Regarding soot, load pressure, and the use of custom dies:
First, load pressure is partially dependent on the case sealing against the chamber walls during firing. I hope we can all agree on that.
Don't worry about soot?
Why not?
For soot to appear on the outside neck of the cartridge, the neck can not be sealing against the chamber walls during firing. I don't understand why you would not want to try to correct that issue with handloaded ammo.
Sounds to me like you dismiss the problem because you do not notice accuracy issues from it, but how would you know if you dismiss the problem? I'm guessing you think that as long as this issue is consistently present every time you fire the rifle it is not something that needs to be fixed? If that is what you are thinking, I am not sure how I feel about that, but I do also wonder what your powder burning between the outside of the brass neck and the chamber is doing to your chamber, but I think benchrest shooters don't expect much life out of their barrels anyway, so I guess it doesn't matter. For my Arisaka, I am hoping to get close to 7,500+ round barrel life, on top of whatever has already been shot out of it by its original-issue soldier and owners previous to me.
More specific to my situaton:
As most of you know, smokeless powder needs pressure to fully burn. If the brass does not seal against the walls of the chamber before pressure starts to drop from the bullet being ejected into the barrel, the powder will not completely burn, resulting in a squib (possibly, probably) and a condition called "powder meltdown" where the brass case is partially filled with congealed, unburned powder. Many factors can contribute to causing powder meltdown, including very cold weather, using LR primers instead of LRM, using a powder that is too fast-burning (which generates the impulse for the bullet to be ejected but ironically does not generate enough overall pressure to fully burn the powder), chambers that are too oversized for the loaded round of ammo, and barrels that are too large for the bullet being used (many people use .308-.311 bullets for the 7.7x58 loading, but .312 bullets are available from Hornady). Attacking this problem can be done various ways, including using dies that allow the rounds to fit snugly to the chamber when loading the round to be fired. Some people prefer to neck-size fired brass, and thus avoid the expense and effort of getting custom FL sizing dies. I did not want to do that even though it is touted by many as a quick, inexpensive solution. My concern is that eventually, those pieces of brass will need to be full-length sized and one may not know that until one is at the range, with loaded ammo that will not chamber. And, however one finds out that the cartridges will not load, one then has to go back and FL size the brass and that sets up a possible powder meltdown when one takes that now-FL-resized ammo to the range. I think most Arisaka shooters would respond to that with, "well, gee, how many rounds are you planning to shoot from that thing anyway?" And as I said above, I want to make and shoot many thousands of rounds. I had the ammo meltdown problem happen with commercially-sourced 7.7 ammo that I no longer use for that reason. I use my Whidden dies, LRM primers, 4064 powder, and Hornady .312 diameter bullets. I can fire my ammo on 32 degree days and have never had the powder meltdown happen with my ammo. Did I need the Whidden dies to accomplish that? No, of course not. I could have attacked the problem with "belt OR suspenders" instead of "belt, AND suspenders, AND better fitting pants", but whatever. It's my rifle and my money and I'm happy.
