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Can't say about cryo, or number of grooves, but some years ago I set up a .223 for LR with the 6.5 twist recommended by Sierra for the 90 gr SMK.
With a 28" Krieger 6.5 twist and a lively disregard for published maxima, I found I could get a 90 gr to 2680 fps as needed to stay supersonic at 1000 yards. Unfortunately that heavy a load would deform enough of the SMKs in flight to make them useless. Similarly, pushing a 75 gr Amax fast enough would blow them up in midair.
Load either bullet down a bit and they were great to 600.
I tried some 80 gr Nosler and Sierra but they were not as accurate in my rifle.
For Long Range I went to the JLK VLD 90 (no longer made) and the Berger VLD 90 gr.
They held up to hot loads and shot well.
Paper ballistics say the 90 gr .224 should shoot with the 175 gr .308 to 1000 yards.
I couldn't do it on the range; I scored better with my F-T/R .308 even though the heavy bullet .223 was more accurate at midrange.
Seems most of the Valkyrie work reported in blogs and gunzines has been at the 7 twist which lets the Sierra live at 2700 fps.
Hang on, I am sure a Valkyrie enthusiast will be along presently with more current and relevant information.
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