I'm working with my new(sort of) .458 win mag.

Currently my groups are 1.2 inches at 100 yds with a pretty consistent 2 and 1 cluster. 2 on top of each other, and 1 off on it's own. So I am thinking there should be some way to dial that last shot into the group. I could be asking too much.
I am shooting a a hornady 350 gr at about 2160fps with 64.3 gr of 4895.
64 gr is terrible, 64.3 works well, 64.6 works well,64.9 just starts to open up a little bit.
So I have a bullet I like at a speed I like in an acceptable velocity window.

The bullet is probably making a bit of a jump to the lands, the pressure is not as high as it could be. Under these conditions, is it probably time to call it good and just load?
What is good enough for me? No such thing really.Maybe minute of groundhog. I like my 300 win mag which shoots sub 1 inch at 350 yds. So the bar is pretty high.

I guess I am also looking for advise on the process.
My experience has been with bench guns where I smoke the bullet and square it against the lands, then start the load dev and chase the groups down to the smallest.This us ususally just under max pressure. So I am a little out of my element.
Sorry for the newb questions, thanks for the help.

Do you get the first 2 shots together then the flyer or 1 shot then the other two or is it random? Have you tried other powders/components? Have you eliminated the possibility of a bedding/rifle issue?
All that being said 1.2 inches is not bad for a big bore but if you want to pursue this further, it would help to have a little more info.

I tried RE7 and that didn't work well at all.
And I think it tends to b the first 2 group together then the last is the flyer.
I don't know HOW to eliminate the bedding as an issue.
Thanks for the response.

I just talked to my gunsmith friend. He thinks there is a good chance this is all about heat.

If you're using IMR4895, maybe velocity change due to temperature is pushing the velocity past the "sweet spot". Sounds like your load is at or very near a "sweet spot" - which is common with leverguns because of all the dovetails, stamping, and bands added to the barrels. Maybe you've got pressure point or two - just a thought or two.

Ok, I'm good.
It is about the heat.

Here is what I did. I went to the range with a load that should perform well. And I waited a long time between shots. Like 10 minutes between. The first shot the rifle was still cold from the air conditioner in the truck. After that the barrel stayed hot from the sun, almost too hot to touch.

frank4570, just a thought, have you considered paper patching? It may not be your cup of tea but the 458 seems to do very well with paper patching.