I have used Hornady buck in 0,00,000,and 0000 when they still made it but have swithced to another supplier foro 4 aught now. I have used it in muzzle loaders and for making small game loads in CF. Hornady's buckshot does have antimony added but it is only a VERY LITTLE bit harder than pure lead. Far softer than most wheel weights.
I have never had a problem with any of the many pounds of it I have used and it is a lot cheaper than buying round ball. I would not be the least bit afraid to use it in your ML but Mic it before you buy. Most of the 4 aught I have bought is a lot closer to .375 than .35 so it could be pretty tight. 3 aught is usually .350 and works good in a