What most buyers of .458 Win Mags don't realize, or know, is how much "free bore" a typical .458 WM actually has from the factory. Unless you are having one put together by a smith and he asks you about it, a standard reamer will create a free bore of about 3/4", the same as most factory products. In effect that shortens your barrel by more than 1/2" already. That's not much, granted, but it's then very difficult to get enough of the "right" powder into the case to compensate for that "jump" to the lands.
When I bought my first .458 WM, a Ruger M77 (22") with a tang safety (which I prefer), I couldn't get decent accuracy from 350 grainers while getting one-hole groups from the 500 Hornady. I contacted John Kronfeld, who was of great assistance in explaining about the typical .458 WM "free bore". I had to seat those 350 Speers out as far as possible (1/4" in the case) to get any kind of consistent accuracy, but even then it was closer to 2" groups than 1 1/2", and that was after shooting several boxes of 350s and trying every suitable powder/primer, etc.
But that free bore served a purpose to defeat the extra high temperatures of African hunting that elevated pressures with the available powders of the era, especially those using factory fodder. The vast majority of safari hunters using a .458 WM was totally unaware of these issues.
The late Finn Aagaard, wrote an article several years ago (which I still have) on the .458 Win in which he had tested most factory ammo in five different .458s. One of those five was his own that had been re-barrelled to a 23", but he discovered in the process of these tests that handloaded with the 500gr Hornady it ONLY had 1/8" free bore compared to the others that averaged about 3/4". In his rifle, the factory loads plus his own, beat all the others, including a 25" Winchester, by upwards of 160 fps, with NO signs of excess pressure. He was getting 2188 fps from the 500gr Hornady in a factory load!
I've since owned a CZ550 with a 25" tube. The weight all up was 11.25 lbs. There was NO difficulty to exceed 2200 fps from the 500gr Hornady with several powders, H335 and H4895 in particular. A2230 wasn't available at the time. Later, before I sold the rifle, I obtained some 2230, and it was slightly faster with a couple of grains less than H335, but I never got it tested for accuracy. That rifle was more accurate than I was. The other powder I used for the 350 TSX was H4198 at 2750 fps. It too shot into MOA when I did my part. The key to that was in being able to seat those bullets long in crimping into the bottom groove.
Concerning handiness: That rifle was perfect in balance, so was never a burden in moose hunting in Northern Ontario for a week (using that 350 TSX). That's vast and rugged country. Didn't get a moose but used the same ammo on a decent bear a few weeks later at 95 yards.
Today, my Ruger #1 in .45-70 IMP (long-throat) is my ".458". It has the original 22" barrel with an "improved" throat, but little free bore using the long 500's or 450s. My bear load last fall was a 300gr TSX at 2645 fps. It's much lighter and handier than the CZ, but the recoil is MUCH greater!
Having experienced a few 1000 rounds through each, I'd recommend a 22" barrel and a weight of at least 9 lbs all up, built on a standard-length action, not the CZ which is a true magnum-length action.
Just my view from who I am.
Bob
www.bigbores.ca 
