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yes the trick is to find CORRECT sized ball....

dont worry about it at all...you CAN fire slugs through full choke...NOT RECOMENDED but you can do it...if you couldnt,you wouldnt be able to buy them or someone who did would sue...

have you pushed one through bore ,with ramrod,inside a wad yet???

trailboss loads in 45/70 generate MORE pressure than lighter loads with other powders....by quite a large amount....
plenty of load data around for slug loads...so its not really untested.
taking...

logic would suggest it will be similar to what the shot was going to be doing...1100fps or thereabouts....it wasn't until steel hunting loads got tweaked that shotgun birdshot loads got above...

doesnt work well...have tried and gave up....the old stories of doing so to increase range of birdshot have grain of truth as it was in days BEFORE plastic wad cups....
as for cut shells....yeah...

are you using rifled slugs in a rifled barrel????? because you shouldnt or will get the leading issues you talking about.... the rifled barrels are for sabotted rounds or indeed inside wadded...

they were for sale on trademe pretty much anytime Ive looked for last few years....you CAN get round ball sinker of correct diameter and drop that inside you wad too...not hard to work out correct...