A round ball certainly does swell to seal the bore lol it's just a hunk of lead... when it's hit almost instantaneously with 20,000 PSI+, it's going to deform (considering you can deform them by stomping on them on concrete lol), and the inertia of the ball resists forward movement, so it goes sideways. Muzzle loading guns aren't magic... they function like every other gun in existence... if they don't, how do I get crisp rifling marks and great accuracy from a paper-patched ball in my shallow groove flint .50 (.004" groove)? Ammo loads very easy for 15 shots, and can load 20 before I start risking pushing the ball through the paper. It has a .502 bore - .490 ball + 1.25x wraps of 20# paper (.004" average) (dipped in 3paraffin, 1crisco, and .15diesel anti-gel), choked & tied with thread at the base. Paper and Ball together come out to .500". With a 100gr charge of 3F, and the god-awful sights Traditions put on it, 5 shot groups averaged 1.75" at 100yards (1.67MOA) (obviously from a rest).
It's currently being fitted with a new, usable laminate stock (traditions stock had a 14.5" LOP and almost no drop at the comb lol), losing some weight in the barrel (hand ground a 1" oct to a tapered oct-round 1" at the breech-0.72" at the muzzle, already proofed), and is getting better sights (was thinking a more primitive no4mk1 sight, but since it's a baller, and won't be taken out past 300yd, I'm now thinking I might just make a set of folding leaves for it).
Pounding the hell out of any projectile deforms it inconsistently, and therefore, it's trajectory will be inconsistent. Just push it down so it just touches the powder, and be done with it. Don't tap it once, twice... throw the rammer down five times or more... just push it down until it stops, and the rod looks like it's about right.
I've said it before: "to the hunting and shooting public, we here practice the "dark arts", and this is even more apparent to the general population, no need adding mysticism to BP and ML shooting in order to make it appear even more like arcane magic "
When you have to pound a patched ball all the way down to the breech YOU have screwed up big time IMHO all your own fault.
I have shot at minimum 50,000 .500 balls with .018 match patching. One whack into the false muzzle using a dead blow hammer) then push it down with one hand on the loading road a 3/8" SS full length rod with a brass muzzle protector.
I have many medals and awards for light bench/buffalo matches NMLRA and local
If you have never shot bench or serious buff matches just offhand shooting you truly have no idea, woods walks and such are very very crude shooting compared.
Either wipe the bore after the shot or use a SMALLER ball and patch and some decent patch lube.