Lead Hardness vs. Velocity?

[Domtomac]

I've been working trying to develop an accurate subsonic load for the 300 AAC Blackout using a cast boolit. I'm trying to match what I had developed for Sierra 220 gr. JRN ~.75" at 50 yrds. I've come close (~2.0" @ 50yds.) using an NOE 240 gr. mold using Rotormetals "Hardball" alloy. I was looking for better and because velocity is low (~1000 FPS) I tried a mix of 3% tin, 3% Atimony. This gave improvement, down to about 1.25 to 1.0" @ 50 yds.

Looking to improve, and again because velocity is low - closer to 38 Spl +P than a rifle load, I tried a 30:1 Lead: Tin alloy I've used for PB loads. Talk about BAD! :cry: The "group" was +7.0" @ 50 yards!!

All other variables were kept the same. Any ideas on the cause(s) of the poor results?

Thanks,

[paul h]

While conventional wisdom is that you don't need a terribly hard alloy for low velocity loads, the caveat is that is for typical barrel twists. The 300 bo uses a very fast barrel twist to stabalize the heavy for caliber bullets and hence that engraving force is going to require a harder bullet alloy than one would typically use. So my guess is your 30:1 bullets were being stripped by the rifling and hence cave terrible accuracy.

Sounds like your 3% tin and 3% antimony bullets were just about there. I'd fiddle with sizing and lubes to see if you can tighten up the groups.

[Houndog]

Try heat treating a few of your 3%/3% bullets and see what happens. Set them upright on a cookie sheet or some other flat bottomed pan (I use aluminum pie pans), bake them at 400-425 degrees for an hour and dump them in ice water. Wait about 3 days to let them season some before trying them.

[jhalcott]

Dropping them right from the mold into cold water might toughen them enough to see if oven treating is worth the trouble.

[popper]

I'd cut the hardball 50/50 pure, no need for the great amount of tin. Your probably running ~30k CUP, my supers run close to 45K, 4% Sb & 0.25% As to get HT to work. The 1:7 needs tougher alloy, mine did better sized 311 with isocore (1/3/96), vs 310. Several have given up on the BT Lee mould. I have no need for subs, 170 is the heaviest I use, close to 50K CUP.

[Domtomac]

Thanks for the replies. The firearm is a Rem. 700 w/ 7:1 twist. There were some lead shavings on the patches when I cleaned it. Makes sence. I'll stick w/the harder alloys & try some heat treating next!
Thanks again.