I've used 2 rifles chambered for a .375 H&H, the first an M70, 24" that failed to make 2600 fps from 300s, though I tried every appropriate powder at the time. My next .375 H&H had a 26" barrel and would easily grant 2700+ fps from at least two powders, one was relatively new -- RL-15 and the other, a much older powder, had similar burn characteristics: IMR 4320.
But the bore of the 26" was off center by .008" and would not give the kind of accuracy I expected, so I had it lopped to 22" in hopes of correcting the poor accuracy (It wasn't until the smith cut the barrel that its off-center bore was discovered). The rifle then gave 2530 fps from the same loads! That was a loss of 185 fps from -6" = 31 fps per inch.
From a 20", .338 Win Mag, prime handloads of 250s attained factory specs at 2660 fps, but another .338 with a 26" tube gave 2840 fps from 250s. That's an increase of 180 fps = +30 fps per inch increase. Then, I've owned several .458" calibers, and while the loss from 26" to 22" is not quite as great as in the faster .338 and .375, it still amounts to about 25 fps per inch, so that was the basis of my thinking in suggesting a loss of approximately 200 fps in a 16.5", .458, while taking into account variances incumbent in a variety of firearms and variable loads.
Also, taking into account that factory loads in .458 WM from 24" tubes usually run about 2050 fps to start with, and several manuals only grant about the same number starting out. So even giving your 16.5", .458, 2000 fps from 500s may well have been optimistic on my part.
Bob
www.bigbores.ca
