Mountain Molds
WW + tin

Personally, I prefer to use straight WW   for big bore bullets because it works and because it is economical.   But a lot of people like to add tin, so I decided to see what all the fuss was about.   Here's a comparison using a 2-cavity XL aluminum block with a 50 caliber, 650 grain GC bullet.

alloy weight bottom band
diameter
WW 638.1 0.5119"
WW + 1% tin 637.9 0.5111"
WW + 1.5% tin 637.6 0.5110"
WW + 2% tin 636.5 0.5105"
50% lino / 50% WW 625.5 0.5116"

Equipment and Procedure   The pot temperature was 600 - 650, give or take.   An RCBS ladle with the orifice drilled out to 5/32" did the honors.   I used the "overpour" technique, dumping the entire ladle into a cavity and letting the excess fall back into the pot.  The ladle was held close to but did not actually touch the sprue plate.   I added the tin in 1/2% increments (and assumed that the straight WW already had 1/2% tin).

Straight WW   cast pretty well.   After preheating the mold by dipping it in the pot for about 30 seconds, it only took a half dozen pours to start making good bullets.   There was noticeable bullet-to-bullet diameter variation on the bottom band and on the check shank, which is pretty common on heavy bullets with any alloy.   The sprues hardened quickly and there was no significant lead smearing when the sprue was cut.

WW + tin   The strange thing was that as I added more tin, more pours were required to get the mold to start making good bullets.  It seemed to take longer for the sprue to harden, and some lead smears stuck to the bottom of the plate and the top of the blocks.   The bottom band diameter shrunk as more tin was added.   WW + 2% tin failed to meet the 0.511" spec for this mold.

50% lino / 50% WW  was included in the test just for comparision.   50/50 is the worst possible alloy for big bore bullets because it is highly susceptible to shrunken bullet syndrome (SBS).   I was expecting severely undersize bullets, but the bottom band diameter was still within the customer's spec, even though the bullets were 100% frosted.

Conclusions  Don't draw any firm conclusions from this test, because it was only one mold and the samples were too small to be statistically significant.   Basically, all the alloys cast better than expected.   Only the WW + 2% tin missed the spec, and only by 0.0005".   There's a lot to be said for straight WW because it shrank less on the bottom band, the sprue hardened faster, there was no significant lead buildup on the sprue plate, it seemed to come up to speed faster, and it's the cheapest alloy.   Save the tin for small calibers.