Barrel temp question

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?

For starters I would recommend using a powder more in line with the cartridge that's also less temperature sensitive such as H4198, Varget or AR-Comp. TAC is suited more for medium sized cases like the .308 family and seemed to be pretty temperature sensitive based on my limited experience with it.

Back to the range today with a new batch of 50gr V-Max and 25.5 gr TAC that shot so well last week….until barrel heated up. Outside air was 94 and barrel/chamber was 88 at start, again measured with an IR thermometer. First five shots were ragged hole at 100 yds. Each successive 5 shot group got gradually more open until 99 degrees and then fliers started going all over the place.

Then I remembered the trick of tuning the aft action screw torque, so I loosened it up a bit and after a few tries of gradually tightening it, had it shooting a bug-hole group again at a barrel temp of 108.

Now just need to see how it shoots cool again.

Any thoughts as to what to expect?

Not sure if I understand your question, but on hot days at the range I have taken "canned air", the stuff you dust a computer key board with. Just stick the plastic tube in the breach and blow air down the bore until the barrel cools. I take a cleaning rod along and run a dry patch through, just to make sure there is no condensation in the bore. But that cost money, so I started taking a mini compressor that I had anyway that did the job a lot faster. I also keep my ammo in a cooler on hot days until they go in the chamber.

But ammo that I developed in the heat is not going to perform the same in the fall or winter, and visa/versa. But I am a low volume reloader and I don't save reloads from season to season.

JMHO :-) ... Jim

Back to the range yesterday with more 50gr V-Max over TAC, and also some 69 gr. Nosler CC HPBT and 69 gr. Sierra TMK's loaded identically with 4320. I found that with regard to the torque of the rear action screw, I think I just tuned the barrel to a higher temperature, which is pretty interesting. The 50 gr. loads shot well only once the barrel temps reached 105 -110 but that happened pretty quickly as the day was already at 98 when I got there.
The Noslers and the TMKs both shot very well at 300 yds, with the TMKs landing higher on the target due (I presume) to their higher BC however, I was not able to log the shots as they landed. Really need a video feed to see at those distances.
Wonder if I can convince the range owner to buy one? :rofl: