45 Colt lead bullet diameter

The "drop weight" of a bullet from a mold will vary due to different things. Tolerance of the individual mold is one, casting technique can be another, but the biggest reason is normally the alloy being cast.

I don't know about other makers, but Lyman tells you in their manual that the nominal bullet weight their molds produce is based on using their #2 Alloy.

Its just common sense that casting a different alloy, with different percentages of lead, tin, and antimony will result in a different bullet weight coming out of the mold.
 
"I have also used Lyman 452454 255 grain from Montana. They also weigh heavier at 267 grains cast .454...why this is so as the 270g SAA 270s weigh almost 290 grains is a mystery."

Part of the problem stems from the fact that the weight is based on a specific alloy. I believe RCBS uses linotype and Lyman probably still uses their #2 formula. It's my guess that most other mold maker probably use linotype.
I have two RCBS molds that state bullets should weigh 250 gr. They do come close if I use linotype but in my alloy bullets weigh out at 270 gr. For the record, both molds are #37-250-FN for the .375 H&H. The 270 gr. RCBS mold for the Colt .45 wold probably run closer to 290-300 gr. using my alloy which cast a BHN level of 11 but can be water dropped to 19 BHN and to 30 BHN using the long heat soak method with a toaster over.
Basically, the reason for the varied weights in cast bullets is based on the composition of the alloy. ;)
Paul B.
 
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