New mold, 45 cal RCBS 300 FNGC for 45/70. 50/50 WW/linotype. Getting bad vent lines and most are casting .457. Thinking that maybe I should go to using less linotype. Maybe WW + a little tin. I use the former alloy in my 405 FNGC and get full .460+ fillout, not much on the vent lines. Also wondering if it maybe that I am running too hot and this may cause some problems, as I am getting frosted bullets and they are showing all minor machining marks.

Thanks in advance and God Bless, Ryan

hat's probably as big as that mold will cast, a 50/50 mix of WW/Linotype is not going to shrink much, so that's probably as close to max. size as that mold is going to drop.


If you're finding whiskers in the vent lines and a heavy frosting, then you are running a bit hot...either cast slower or turn the temp. down on the pot (or both). A little frosting isn't a bad thing but I do try to avoid heavy (pitted looking) frosting.

I might have to send this mold back to Midway or RCBS. I will try 1 more time with it, but have never had this problem before. The bullets' shanks/bases will not hold a gas check, so they are useless (this is only about half of them, so I assume it is only 1 cavity).

Does sound like it was cut with a worn cherry. If you see vent lines, it's not for lack of filling (what linotype is made to do). Other than direct measurements with the right toolmaker's gauges, I think you have what you have. RCBS used to recommend a 10% tin, no antimony alloy for their molds. You could try that just to be sure, but as Ribbonstone said, you're probably doing pretty well on fill.

This is an excellent general background article (http://www.lasc.us/CastBulletAlloy.htm) on casting alloys and considerations in choosing them.

I have sent RCBS an e-mail and am awaiting their response. Hopefully we can get this resolved soon.

EastTNHunter,

I have the same mold and I will say that g.c.'s were a loose fit on mine as well and thus did fit as original gas checked bullets were designed back in 1906.....so that the g.c. fell from the bullet at muzzle exit.

I did put several coatings of NEI Mold Prep across the vent lines on the RCBS molds that I use with a bottom pour pot. That reduced the vent line enough to eliminate the whiskers.

I eventually opened up the g.c. shank for a tight fit but.....they didn't shoot any better. I will say that there is much less of a chance of a loose g.c. hitting the chronograph.:)

Fortunately, bullets in w.w.+2% alloy drop @ .459" from my mold.

Hope everything works out ok with RCBS.

John