old guy using SX for decades
I know this is an old post, but as a long time user and admirer of Hornady SX bullets, I had to reply. I was searching for some 55SX on the web (with no success, Hornady has quit making them, at least for now) and this thread came up in the search.
I have used 50 and 55 SX bullets for 40 years +. Have shot them in rifles with twists from 1-14 to 1-9, always in 222 or 223 cartridges. Have taken coyote, fox, ground hog, crow, coon, and blackbirds with them. In no case have they failed to deliver top accuracy and instant kills out to 250 yards.
Hornady designed them for 1-14 twist at no more than 3500'/sec.The faster the twist, the slower the muzzle velocity must be to prevent disintegration. Currently I am shooting the 55 SX in a 1-9 twist bolt gun at 3200'/sec, with no lost bullets. Took a large male coyote last month with a chest shot, and he dropped like a rock. The exit wound was the size of a baseball (yes, there was an exit wound). My current load is 25.3 grains of 3031, in a rifle with a NATO chamber and throat. In a rifle with a commercial chamber, this load would be over max, so beware.
My favorite groundhog load is a 50 grain SX with 25 grains of 3031 in a varmit rifle with a 1-12 twist and a commercial chamber. Very effective out to 275 yards.
They should work just fine with your intended load, unless you rifle has a very fast twist. A 1-7 twist might require as low as 2700-2800 '/sec max velocity.