I do ream the cases, but I tell you what I think about it.
I do believe the closer you get to full bore .458 in the original case, the less and less important this reaming becomes IMO.
In the case of smaller calibres such as 257 Weatherby Magnum or 7mm Remington Magnum & 300 Win Magnum, the brass does have a slightly more inwards taper commiserant to the smaller & steeper agle to the more narrow and smaller rifle bore. Also varying brass manufacturers have differing thicknesses of case wall thickness right where the case neck crimps the bullet. Often too, I think a big 'help' in requiring no/less reaming is the more blown out and straighter case wall of Weatherby Magnums. I think 375 Weatherby Magnum requires less work to bring to spec than say the 300 Win Mag, again cut brass pieces from the neck reamer are the key to this.
In other words if I took R&P brand 458 Win Mag brass, I don't really think one has to ream them. Just cut to proper length. I say this because the RCBS reamer for thier conversion die removes less brass from this converted case than any of the other smaller H&H magnum calibres.
And you know what, after you've fired these converted cases just once with your big ole .458 bullets I really doubt us big game hunters could any the difference in neck crimp tension at 100 yards. I suspect alot of this has to do with wildcat chamber differences and if your rifle chamber will 'tolerate' this slightly bigger 'bulge' of the bullet pushing outwards on ever so slightly thicker unreamed brass versus reamed brass.
In my example I do believe I've accidently handloaded cut & unreamed brass for my strong mauser actioned rifle, mistaking them for cut & reamed brass. After the first firing, it's fireformed anyways now matter how you look at it. So if it will feed and chamber and go bang, that's the first part of the equation resolved right?
I could not tell the difference in either of the two examples while sighting in for hunting season. And in chambering, firing and ejecting, there was not one iota difference in observed performance IMO. I discovered this purely by accident after tumbling the fired brass and observing the inside cutting marks that were missing on some recently fired empties. My bad with no repercussions.
Mind you maybe this could be a prime subject for chronographing to see if there is a pressure spike in velocity of sorts?. Speaking of course, to reamed versus unreamed casings.
I tried Winchester brand 458 Win Mag for conversion and the thinner neck walls just collapsed under the sharp hacksaw. Waste of time.
Older Norma manufactured brass (very old) had the primer pockets come loose too early (one or two firings)
I think the Federal brand 7mm Rem mag & 300 Win Mag will follow the Norma weakness the soonest.
The best brass for conversion in my experience was the bigger calibres, R&P 458 Win Mag & 375 Weatherby Mag. Hard to tell between these two for the best ones. Slight preferance for the higher quality Weatherby brass I think.
Sorry for the long winded reply.
