possibly you.
Here's what I would do, t'were I thee....
Buy two 5 packs each of the slugs you want to test. Stick to 2 3/4" loads.Use the Big Three, plus both the Rottweil and KO Brennekes. Don a wearable recoil pad like the PAST pad, take a dose of your favorite anti inflammatory, and start testing.Use your best bench rest technique, and take your time.
After firing one three shot group with each slug, compare the results. Discount/reshoot any called flyers, of course. Compare, then fire another three shot group with the best two. Confirm the best with a last group, then go buy as much as you can of that slug, especially from the same lot. Chances are, it won't get any cheaper and lasts for decades if kept clean,cool and dry.
Shoot up the other, rejected slugs for fun, or donate them to a buddy or your gun club.
There's no ineffective 12 ga slugs. just ineffective slug shooters. Make sure YOU'RE up to the task by practicing, and keep your shots close,knowing what your limitations are.
Hope this helps....