Browning offers a rifled choketube for their inv.+ barrels for use with slugs.

Can you expect these tubes to make slugs more accurate?
If so, what spread might be expected at 50 meters?

I purchased a Bennelli last fall, and bought a rifled choke tube for it. Using the bead sight on a ribbed barrel, I get 3 to 4 inch groups at 75 yards with sabot slugs. With rifle sights I could probably do better, and with a shotgun scope the groups would shrink more. I have a Hastings rifled barrel on an 870 winchester, and the choke tube groups about as well as the Hastings barrel. Hope this helps.

While I have never used a rifled tube, I can tell you that Browning has been very succesful with them in their guns in the past. In fact as you may or may not know the famed Browning A-bolt rifled shotgun was not only produced in a fully rifled version but also a smooth bore version available with a rifled tube.

From most accounts the smoothbore version with the rifled tube was actually very accurate even when compared with the fully rifled version. Clearly the overall accuracy here was a function of the superior design of that particular gun. However, my point is, assuming all other factors are held equal, it is possible for a rifled tube smoothbore to attain somewhat comparable accuracy to fully rifled barrel.