Ouch,
Confession time! Way back when cast bullets were a haphazard event that sometimes worked and often times didn't in my life, I began casting and shooting for the .458 Win Mag. I was on a really tight budget, didn't know beans about bullet mold selection, or what made a good bullet for that matter, and so went for the cheapest route possible to feed my new custom mammoth slayer.
The combination was a Lee .457"-510g RNFP in gas checked guise (single cavity), and Lee Liquid Alox for lube, along with a Lee Push Through type sizing die. (Before they had the dies that fit on your loading press.)
Well, results were miserable, and a kindly older gentleman at the rifle range where I was shooting explained that the key-holing that I was experiencing was due to undersized bullets. He showed me how to lap out the bullet mold cavity, and how to lap out the sizing die to get a larger diameter bullet for my gun.
I tried his fix, and ended up with bullets that dropped from the mold at .461" (I overdid the lapping of the mold), and the sizing die was lapped to size the bullet to .4595". I loaded those bullets in fresh .458 Win Mag cases with 73.0g H335 powder and Federal 215 primers. Once again (against the better judgement of the fellow giving me advice at the range) I used the Lee Liquid Alox for bullet lube. Those loads shot into under an inch and a half at 100 yards!
Nope, this wasn't a fluke. That gun and bullet/load combination would shoot just over an inch at 100 yards on demand. Chronographing some of those loads years later showed that they were pushing just over 2,000 ft/sec. I've still got a few of them sitting on my ammo shelf from a decade and a half ago or longer and yes, they still shoot the same... I fired some last summer just to free up some brass for use with other bullets.
No, I'm not advocating the Lee Liquid Alox, as it's a very sticky, messy proposition, but for this application it worked, as well as some very accurate .30-30 winchester loads that I made up during that same time period.
Perhaps you'll find this info helpful.
God Bless,
Marshall