Can a semi-wadcutter (no grease) be used to slug a barrel on a 357 Marlin rifle? Can a lapping bullet (no lapping compound on the bullet) be used to slug a barrel on a 357 Marlin rifle?

Both are OK if they're big enough. You'll know after pushing one through as the bullet should be polished from contact with the bottom of the grooves all the way around. Use the softer bullet of the two.

If they aren't big enough, squeeze the bullet length-wise in a vice or press to expand it a couple of thousandths. This isn't a job for a bigger hammer :) Go slow.

Bye
Jack

Be sure to apply some grease to the barrel bore prior to tapping the slug through!

The proper sized slug should leave a small ring of excess lead at the muzzle as the slug is tapped down bore.

Can a semi-wadcutter (no grease) be used to slug a barrel on a 357 Marlin rifle? Can a lapping bullet (no lapping compound on the bullet) be used to slug a barrel on a 357 Marlin rifle?

The prior answers are excellent. The only thing I can add is that I like to put a wood rod in the bore (when you have access to the breech) and another in the front and tap it once or twice to upset the soft slug in the bore. I usually add .0005 to the answer. Hope that helps....