Some of this you may already know. What is the bore dia. vis-à-vis the bullet dia.? Many of the trapdoors can be found w/ what today would be classified as oversized bore. A quick & dirty way to check is to place a bullet into the muzzle. There should be some resistance @ the shoulder.
Since youre utilizing store-bought bullets, do you have any idea of the alloy? Youll need a soft bullet so it would upset upon firing & thoroughly seal the bullet form gas blow by, which will cause leading.
Check on the condition of the crown. Many crowns are usually not the best & since you have a cut down carbine, there still may be some burrs in the lands & grooves. Observe the condition of the rifling @ the crown.
I would thoroughly clean & scrub out the bore. You may have some leading by now.
The carbine load was a 405-grn bullet w/ a 55 grn powder charge. Stacked card wads took up the resulting space.
After shooting a few shots, look @ the end of the muzzle, if your lube is working properly, there should be a wet looking & soft ring of lube around the crown.
If you use a blow tube, make sure that it properly fits, you dont want any moisture in the chamber.