I think I saw a YouTube video where a guy was doing that by necking down the casings.
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I was recently given about 5,000 old Slobovian manufacture 80 mm lead hollow-point slugs
Oh, those cases full of "spam cans". I'll bet he got those from J & G Sales.
Sometimes you get duds with that old East Bloc ammo, but the biggest problem is that it's so dirty. Just be sure and clean the mortar muzzle with Windex out in the field when you're done for the day and you should be fine.
Thanks.
;-)