Wm Cook
posted this
08 November 2025
Summary: Casting 20-1 went better than I thought. The roof didn’t cave in and I walked away from the bench with all my limbs and fingers. The work was done with RotoMetal 20-1 and ladle cast out of a PID controlled 10# Lyman pot. The mold was from Accurate. It’s a single cavity iron mold dropping a ~210 grain bore rider bullet.
I cast two small lots of ~50 each. The first was to get a feel for the cadence and to see how the mold and alloy behaved. The second lot was an attempt to see if I could cast to be what I consider match quality 20-1 bullets.
The first lot was cast with an average alloy temp of 752.6 (+/-14) and an averaged mold temp of 447.6 (+/- 18.5). I started with a high alloy temperature based on recommendations from Ric, Bud and others. Mold temperature was dependent upon the cadence. I used the counts of 5, 10, 15 between frost and sprue cut as the variable and just tracked mold temperatures based on that. The sprue cut felt a little squishy (compared to Linotype) with all three cadences but there was no tear, no smear. The weight distribution (ES) was greatest with the 15 count and the mold temperature was falling off as well. With the 15 count the appearance was textured such you could barely see the seam from the mold. It filled out fair but the base wasn't sharp. The 5 count lot had a lower weight distribution, a better appearance and a sharper base, so I used 750 degree alloy and a 5 count for the starting point on the second lot.
The second lot was about 50, 60 pieces. The mold temp was controlled with a 5 count before the sprue cut and the speed to ladle refill was quickened. I sped up the return to ladle intentionally in order to increase the mold temperature and improve appearance. The mold temp averaged 472.5 (+/- 7.5) for this group. A clear warning needs to be given about mold temperature; at this temperature a brass mold will warp. To repeat myself, I’m using an iron single cavity mold. Iron gains heat slower than brass and aluminum but retains heat the longest (over aluminum & brass). Brass mold can be damaged with this high of temperature. The rejection rate with this second lot was 11%. Rejections were based on weight and appearance. Overall appearance improved with the 470 degree mold although I could now see the mold face seam, but it’ll never be mistaken for a Linotype bullet. Bases came out flat and you could feel the gas check snap onto the base. So there was enough fill on the base to look and feel right. I considered them as being match grade.
Learnings:
- The 20-1 alloy cast a nose diameter .0009 smaller than the 20-1 alloy. I didn’t expect it to be this great. My expectation was that it would only show about .0003 shrinkage. The as cast weight difference showed Linotype at 198.62 grains & the 20-1 at 213.51. With a 7% weight gain my initial thoughts of running the same load I was using with Linotype have changed.
- To see how soft the bullet was, I press checked (Lyman 450 using their gas check seater) using nominal pressure on the ram to simply crimp the check on the bullet. After measuring 10 samples, before and after, I found that the Lyman had bumped the nose with the 20-1 alloy from an average .3018 to a 10 bullet average of .3079. A gain of .0061. To be clear this was same day casting and same day adding the check. In comparison to casting and sizing Linotype the same day, my last lot of Linotype had the nose size increase from .30208 to .30296 (10 bullet average), showing a gain of ~.0009. Interestingly enough, with the 20-1 I had expected the nose to crinkle but it surprised me how the base went big as well, going from .3108 to .3150. A gain of .003. But the bullet growth (nose and base) wasn’t a uniform bump. Measuring before and after adding the gas check was added, showed the nose of individual bullets (again within the 10 bullet sample group) varied from .3050 to .3087 with an ES of .0037. All measurements were averaged based upon measurements at 12:00, 3:00, 6:00 and 9:00.
As a general practice I normally cast, size & lube Linotype with this mold on the same day because I need the .001 bump. My experience is that if you let Linotype set a couple days there will be little dimensional change. For #2 I usually wait a week. I don’t know how long it’ll take 20-1 to set before it’ll be hard enough to size. I don’t even know if it's possible to but gas checks on 20-1 or 25-1 on alloy bullets? When the dust settles I’ll be putting the gas check on with the Lyman 450 gas check tool, applying lube in the same “as cast” sizing die and eventually push sizing it with the Lee die. The bullet drops at about .311 and I am sizing to .3098 so there isn't a lot of stress put on the bullet.
The next cast I’ll repeat the 750 alloy temperature, go to frost, take a 5 count and expect to keep the mold around 470. I believe that an alloy temp of 725 and a quick cadence could keep the mold temp up to make good bullets without the dross buildup and I'll play with that some.
I’m prejudiced in believing that, within normal operating temperatures, manipulating the mold temperature as a variable has twice as much affect on bullet quality as changing the alloy temperature. Within a reasonable operating temperature range, 20 degrees difference on the mold has the same effect on bullet quality as 40 to 60 degrees on the alloy. How fast you return the mold from sprue cut to the spout on a bottom pour or how much overflow you put onto a ladle cast can change temperatures quickly and significantly. As everyone already knows it's keeping the same cadence throughout the casting session so as to produce similar bullets.
Load development will be interesting. As I said, the initial thought of starting out at 1700fps might have to be set aside and I’ll have to start working from about 1400 fps and going up from there. I’ll honestly tell you that load development with 20-1 alloy is a black box to me. And maybe a bore rider is a poor choice to learn with.
All thoughts contrary about what I did or the assumptions I made are absolutely welcomed. I would appreciate it if some of the PB BR shooters would chip in. They’re running in the 1450 to 1600 fps range and shoot some tiny groups. Thanks for listening, Bill Cook.
. My standard temp setting is usually 725. 
