Filaman,
One major comment before discussing powders: As I have mentioned a number of times before in several magazine articles and books, there is no way 7x57 reloading data can be standardized in the same way as it can for, say, the .30-06, .270 Winchester, 7mm-08 Remington and most other cartridges. This is because the 7x57 was introduced early in the smokeless-powder era as a military round, and like most other early smokeless military rounds it was designed for a heavy round-nosed bullet, so the chamber throat was very long. (Apparently heavy, round-nosed bullets were initially standard because that's what were primarily used in the black powder military cartridges of the day.)
Not too much later, most military rounds were switched to lighter pointed bullets at higher muzzle velocities, which flattened trajectory and reduced wind-drift at longer ranges. In some of the older military rounds, the chamber throats were shortened to better accommodate the lighter spitzers, good examples being the 8x57 and .30-06. But the 7x57 never went through this in its military days, though eventually when it was primarily being used as a hunting cartridge many rifles were given shorter throats.
There was never any real standardization done, even though organizations regulating such things, such as SAAMI (the Sporting Arms and Ammunition Manufactuers Institute) in the U.S. eventually settled on a much shorter chamber throat than the long military original. But even after that, American-made 7x57 throats have varied. I've owned several 7x57s made by Ruger, and throats ranged in length from the present SAAMI throat (which is relatively short) to one perhaps even longer than the original military throat. Plus, plenty of other companies have come up with their own variations. I had one custom 7x57 made by a company that uses a reamer with a throat even shorter than the SAAMI specifications.
As a result of all this, there is no way the same powder charges are going to produce similar results in all 7x57s. I've seen as much as three grains difference in the amount of IMR4350 required to produce the same velocity with 140-grain bullets. Any suggestions about powder charges on this thread can results in such widely varying velocity--and pressure, because pressure is what produces velocity. This is aside from the usual variations in powder lots, brass thickness and bullets themselves, and is exactly why I've long advised loading the 7x57 for to a general muzzle velocity for various bullet weights, rather than sticking to a specific listed powder charge.
Yes, there is something different about Reloder 17: It's a very progressive burning double-based powder, which is different than the single-based powders often recommended in the 7x57, such as IMR4064, IMR4895, IMR4350 and H4350. The dash of nitroglycerine added to double-based powders (which is why they are called double-based) provides a little more energy.
But 17 is not alone in this: Another excellent, modern double-based powder for 140s in the 7x57 is Ramshot Big Game, which is widely known as a "wonder powder" for the 7mm-08. It works just as well in the 7x57, and in my experience is more temperature resistant then RL-17. In my last 7x57, made by the company with the short-throated reamer, 48.5 grains of Big Game resulted in just under 2900 fps from the 21-inch barrel with the 139-grain Hornady Spire Point Interlock, and excellent accuracy.
But that does NOT mean Big Game will produce that much velocity in a 7x57 with a longer throat--and there are many such 7x57s out there, even among modern commercial rifles.
