How much does alloy change diameter of bullets in .408 caliber? For example, if 1:30 casts .408, what size will it come out if I use 1:20 or 1:15 tin to lead mix?

The diameter of the mold determines the diameter of the bullet. It will effect weight ie might go from 265 grn to 245 as different composition will effect weight. It may effect the hardness of the bullet as well.

How much does alloy change diameter of bullets in .408 caliber? For example, if 1:30 casts .408, what size will it come out if I use 1:20 or 1:15 tin to lead mix?

Guess you'll have to cast to be sure..some of it depends on what alloy the mold was cut for.

This if for a mold cut for 1:10 alloy (pretty hard in comparison to your 1:40 or 1:30) and a .430" bullet:
1:10 Nominal (.430")
WheelWeights: very close to nominal values
pure lead: -.0016"
linotype: +.0009"
{Borrowed from the RCBS Cast Bulklet Manual...but Lyman's figures ae close.}

Basically, the herder the lead, the LESS the shrinkage and the larger the diameter of the cast bullet...and the less the weight.

IF a mold is cut to thow a 200gr. bullet using 1:10 alloy:
1:10 = 200gr.
WheelWeights = +1.3gr.
pure lead = +8.9gr.
linotype = -8.2gr.

Thanks for the reference.

I've tried to figure this out for years. Probably cast two dozen different bullets using several alloys, wheelweights, pure lead, 30 to 1 and linotype. What I finally decided was that dimensional variations between alloys was so small that the differences were right at the minimimum accuracy my digital caliber could measure. Said another way, the error in my measurements was large compared to the difference.

I don't doubt that changing alloys changes bullet dimensions slightly but the differences are so small as to have no practical applications.

Will toos anothe rlog on the fire..found the page in Lyman Cast Bullet Handbook 3rd. edition (p.58) that charts their predicted diameters. Sitll no 40cal, but will list the .458" data.
Lead = .4575
Wheelweights = .4583
#2 alloy = .459
Linotype = .4595

Now interestingly, they also list the predicted diameter after sizing...the harder alloys will exit a sizing die a bit larger tahn the softeraalloys, even though they've both been run through teh same sizing die. For the .458" bullet, that ranges from .4566" to .4574" (using a .457" die).

I honestly dont think it would make that much difference to accuracy so long as you keep the bullet alloy consistant...mixing bullets fro didffernt alloy mixtures wouldn't be a good idea.