I did an earlier post on cast bullet weights that were heavier than I expected. (thanks for the response on that question)upon further investigating, I was looking at the mould, a single cavity 429421, noticed that it had the large mould handles on them. Checking the fit of the moulds on the bottom I could see a tiny opening along one corner. Held the mould up to the light and could see a little light coming through that portion. Changed to small mould handles and the fit was perfect, melted down the bullets that I had cast earlier, (which were running around 265gr.) and recast 50 more using this setup. Out of the 50 cast, 47 were between 254 and 255 gr. The other three were slightly over or under. Lyman book says this mould should cast a bullet from wheelweights at 252gr. Would not have guessed that the mould handles would have made that much difference. Maybe this info may help someone else. Think I'll go cast some more.

If you weren't getting finning at the parting line, look somewhere else for an explanation. If the blocks were not mating by enough to result in that great a weight change you would have at least severe finning, or outright leakage.

If you really had a set of small blocks, from when they used the samll block for single cavity moulds, then it is likely that the gap would have been on the order of 1/16" if you put them in large handles, I've done it before.

I agree with the above poster. I just got two new RCBS molds, a 180 FN gc in 30 caliber and a 158SWC gc in 38 caliber. I had to file down some Lee six cav. handles to work on them and noticed that they weren't meeting perfect and had to make sure they were closed good or I got finning. Anyway, the first casting I did with them I was using 20-22bhn melt and the 180's were 190+grns and the 158 were a tad over 158grns without the lube or gas check.

I can see light through my molds closed too, a tiny bit. They've got air vents to set up good. But if they're not closed good, you'll get the finning around the mold seams.

Bill

I did get a little finning on one side with the large handles, but not on all the bullets. I've since cast 50 more using the right handles and all but six weighed between 254 and 256, no finning. I have no other explanation for the 10gr. difference in weight from the first ones I cast other than the mould handles. Done nothing different. Thanks for your input though.