One problem with lighter (below 68gn) 0.224" bullets is that there are no G7 based BCs around, at least none I've seen to date. The G1 BC that Bob quotes and the Sierra Infinity program uses, will likely only be accurate enough to 500 or 600yd with this combination. Because G1 BCs are velocity sensitive, ballistic table and program results are usually over-optimistic at longer ranges. I would therefore expect the retained velocity at 800yd for 55gn bullets to be lower than shown, and also the wind drift to be greater.
The two great ballistics evils that affect long-range shooters are wind-drift, which applies to any calibre/bullet/MV combination and which one seeks to minimise in order to cope better with wind strength and direction changes between shots. The Infinity results for the BlitzKing at 800yd are large even if they were 100% accurate, so you'll get a lot of movement on the target with each and every wind switch - still on the plus side, you'll learn a lot about wind-reading fast! The other evil that only affects some combinations is terminal velocity falling close to the speed of sound (1,120 fps under standard ballistic environment conditions). Although the Infinity results look well above that, I suspect the real 800yd figure will be significantly lower, enough to see the bullet well into the transonic zone. That is speeds just above the speed of sound from around 1.3 Mach down to 1.0 Mach. This risks increased airflow turbulence around the bullet, usually getting worse the closer you get to 1.0 Mach which slows the bullet at a greater rate than the BC says it should, and also risking it becoming unstable. With the longer, more ballistically efficient 55s being only marginally stabilised in the .22-250's standard 1-14" twist rate anyway, this becomes a real possibility at these ranges, seeing bullets that shoot OK even at 500/600yd all over the target frame and making oval holes. (I had this at 600yd some years back with 55gn bullets from a .222 Rem also using 1-14" but at lower MVs of course, that had shot the combination brilliantly at 300yd.)
.22-250 has been used successfully in beyond 600yd match shooting, but with faster rifling twists and using heavier bullets. It gives rather short barrel accuracy life though in this form so has largely dropped out of favour.
At the end of the day, there is only one way to find out how your rifle will do - go and try it. The worst that can happen is that you'll have a frustrating afternoon, but you'll learn something. What I would advise strongly is to try the rifle and your favourite load at shorter ranges first - 600yd will tell you a lot and give you an accurate scope zero to start from. If you go to 800yd straight from a short-range zero, you'll very likely have considerable trouble 'finding the target', especially if it's windy!
Laurie,
York, England