Arrived at the range with my mod 12 bvss in .223 in the case, box of 50 gr. V-max in three different loads in half grain increments, 10 rounds each, and 50 gr Hornaday SP using same loads. Also brought my IR thermometer.
Set everything up, checked the outside temp (OAT), 94F, and gun barrel 87F. Began shooting first 5 rounds of first load checking barrel and chamber temp before chambering round. Recorded everything.
Groups started fairly open as expected, barrel slowly warming to ambient temps, and tightened as load neared maximum velocity being tested (3500 fps). Used 24.5, 25.0, and 25.5 grs of TAC for all loads.

When I got to the first five rounds of V-Max group 3 (25.5 gr) at barrel temp of 95 to 97F, the OAT was 96 to 97. Five holes touching, group size .365

Repeated the test working from lowest velocity upwards again shooting rounds 6-10 of each load ( 3 loads each of two different bullets). The slowest load showed remarkable improvement over first shot, barrel being 10 degrees warmer and exactly equal to OAT. But the previous best shooting round opened up considerably when barrel temp hit 99 to 100, measuring .910. All other groups opened up at 99 degrees too. BTW: measured the (laminated) stock temp too. Of course it did not warm as quickly.

During this test, I did not chamber the round until ready to shoot, and fired within 20 seconds.

Barrel is stock with about 1800 rounds down the tube, throat eroded about .25 inch from new. Bedded it two months ago, and noticed improvement in temp sensitivity, but this was first time I actually measured it.

Is there anything else I can do to improve temp sensitivity, besides buying a new, stress relieved barrel? Or just wait for Fall to arrive with lower temperatures here in HOT ol' Texas?