littlejack:
I'm not expecting rifle like accuracy at 100 yards from a smoothbore shotgun with slugs or RB's. In fact I am pretty sure that decent hunting size groups past about 70 yards is a no go. The smoothbore musket shooters can compete with rifled guns all the way to 100 yards so I have read and been told. I suspect that is a good marksman with a carefully worked out load and patch/ball fit shooting against somewhat more average shooters with their rifles but the fact seems to be that a smoothbore muzzleloader can deliver decent hunting accuracy pretty much out to 100 yards.
You are certainly right that when rifled guns came into existence they were far more accurate than smoothbores but bear in mind that the custom in shooting smoothbore muskets was using a loose ball for easy loading. The military muskets used quite grossly undersize balls so they were fast and easy to load. They did not expect to be able to hit a man with an aimed shot past 50 yards. Massed volley fire was the military method... get the range and lob lots of lead!
The cloth patched round ball rifle was significantly more accurate than that but "smooth" rifles ~ that is long arms that looked like a rifle with rifle sights ~ were also common and could be quite accurate with tightly patched ball to common hunting ranges (to qualify, I have read and been told this but not done it or personally seen it).
Now having said that we are using new fangled modern cartridge loading firearms but we have to deal with oversize chambers, inaccurate (in gun tolerances) cartridge to chamber fits (read sloppy), opening a crimp and the RB jumping through the forcing cone to bore which the smoothbore musket shooter does not. He has a tightly patched ball guided all the way to the muzzle.
So, I don't expect round balls from smoothbore to be terribly accurate at 100 yards. However, since I can generally count on 3" to 4" groups at 50 yards with good round ball loads and some slug loads, if the slug is well stabilized and that group size doubled or double and a bit by 100 yards it still wouldn't be that large ~ 6" to maybe 10". I'd take that! Especially with a decent slug (Brenneke, Gualandi, Vitt-Boos, etc.).
I have shot several sub 6" groups with factory loaded Foster slugs which I find frustrating! The factories can seem to produce such consistency in their loads that they can achieve a decent level of accuracy to 100 yards but I have as yet to do that consistently with home cast and loaded slugs. Makes me feel like a failure!
In rifled gun, I have to think a round ball either groove diameter (or a hair larger) or a ball in proper fitting shotcup should provide darn good accuracy at 100 yards. The only issue I see with modern rifled guns is that they have a fast (for round ball) rifling twist so over spin the ball. A 12 ga. round ball wants something like 1:110" twist not 1:36" twist. I can't say that spinning the ball 3 times as fast as required is detrimental but I can think that transferring that twist rate to a RB in a shotcup may be problematic due to small contact area and lack of friction. If the ball isn't spun to the same rate each time or if it rolls a bit as it is spun then accuracy will be less precise.
If I ever finish my slow twist rifled choke tube (restarted that project again) I will do my best to find out. My goal is consistent sub 6" groups at 100 yards. My rifled choke tube will be 1:72" twist which his still fast for a round ball but much more reasonable for accelerating the ball without skidding. I think anyway.
I just asked on another thread for some info on the Lyman 525 gr. slug accuracy in smoothbore and rifled guns. I do not have that mould (yet) but would like one... but I am not convinced it will get me anywhere my Lee slugs and slugs from my home made moulds won't get me. Your 5 shot 2 5/8" group at 50 yards is very good and if that held up to 100 yards you'd have groups between 5" and 6" which would make me happy! I'd think those could be tightened up some with a bit of work too so quite acceptable from a shotgun in my opinion.
I still have to think that a tight fitting round ball in shotcup should do as well. A full bore round ball should too and from limited experience shooting some 0.735" RB's from a borrowed rifled gun I got several 5 shot groups of similar size at 2" +/- a bit at 50 yards. I didn't get a chance to shoot to 100 yards to see if accuracy held up but recovered balls showed nice clean engraving with no sign of skidding.
Anyway, I'm rambling on again!
Back to topic... If we can provide a solid, non tipping platform that will keep that ball centered and well guided into the forcing cone then we should be able to compete with patched ball from smoothbore musket and produce hunting level groups (say 6" to 8" groups on demand) to at least 70 yards or so. Not sure many (if any) here can do that.
I am pretty sure I will have to break down and buy a rifled barrel though.
Longbow