Here is the deal. I have a .416 rem mag. I would like to squeeze as much accuracy as possible without sending it to a smith for rechamber and all the stuff that would go with a rechamber. In my more precision minded guns I have a tight neck and I neck turn my cases to make them more centered, a common precision routine. I'm thinking I can get .458 win mag brass and neck it down to .416. This would leave the neck extra thick. Then neck size it just enough to fit a loaded round in the chamber.
I'm sure you all are aware there is a little more to it than just what I have mentioned, but that's the basic idea. The only problems I can see is that the stronger neck would increase neck tension, I don't know if that would be good or bad. And if the neck is too tough the bullet will probably crush the case as I try to press a bullet into it, more caution might take care of that, I don't know.
There may be things I haven't thought that make this a terrible idea.
What do you think?

Iirc the 416RM is a long case, the 458wm is a short case. You would need 375H&H length brass.

That said, my limited experience makes me think the 416RM is moa accurate in most rifles with factory ammo if you sort through the
the ammo. Beyond that, in most areas of Africa whete the 416 would be a good choice, range is limited by terrain or cover.

Fwiw, I have extensive Arfrica experience...

I don't currently plan to use it for africa, at least not at the moment. If I can use full power loads for coyote up to 200 or 250 yards, I guess I could live with that. Or it might just teach me to flinch. I have some machined bullets from Cutting Edge bullets which are light for caliber but still long. You may be familiar with them.  225gr brass, they should be quick.
I can work up to dangerous game loads in the future, right now I just want to be able to pick it up and point and shoot, basically.

I guess I would need to rethink my brass idea anyway. Maybe weatherby mag.

I too have extensive Africa hunting experience. with the Remington .416 in a Remington 700.  I shot my handloads using the excellent Barnes X bullets. The rifles provided sufficient accuracy for the ranges we hunted.

And we never did recover an X bullet, not event from Cape Buffalo

I purchased this stainless winchester with the idea of it being my kind of "go to" rifle. I got it from a guide in dangerous bear country. I'm supposed to take it to africa eventually. But for now it's my east coast gun with lighter loads. It's not a heavy gun. It's great in the dense brush and that's why I wanted it. But it doesn't soak up much recoil at all. I have not yet got around to installing the magnum recoil pad.

I recall doing load development It was pretty brutal.

I did load development work for a bud's 416R, which he killed an elephant and a cape buffalo with. It is a brisk recoiling round, but not abusive.

As a "go to eastern rifle" it is WAY more than required. A 243R or 257R is sufficient.

If you want to stick with it, I suggest down loading to a relatively soft bullet about 350grd at 1850 or so.

Btw, wrt recoil, you can get used to it. Build up recoil tolerance by shooting a few than quitting. Next time, a week later - no more, shoot a few more.... Repeat....

Op. Are you capable of out shooting it as it is ? I doubt it it. Chasing a fart in a wind storm.
Lock it down in a rest. If you can physically shoot as well as the rest then chase something if you feel compelled.
Triggers, bedding, free float, sights, fit and consistent loads will get you way more than screwing around with 1/10000 of an inch in the chamber.

I'd neck turn the cases, fire a round so I could measure the neck diameter, then order neck bushing dies and the appropriate bushing (or a custom die).

No idea if Redding makes neck bushing dies for a .416 Remington Mag.

You are probably right about this. Neck turning is just one of those things that I definitely always do on a gun I want to be accurate, of course that's in a tight chamber. It BUGS me that neck turning brass on this will simply cause it to slosh around even more. I can sort bullets, control primer choice, control powder charge, control OAL, measure for out of round, etc etc. I can do all of the usual stuff in an effort to make it shoot the light fast loads with as much precision as possible. The neck turning just bugs me.

Thanks.

We are on the same page. That's how I stocked my freezer this year. 325gr at about 2000fps. I swear I can almost see the big slow bullets going downrange.

I'm going to make a note of your suggestion.
I waited a while to find just the right rifle based on the suggestions of the smith at Great American Hunting Rifles. The winchester in .416 rem mag has a habit of being inherently smooth and fast compared to other dangerous game rifles. Operating the bolt very smooth and fast is what I have been working on. Managing recoil will be part of that same equation. Fast, accurate, powerful, (that's the idea anyway).

Ball and dummy drill

I'd skip the idea of neck turning all together for a hunting rifle. Gaining an extra .1 moa likely won't have any great impact on hit probability at the ranges you'll be shooting a .416.

Also, what kind of accuracy are you looking for? 1/2 moa or better can be achieved without neck turning by just finding the proper load in most rifles with a decent barrel.