Two of the PH's I've worked with in the past have these rifles. I've used them quite often on hunts prior to having my own DG rifle built. While planning for my own rifle I used quite a lot of different cartridges and rifle makes to learn about them which helped in my decision.
With this I came to a few conclusions, that are clearly my own opinion based only on my experience, for what thats worth.
You know there are certain groups of cartridges that we can make generalizations about. Like the 7mm, 280, 270, and maybe even add the 264 and the various 25 caliber cartridges made. They are all so similiar in performance that for a deer species would there be a whole lot of difference between any of them under 200 yards? Really thinking about it would there be much difference if you added in the 300 savage, 30/06 and 308?
I kinda felt the same way with the DG rifles I was using. Once you get above 416 most of them shooting a premium bullet of 450-550 grains over 2200 fps ended up providing the same results. When a 500 grain bullet can be driven fast enough to penetrate anything it hits 4-8 feet deep I'm not wishing for more then that.
The 500 jeffery is a great rifle, a little heavy and very expensive to shoot enough for lots of practice. The bullet diameter is not an off the shelf projectile, and the Brass is difficult to get too. I don't feel I can become "expert" with a cartridge I struggle to find componenets for. Especially when It has not shown me yet any exceptional stopping power advantage over a .458 diameter bullet of 450-500 grains.
Since shot placement is the single most important issue, the practice I get with a 458 due to easy brass and bullets gives the edge to the 458 diameter in my opinion.
I chose to have a 458Lott built. It's about 1.5 pounds lighter then either of the Jefferies I have used. The brass is as inexpensive as any brass around( 375HH brass works fine)
I can shoot any 458 diameter bullet, and I can shoot 458 Win mag factory loads. Best of all it will shoot a 500 grain bullet at 2300fps, and a 450 grain BarnesX bullet even faster! That bullet has an absolutely stunning effect with head shots on everything I have hit with it. I've only recovered a few, the rest have exited even when shooting end to end on Buffalo. Not sure how the Jeffery will surpass and end to end with an exit leaving a 1" hole on the way out?
Probably the single biggest factor in my decision was the amount of other PH's also choosing this cartridge or having their existing 458 win mags rechambered to this round. It's an extremely popular cartridge in Southen Africa for PH's now with a growing interest.
Nothing wrong with the function or killing power of the Jeffery, just not a practical choice for frequent shooting and long days carrying it in the bush. My 458 Lott is 9.3 pounds loaded and ready to go. The Jefferies are all in the 11pound range. You may say that's only a pound to a pound and a half......so what! Well looking another way, it's over 10% more weight. Your arms get really heavy after 6 hours of packing that thing, add a scope and it's more weight, and uncomfortable to carry over the shoulder, and worse yet with a sling!
I'm not sure I would want to shoot a full power Jeffery under 10.5 pounds either!