I sure can,t begin to dispute the excellent results your getting .. CONGRATS
nor the fact your correct that the bullet diam expands into the rifling if the leads soft and powder charge,
is sufficiently high enough,
Ive just pointed out accuracy can also be achieved,
with a full diam projectile if proper tools and procedures are used.
this false muzzle route is not commonly used anymore , but a few of use do still find it works
btw
If you build your own target muzzle loaders ,you generally drill the 3-4 outer index pin holes, so the pins only line up if the false muzzles properly indexed
and then cut about a 2" section of the original barrel,off
too fabricate the false muzzle , insuring proper bore & rifling alignment
other gunsmiths would get similar results,
The barrel was bored then a section was cut off and faced off to which pins were fitted to rejoin it to the barrel.
The reaming and rifling could then be done so they are a continuous smooth transition from false muzzle to bore.
The false muzzle is opened up at the front to allow the bullet to be inserted with out damage.