The .416 Rigby in factory-ammo guise is quite a different puppy from the same round, handloaded. At least, it CAN be different.
My own .416 Rigby happens to be a Ruger #1H, and it will deliver a 300-grain X-bullet at 3000 fps, chronographed and consistently. However, it's flat-out miserable on the back end, and unless the elk start growing armor-plate, I don't need such ballistic performance. Same applies to the 400-grain bullets, which easily reach 2600+ in my rifle.
In a handloader's rifle, the .416 Rigby is a .416 Weatherby, or VERY close to it. This is logical, since Roy based his .416 on the Rigby case. Use caution as loads are increased, as always.
I also own a post-WWII .404 Jeffery on the standard '98 Mauser action, and it's not difficult to exceed 2400 fps with 400 grains in this one, either. (Factory .404-400s ran at 2150 fps.) Remington .375 Ultramag brass makes perfect .404 cases at a fraction of the cost of Norma- or RWS-made cases.
Whatever the differences among these various .416s (and .404s), it's safe to say that ALL will deliver more horsepower than logically "needed" in North America. So, what does "need" have to do with it???? If any shooter wants to mess with the .416 in any flavor, more power to him ...literally.
It happens that my two rifles fire rounds designed around 1910, but the fact that NEW cartridges are coming out which essentially duplicate the performance of 100 years ago simply means that those long-gone designers had things right. Current designs are fine-tuning the basic package of .416/400/2400, but oh my, does the "basic package" ever WORK.
As a dedicated bullet-caster, I must add a plug for that part of the hobby....the plus-.40 calibers offer wonderful performance with cast loads, and often deliver "accuracy loads" much easier than will smaller diameters. The RCBS 416-350 mould is readily available, and does great work in my .416. At 2000 fps, it's a viable 200-yard proposition for hunting. Earlier this year, I was shooting my cast hunting load of RCBS .416-350/2000 fps on the same day I was firing some .223 trial loads (with jacketed bullets) from an Interarms Mini-Mauser...and the .416 shot TEN-shot 100-yard groups SMALLER than the .223! That is, just under 7/8" for the .416, and right at 1" for the lightweight .223. I will admit that there was a slight difference in how the two rifles felt on the benchrest, however....