Happened to my buddy who was shooting REAL bullets. He was shooting something like 120gr., checked the left barrel upon my instance after firing 2 sucessive shots from the right barrel, and found the bullet had moved forward 1/2" or so. The reason we did this test, was to show that to shoot at a moose, then reload, follow up the moose, he'd be shooting the right barrel first again, then the left if needed. Finding that the left barrel's bullet had become an obstroction was enough to make him switch to patched round balls - that and the better accuracy he got from round balls. I now have that gun, and it regulates with 100 and 110gr. 2F with .562" and .570" round balls - whichever are handy.

This is the reason I've not tried any slugs in my 'new' .58 DR.

As far as recommendations from the manufacturer, Savage sells a smokeless powder, bolt actioned inline muzzleloading rifle and recommends some smokeless powder loads for it.
There have now been quite a few that have blown up. Go figure.

Of course, the manfacturer can claim their directions weren't followed or loads were exceeded - no way to prove either way. Go figure.

I wouldn't do it in any double, and refuse to do it in a single as long as the round ball comes out superior or equal in every way - which it has - generally superior.

It wasn't until fixed ammo could be loaded in the breech, that hardened slugs could be used. At that time, they started out penetrating round balls and showed some superiority in absolute penetration - which was rarely needed. One simple goes to a larger bore, shooting a larger, heavier ball that penetrates more deeply.

Slugs eventually foul the bore with powder fouling, and quite often with lead fouling. Round balls with cloth patches do neither- as long as they are loaded to the 'perscription' which works in all muzzleloading rifles with descent bores.