You have several options the others have given
A ball puller. PITA
or co2 or even compressed air, just make sure it is pointing in a safe direction.
Depending on the nipple, drum / bolster setup.
I've had to do this a couple times with bad loads. In the field.
Carefully remove the vent cleanout screw. dribble a couple (2) grains of fffg or ffffg in the
hole. replace screw carefully (no sparks allowed) cap it aim in safe direction.
The cap will ignite the bp the bp will ignite the pellets.
This is provided the nipple isn't clogged.