Yup, STP's messy. GONRA suggests trying TriFlow Synthetic Grease (bike shops). 3 oz tube does 1200 .50 BMG scrap dealer mangy brass.

While we're at it, you GOTTA verk up a way to check for dreaded Case Stretch with scrap dealer brass. Get a "Granite Chek" with dial indicator from
yer tool supplier. Womp up few stray parts in yer shop.
Use a "Ruby & Roller" for First Class Setup.....
More or less copy smaller commercially available units.

With low class brass I had access to, expect >30% losses...