I have two 6.5 x 55 rifles, one is a modern, strong action Zastava M70 designated 6.5 x 55 SE (Commercial M98 Mauser); the other is a 1900 M96, built in Obendorf, it is small ring with the stripper-clip thumb-cut.
I have been sticking to "standard" loads for both as I rarely shoot beyond 200 yards with the Zastava, and only shoot the 96 at our 100 yard range.
Most accurate in the Zastava so far is 120 gr Sierra HPBT Match Kings in front of Dominion D4350. For the Swede, I am presently loading PPU 158 gr Jacketed soft point semi-spitzer bullets in front of Dominion D4895. Both are a bit short of the max loads listed for the IMR counterparts to these powders.
In the Swede there is little different between the 120 gr Sierra HPBT and the 158 gr JSPs. Interestingly, at 100 yards, the point of impact in this rifle is virtually identical without changing the elevation.
In the Zastava, both loads are shooting 1" high at 100 yards (where I want them to be) but the 120 gr often give me 1 MOA or less groups on center, the 158s about 2 MOA about 2" left of center. Why? I don't know, but am guessing barrel harmonics.
The Zastava has ample free-bore to load longer for minimal "jump" with bullets of 120 gr or longer. The Swede has a lot more. S & B ammo is known to be "hot", but with the long free-bore the extra pressure in the European Ammo (Norma, PPU, S&B) don't seem to be too much of an issue. (Weatherby used this to control pressure spikes in his magnums). Make sure your rifle is in good condition, and verify your headspace.
I have some 85 gr. Sierra Varminters that I have yet to try, but will be pushing the pressures up a fair bit in the Zastava, hoping for velocities of about 3200 FPS or a bit more.