Yeah, I feel your pain, having been there before.
Have never had that happen with TAC, and I've gone through several 8-pounders not just in .223's but the .204 Ruger and .17 Fireball.
Also tested TAC at 70 and then at zero and in the 90's over a chronograph. Per usual in my tests, EVERYTHING (not just the ammo) was cold or warm, just as it would be when hunting. Am not going to look it up right now, but the TAC loads was around 26 grains with the 50 Ballistic Tip, and got just under 3300 fps from an Oregon Kimber I had back then.
At zero it averaged EXACTLY the same velocity as it did at 70, which was obviously due somewhat to chance, as even when chronographing several strings of the same load, on the same afternoon, they won't be exactly the same. But it did impress me some.
At 90+ the load was in the low 3300's, but have seen many other powders gain a lot more velocity in warmer weather. In fact have never chronographed any load, with any powder, that didn't gain some velocity at much over 70 degrees, even the Hodgdon Extremes, but with so-called temperature-resistant powder the rise in velocity is less than with what might be called conventional powders.
Have also "greenhoused" a few rounds in a Ziplock bag, along with a small thermometer, when doing warm-weather tests, to get the ammo temp up to 120 or so. Then have shot them in a "cold" barrel and also one that had been warmed up with a number of rounds. That can get interesting!
