A:
Lightfield would suggest that you only use 2 3/4 in slugs in guns chambered in 2 3/4 or 3 inches. The reasoning is that there is enough gap between the end of the shell and where the chamber necks down to the barrel on a 3 1/2 chamber using a 2 3/4 in sabot slug that the sabot could be push out and separate from the slug, possibly causing pressure issues. From My understanding lightfield is the only manufacturer warning of this possibility. They are also one of the few manufacturers making sabots in 3 - 3 1/2 inches. I think they recommend using the 3 1/2 in slug (or at least the 3 in) in 3 1/2 in chambers. I have never heard of anyone having any problems with this issue. I talked to someone associated with lightfield and they said that they had seen some guns that have erratic patterns with undersized slugs.
I have personally used 2 3/4 in lighfield slugs in my 3 1/2 in chambered Mossberg 835 Ultimag 12 gauge with rifled barrels for years and not seen any issues myself. Accuracy is great. I wouldn't use it with a smooth bore though as sabot slugs are made for rifled barrels. And if Lightfields warnings worry you then just play it safe and use a 3 1/2 in slug with a 3 1/2 in barrel.