Well, you've been given a lot of good advice. It's all standard handloading precautions, don't push the envelope.
If you go onto the Norma Reloading site, they caution you about using high pressure loads in the 96 actions.
As far as the 98 actions go, they are quite tough. When the right powder and bullet combinations are used, a lot can be done to improve things. Improving velocities is another issue. It always means MORE PRESSURE.
The 6.5x55 has a fairly large case for the weight, diameter, length of the largest bullets available. the tight twist in the old Swedes doesn't help anything either. In the K98, you should be able to slightly increase powder volume for a very small increase in velocity but a large increase in pressures.
The cartridges built for the original rifles used a fast flake style powder and velocities were much more conservative than what we expect now. For its day, the 6.5x55 was a real hot rod. Maybe even ahead of its time.
Check out the velocities offered by other cartridges in the same medium case class. Their velocities are very similar and usually slower.
The 260Rem is a completely different cat and was designed for use in modern rifles, made of modern steels, using modern powder, bullets and cases.
You need to find which load is the most accurate in your particular rifle. Velocity increase would be nice but not always possible.
You also have another conundrum, Hornady is no longer making or marketing 160 grain bullets. You will just get used to how your rifle shoots them and there won't be any more, unless you scour the gun shows and garage sales.
Norma, RWS and maybe a few other European bullet makers are likely still making the heavy for length/diameter bullets. Getting them is a whole different story.
As for your action being weak because it's old??? Maybe, but I doubt it has changed much or at all from when it was made. I have a GEW98 action that is shooting full pressure 257 Ackley Improved with a 40 degree shoulder. Extremely accurate rifle but not much faster than a modern high pressure load for the 257 Roberts. A bore will only burn so much powder efficiently, until it reaches a saturation point. The rest just turns black and ends up on the ground or in a huge fireball at the muzzle. Back in the day our Gew98 actions were made, a lot really depends on when and where they were made. Just like the 1903 Springfield, the early actions were surface hardened and used a different metallurgy than just a few years later. Say 1910??? That doesn't mean the earlier actions were weaker, maybe just more brittle. Not likely to blow up during firing as crack when dropped on hard surfaces. They didn't have optical pyrometers back then either. A master gunsmith, passed on his experience to an apprentice on how to gauge temperature by judging the color of the red hot metal. The real issue with this was the lighting you were judging the color shade by. If it was darker, it appeared to be hotter than it was or if it was lighter, it appeared to be colder than it was. Somehow, they managed to get it pretty close most of the time.
I've always been impressed by how much can be gotten out of a small case capacity or medium case capacity and how little increase is gained by going to a magnum case with half to twice as much powder but twice the recoil.
I have a 6.5x55 in a M38, M96, k98, Tikka T3 and Winchester Mod 70. The powder charge I use in all of them, is 47 grains of H4831 with magnum primers and 140 SSTs. This load is likely a bit hot for the Swede actions but pressure signs aren't present. That doesn't mean you should immediately jump on this load for your rifle. In fact, you should start at 42 grains and work your way up. The 140gr SSTs are as long as most 160gr round nose bullets so they need a fast twist rate to stabilize.
H4831 is one of the better powder for the 6.5x55. Pretty hard to go wrong with it, even over Standard primers. One thing about it though, when the pressures start to climb at the max limits, they don't go up exponentially by weight or volume, they spike and do it very quickly beyond a certain point.
I've been told you can't get enough H4831 into a 6.5x55 case to cause problems. Don't believe it.
