[Anthony]

Well, given that a hand grenade has no propulsion system, it seems like a reasonable model for the capacity of a warhead. Extrapolating from the sizes there, the total weight is (size/64mm)^3 lb, or 0.013 lb for a 15mm. Add a case weight and figure you can put any reasonably durable 0.01 lb object into a 15mm projectile. Halve that weight if you want an armor piercing projectile (i.e. APEX).