I recently bought a Mossberg 24" rifled barrel made for the Remington M870, it looks like a well made barrel since it has thicker barrel than the 20" smooth bore barrel that I ahve.

I did a search for previous posts on this subject and there was very little.
Has anyone fired normal foster type or Brenke slugs through there rifled barrel. Someone told me the rifled barrel are made strictly for Sabot Slugs. I micrometered the bores at the muzzle of the mossberg and my std remington slug barrel and there is very liitle difference?

I have a Hastings which is on the same order as yours. I have only shot the sabots out of mine but get good accuracy, around 3 inches at 80yds.I don't think the foster slugs will work since they are smaller than the bore and probably to soft to hold the rifling even if they did touch the lands. The Breneke might work if the felt expands enough to bite into the lands. I think your best bet though is to use sabots. I know they work.

Among other issues, the Brenneke slugs I fired through my rifled slug gun would occasionally keyhole. However that was in -my- gun, the only thing you can do is try out some different loads to see what your barrel does.