It will be close to 28 BNH quenched. The antimony is what gives the alloy the ability to be quench hardened. Your antimony will be about 2.5%, and those alloys quench to about 28 BNH, when quenched from a 450 F starting temp. Pretty darn hard for most purposes. The sprue may indeed be hard to remove with a vibratory cleaner. I vibrate loaded ammo with BNH 9 bullets and walnut hull media, and the noses do not deform, they just get a little bit of burnishing and a few minor scratches on the nose!The alloy calculator said 13% tin with a Brinnell of 14.6 aircooled.
What do you suppose it is water dropped?
Vibratory cleaners are intended to clean rather than impact the product. Any buckshot made harder than about 7BNH may be difficult to "vibrate off" the sprue.
good luck, garrisonjoe