Cast bullets labeled .358" 180gr. TC, but weigh 170?

Uncovered a stash of ABC cast bullets from years ago. They are labeled .358" 180gr. TC but weigh 170. gr.  Would a high alloy mix account for 10 gr. of weight? Do I load for 180gr. or 170gr.? Planning on using 13 gr. of 2400. Sound right?
Thanks in advance, dgang

Yes, the composition of the alloy can make a noticeable difference in the weight of a bullet. For example, I have an RCBS mold for a 250 gr. .375 caliber bullet. RCBS bases their bullet weight on IIRC, linotype metal. My alloy which has a higher lead content than linotype cast out at 270 gr. which is fine by me. Looks to me like your bullets have a fairly high tin content which would explain the lighter weight.
Paul B.

Load for the known 170 gr., 13.0 gr 2400 is a good starting place. Alliant shows 14.5 grs with 170 gr GDSP.
gramps

The alloy does play a big part in the weight. Paul gave good info!

Hi,

That weight difference works out to be about 5%, which is probably within the range of alloy differences (in this case, as already noted, yours would be higher in antimony and/or tin than what the mould maker used when developing the mould.)

For recipes, if you have some for 170s, you should be ok. It's generally accepted that in cases like this with small variations in bullet weight you can load a lighter bullet with the next heavier bullet recipe, so your 180 data should work, too. In any case, don't go the other way: for example, DO NOT try to extrapolate 158 gr recipes. All this assumes, of course, reputable published data. Remember, start low, work up with ANY change in the recipe.

Rick C

Yep alloy does make a big difference in finished bullet weight. Basically the more lead in the alloy, the heavier the bullets will be. A bullet of 97% lead 2% tin and 1% other stuff (antimony, arsenic, etc.) will be much heavier than a bullet of linotype alloy, 84% lead, 12% antimony and 4% tin...

What everybody said .

Making me feel old know , but back in the medium day , I purchased .358 nominal 180gr T/C bullets from Nat'l Bullet Co .

A whole lot of Commercial Casting companies use the equipment mfg by Magma Engineering . Other brands of molds can often be adapted , but a whole buncha Bullet mfg use the molds made by Magma Engineering , and many of their designs can be easily recognized.

And the seeming digression leads us into the fact the Magma 180 T/C was apparently spec'ed with somthing like 1:20 Lead/ Tin , because with modern " hard cast" alloy, they all weigh 10-12gr less .

Not even a slam on the bullet design, or Nat'l Bullet . The design performed as expected for cast T/C , with good accuracy. They just weighed 169gr instead of 180 .

Shooting lead I worry more over dia. of the boolit than the weight .

Cast size/lube then load accordingly.

You`re safe using loads for 180 lead boolit loads NOT jacketed bullets !!

Lead seals the chamber & barrel better & using starting jacketed loads will put you at or the least close to max loads using lead.

By the weight/358" I`m assuming 357 magnum ???


GP100man

Could be your scale is not properly calibrated too. Or is "off" somewhat at that particular weight. Something rare but possible especially if you are using an electronic scale.