Rcbs .38-150swc mould and 20/1 alloy.

[JNG3]

I know that moulds are designed to drop bullets of a given diameter and weight with a particular lead alloy and that will change when the alloy differs. What I don't know is what alloy Rcbs designed their .38-150-swc429 mould around. I want to use 20/1 (95% lead 5% tin) alloy and NEED a diameter of at least .358". Before I spend $100 on a mould plus handles I thought I'd tap into the expertise here. I'm pretty set on the lead/tin alloy and needing at least .358" before sizing. Before someone suggests using another alloy, well today I scored a large quantity of this alloy and it should work well for my 38spl based on the pressure of the load and bhn of the alloy.

[dubber123]

While I do not have that particular mold, all of my half dozen RCBS .35 molds cast at least .358" Good luck with your new mold.

[Tatume]

While I do not have that particular mold, all of my half dozen RCBS .35 molds cast at least .358" Good luck with your new mold.

Agreed. You won't go wrong with RCBS.

[lightman]

Not much help here, but I have that exact mold. It cast good bullets from the beginning without requiring any tuning or tweaking. All of my RCBS molds have been that way. I have not used the alloy that you have so I can't help with that.

[runfiverun]

IME ww alloy casts pretty close to 20-1 alloy.
you might be able to detect a difference if you strictly control the mold temp within 5* at one end of the heat scale for one alloy and at the other end for the other alloy.
but I don't get all that anal about it.

[Scharfschuetze]

I also have that mould and I've never had an issue with under size boolits. In fact, it still casts probably the best 38/357 calibre projectiles for me.

When I used it the most in the 70s and 80s, I used straight COWW alloy through it. I believe WWs had a bit more tin in the alloy back then compared to now; but antimony has generally been high in them both then and now. How that relates to your 95% Pb and 5% Sn alloy (20 to 1) I don't know, but it always seemed to me that alloys with high antimony content shrunk the least.