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Old July 28, 2021, 07:17 PM   #17
Join Date: March 4, 2005
Location: Ohio
Posts: 19,301
All the mold designs and materials have different advantages. For aluminum, thermal conductivity is a plus. It is about two-and-a-half times greater than brass and five times greater than iron. This makes it easiest to heat quickly and uniformly. Brass is the easiest to machine with high precision. Iron is toughest. Aluminum will also cool the fastest, so it demands the most consistent production rate to keep the temperature even. It also has the highest temperature coefficient of expansion, meaning it will change size most with temperature, where iron changes the least of these three materials; less than half as much as aluminum. The practical consequence is an alloy that casts a hundred degrees hotter will make an aluminium 35 caliber cavity about half a thousandth bigger, while iron will be only about a fifth of a thousandth bigger. This is before considering how much the casting alloy itself shrinks as it cools.

Personally, I like the tumble-lube designs. I got one of the Lee six-cavity molds for 148-grain wadcutters for my K38. In alloy about like Lyman number 2, they cast 0.3585", and compared to any of several brands of commercial match 148-grain wadcutter ammunition I've tried, the groups I get from them are half the size. I shoot them as-cast. Letting the gun do the sizing is a successful time saving step, and seems to produce less leading than sized bullets do. I haven't powder-coated them as they shoot so well that I don't want to change the diameter or start sizing them to accommodate the coating thickness. Plus that adds two steps. The penalty is some periodic lead cleaning, but it's not bad, given the modest target ammo pressures involved. Shooting them without lube seems to work just fine, as long as the bore is smooth.
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