One reason is to keep operating pressures lower by using a larger case.

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Alf,

Not everyone has test facilities for testing their rifle!

Even Hornady recommends loads for getting 2150 fps in the 458 WM. I had no problem in getting this velocity in 22" barrels. Though every rifle and bullet brand are different, and hence one gets variations in velocities, one just has to load 'em up and if there are no signs of high pressure, away you go.

Yes, looking for pressure signs is key. However, care to be taken working up loads in cold weather and taking ammo to tropic.

ALF- Go to quickload ,look up .458 win. Pick barrel length and bullet of choice. Find powder for 2100fps under 65KPSI. Usually ball, Check with Oehler. Trust me it works , Quickload usually underestimates pressures in my experience too. Yes there are many loads that will achieve that velocity and more. A long barreled .458 win can give a short barrelled .458 Lott a serious run for the money. Neither will match a Ackley though! I've also found that despite very high pressures ( you know how far I've gone), straight walled cases rarely give extraction problems. I suspect its due to my observation that straight walled cases tend to be thicker than other types particularily in the shoulder area.-Rob

Hornady shows their Heavy Magnum .458 Win Mag loads, both soft points and solids, at 2260 fps. They attribute this to a "cooler" burning powder and a special loading technique. Even assuming a shorter barrel velocity should still be around 2200 fps.

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I would say the 458 Lott. It can push a 500 gr bullet to over 2300 fps. Thats alot of FT LBS