A friend just got a bushmaster with a 1-9 twist. He then got 100 hornady 75 grain BTHP bullets. I tried loading some with Varget, Win. cases, win srp, 20.0 to 25.0, 5 loads in 1.0 grain increments. They shot all over the map,(6 inch groups). The rifle shot some UMC 55 factory loaded fmj's quite accurately,(1.3" @100 yds). The goups got better at the higher load levels but still were around 3.0 inches. The 25.0 load was hot, maybe too hot.
Case head expansion along with primer appearance showed I'd better back off a half grain or more from 25.0.

The Hornady # 5 manual has the 68 and 75 grain bullets listed under 5.56 x 45mm right next to the .223 rem data. They state the rifle used was a colt AR-15 20" citadel bbl. with a 1 in 9 twist. All of the bullets went in point first with no visable keyholes. My guess is this twist is too slow for this bullet. Thoughts, advice?

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if you run, you just die tired

Shoot at longer range to see if you eventually get keyholing. A buddy of mine has a 223 bolt action that has a 1-9 twist barrel on it. He tried a number of the heavier bullets with poor accuracy at 100 yards but no sign of keyholing. When we backed off to 400 yards one day most of the bullet holes showed keyholing and were all over the 6 ft square target backer.
1-9 is a bit minimal for the 223 due to the muzzle velocity. Try some 69 grain Matchkings and see if the problem goes away. They are excellent bullets and have a much higher BC than the lighter bullets.

Two quick choices...change the bullet to the 69 grain sierra or try RL-15 in the same loading range.

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