Is it possible to neck up 300Win Mag or 7mm Rem Mag brass to use as the .416 Taylor? I've heard of using the .338 Mag brass but I already have a large supply of 300 and 7mm brass. Any thoughts or experience?

Not so much... you'll have quite a few split necks

I've used a lot of Winchester 7mm Remington brass to form 416 Taylor. Never split a neck with the 7mm's.

The 300 Winchester brass, I split about 1/3 of them necking them up.

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I've used a lot of Winchester 7mm Remington brass to form 416 Taylor

I used to make all my 416PDK brass by necking up 280 brass to 416 and fireforming. I don't know how many steps fireball uses but I used 5or6. I would lose a case now and then. The 300 will require the shoulder to be moved back.

Your 7mag & 300 brass started out at the same spot as 458 brass. The neck when expanded back to 416 will be thinner and shorter than when 458 is necked down to 416.

Necking up from 7mm is possible but to me a royal pain. If it were my time and $$ I would sell the 7 & 300 Mag brass I didn't need and buy some 458. Run it in my 416 die and be done. Even better buy 416Taylor headstamped brass.

I necked to 308, .338, .375, then to 416.

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I necked to 308, .338, .375, then to 416.

If my memory is working at all I had a 358 step at first. It was between 338 and 375 I had some trouble with necks splitting. At the end I made my own expanders with a long taper from 338 to 375 and 375 to 416.

I figured that approximately .030 a step probably wasn't bad....the 300's generally gave up the ghost at the .375 stage.

Thanks for the information. If I neck down the 458 brass will the wall of the neck need to be turned down? I just noticed that Graf has some Jamison brass headstamped for the 416 Taylor. You're right I should save myself the aggravation and just buy the brass, but what fun would that be? I can't wait to make some of these bad boys.

I belive the 300 Win cases will end up too long.
The Lee dies have a nice tapered expander and are cheap. You can get a few different sizes for stepping up. I tried some with .338, 358, 375 and .416. The few I did turned out fine. I tried going straight from 300 win to .416 and did split a couple of necks.
I think I'd rather have the longer neck from the 300win brass. You'd just have to cut the neck and throat deeper on your barrel.

I haven't tried necking down 458 brass, but the description in COTW says cases are made by necking down 458 brass, but didn't mention any neck turning. I wouldn't think you would, it's only a small step down.

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If I neck down the 458 brass will the wall of the neck need to be turned down?

I made my 416 Taylor brass from 458. The necks were fine no turning required. Midway also has A-Square & quality 416Taylor brass.

http://www.midwayusa.com/ebrowse.exe/browse?TabID=1&Cat...*652***670***9013***

When I was loading for the 416Taylor, I emailed RCBS asking technical advice on whether to use 338WM or 458WM brass. Their advice to me was not to use 338WM but to use 458WM instead and size down. That is what I did and had no problems.

GVA

For some reason I've always had more uniform neck thickness when necking down so thats what I would normally do.