Slugs and choles
Having grown up in this area, then left to see the world, I returned regularly over the years to hunt deer here on the family farm....I grew up shooting slugs in shotguns and since that was the only legal deer long gun back in those days and still is unless you go with a BP rifle or a pistol caliber carbine or rifle....I heard all of the wives tales too over the years...bottom line is: that foster slug is soft and will conform to whatever opening it gets shoved out of.Steel is harder than lead, so it is possible that the slug could be deformed and thus not as accurate. In theory, the cylinder bore or improved cylinder should work best for the foster type slug because it will not deform the soft slug....having said that....I have a 16 ga. bolt action JC Higgins that has a modified choke....and I harvested a bunch of big bodied Indiana soy bean eating white tails with that old gun......An uncle had a JC Higgins 16 ga single shot / full choke - and he killed deer with that gun every year.....In my experience, if you are setting up a slug gun, then you need to shoot different slugs thru it and see which loads that particular gun likes the most.....nowadays, I deer hunt with a 40 year Mossberg 550 with a 24" slug barrel ( smooth bore / cylinder bore).....or a Savage 220 bolt gun that has a fully rifled bore that shoots sabot loads moa ( or less) out to 150 yds......if we are talking tactical shotguns....a cylinder bore or improved cylinder....with slugs or buck...as far as bird hunters carrying slugs.....I never have....hunted birds a lot out in Arizona, and still do around here..I might carry a couple loads of buck - #4 or 00...just in case..... but if I need to deal with "varmints", I have always have a sidearm....
As far as buckshot.....I haven't done any pattern board testing with buck shot thru tighter chokes, but I would think that yes, it would influence the pattern.....but....back boring and lengthening the forcing cone will produce amazing results in regards to shrinking pattern sizes with buck shot.....I have a Remington 870 worked over by Vang comp, and this shotgun with a 18" bl will group #4 buck and 00 buck into amazingly tight patterns.....to the point where I can take almost surgical head shots at 25 yds....15 yds...rat hole.(Vang has a web site- - check out the DEA pattern testing photo's of 00 buck and you will get an idea of what I am talking about)...typically with a out of the box police 870 I always saw "an inch per yard"....7 yards - 7 inch pattern....15 yds - 15 inch pattern - 25 yds - 25 inch or larger pattern......taught police shotgun for thirty years and did this demo annually every year to impress upon the officers of their liabilities with our duty loads as the range increased.
Hope this was helpful Kaaskop....be safe brother.