Well if it ain't the brass, it could very well be the rifle. You said the bore is "not in good condition".
That could be the cause of the problems w/ lead bullets. Less than perfect bores often times have difficulties w/ rough, pitted, etc., bores.
Some Shooters claim that after lapping a rough bore, it shot better & cleaned easier.
What is the condition of the crown? Often times the crowns on lever guns are worn from mismanagement of the cleaning rod. If the crown is damaged, a good gunsmith can re-cut the crown.
You could try seating the bullet out to engage the rifling, but too long an overall length will not cycle through the action & you'll wind up w/ a single loader.
FFFg might work, but too much velocity can cause leading --trail & error to find what works.
Myself, I would spend some time scrupulously cleaning the bore. Maybe even use some JB Bore Paste to help smooth out any rough the spots??
If any case, it is advisable to throughly clean the bore when switching between cast & jacketed bullets.
I would also stay away from the shorter brass. Short brass causes the bullet to upset to fit the chamber, then it is swaged down as it enters the lead/bore & all of the lead has to go some place. Beside a pitted, rough bore, it also could be leaded up.
The most important aspect of this signature line is that you don't realize it doesn't say anything significant until you are just about done reading it & then it is too late to stop reading it....
Grand Poo Bah
WA St F.E.S.