Most modern slugs, Fosters and others are designed to pass through a full choke without damage to the gun - that is why some folks and indeed, the slug manufacturers themselves call rifled slugs. They have collapsing vanes impressed into them. Brenneke's are the same - the vanes are designed to collapse to prevent damage tothe tubes and chokes - they do not spin the slug for accuracy- that's an old man's tale based on speculation based wholy on the slug's unfired appearance. After travelling through a choke or even cylinder bore, the vanes are pretty much flattened, smashed flat and not evenly, either, however they do shoot well in some guns. They also lead the bore, in some guns - especially older ones with some pitting.
: One cannot pick a slug and demand it shoot well in his gun - it may, or it may not - that is why you should buy a variety of different makes and try them to find the one that shoots best. Federal makes or at least used to make a 1 1/4 ounce slug(12 bore). This one, at 547gr. is better than the 1 oz. or 7/8oz for better penetration, but as always, one must shoot it to see if his shotgun likes it. My double didn't, but would put 2 lefts and 2 rights into 8" at 100 yards with round balls at 1,550fps MV.
; Cylinder or improved cylinder chokes are usually the most accurate - but not always.
; I would find some plastic shot wads with a cupped(rounded) gas-sheck base and I'd load round balls in that gun. Measure the choke at the muzzle and pick a round ball that size or sligtly smaller. The gas check is cut from the base and loaded cup-up, underneath the ball. The cup shape holds the ball in the middle of the bore for good accuracy - such were my 12 bore loads - 8" at 100 yards - lefts and rights. Since you are using 2 1/2" cases anyway, you probably have to re-load - might as well use the best slug there is, a solid WW alloy round ball. It will outpenetrate any hollow-based slug. Sorry about the above noted heavy slugs - I'd misread your post and thought you were talking 12 bore. In a 16 bore, your ball weight should be over 400gr. for any choke more open than full. A full choke sized ball might be only 320gr. to 380gr. they'll still go lengthwise through a deer.