I loaded up some hornady 68gr BTHP bullets with 23.5gr of H335 for my AR. The groups were really bad, I had to try some m193 just to make sure it wasn't me. Should I drop the load down a bit or change powder? I got around 2820fps with the H335.
I use H335 for 55, 60, 62, and even some 69 grain boolits. Can knock the crap out of a 4" round plate at 500 yards. I use mainly CCI small rifle primers, but WW's work well too. My twist rate is 1/9. Keep in mind, a 1/9 will sometimes not stabilize boolits over 65 grains (velocity dependent), and projectile length dependent. A bullet is not necessarily stabilized by twist rate, but instead by RPM. It's a function of bullet length, material density, twist rate and velocity. So- if you're not stabilizing with your 1/9 twist, and you're on the ragged edge, try some more velocity. That MIGHT do it.
I find that H335 does better with lighter weight bullets (in my rifles). It meters fantastic, so I use it for 55gr plinking rounds loaded on a XL650. For my service loads I use Varget behind a 69gr SMK, 75gr Hornady BTHP or 80gr SMK loaded one at a time on a RL450.
I had problems with H335 also shooting a 55 Gr Ball. It was suggested on another site I frequent that it is temp sensitive.
When it was about 50 degrees outside I was shooting about quarter size groups at 200 yards. I later went back trying to replicate the groups but it was in the 90's and my groups spread out to two and a half moa at 200 yards. I thought it was me also. I switched to Benchmark and my groups have come back down to what I want.