IR thermometer gun. If you leave the ammo in a temp environment long enough it will normalize to that temp, including the powder. The case, the bullet, the primer, and the powder. So the ammo needs to be in that temp environment long enough for that to happen. If you load the ammo in a 70* room. Leave it in your 60* garage overnight, throw it in your vehicle and head to the range where it's 80* .... are you really testing the powder at 80*?
Here's my experience at the last four matches I've shot.
December 2020. 6GT, 38.3gr StaBall, Hornady brass, 107SMK. I developed the load in the weeks proceeding at an outdoor range. Temp was average around the 50's. Day of the match, it was cold, windy, and had just blizzarded. I wore two jackets that day. No elevational issues. Tied for first.
I was gone for 5 months and the next match was June 2021. 6 Creed RL23. 2985. I had to re-develop a load because I didn't take good notes. It was in the 70's to 80's the entire time of load development. Loaded mmo the night before, drove 2.5hrs to the match, cool morning, constant sun all day, warm and sweaty in the afternoon. Kept the ammo in my backpack and out of the sun. No elevational issues, second place score; two guys ties for 1st so ended up in third.
July 2021. 308W, 43.8gr BL-C2, 178 BTHPs. 2665fps. Loaded the night before, stages ammo in my truck in the garage. Probably 55-60* overnight and 40* outside on the drive down, but 68* or so inside the cab. Drove 2.5hrs to the match, kept the ammo in my backpack all day, kept it out of the sun, and was diligent about trying to keep the ammo as cool as possible. No real elevational issues but had 3 wierd flyers/ misses. I took .1 out of my data 2/3's of the way through the match and then .2 during the last two stages. The load chrono'd 2660 the week before, 2665 the morning of the match at the sight in range, and then after the match it chrono'd 2698 with some extreme individual rounds over 2700 in the 2720's. It was hot that day, sweat running down your back in your butt crack, change clothes for the drive home, sunburn,etc.
July 10th. 6 Dasher, 34.6gr H4350 2915fps. I loaded 250rds for a two day match in Aug 2020 but ended up not shooting the match. It was in the 90's when I did load development and I had to bring my normal charge down from 35gr to 34.6gr. it still chrono'd 20fps faster at 34.6 than it did at 35grs at that time. I used this ammo for the match last weekend. Checked zero and chrono the day prior. 80* Ammo had been sitting in my reloading room, climate controlled at around 70*. Still chrono'd 2915 or so. No zero change almost a year later. Next morning I drove 30mins to the match, checked zero and chrono. No real change. It was hot as shit throughout the match. In the 90's, sunburn, hood on all day, mags, barrel, and scope hot to the touch. I didn't take a backpack and just carried a 100rd Alpha Munitions plastic ammo box around from stage to stage in my hand and set it down next to my tripod. I tried to put it in the shade when possible but that was 2 or three times. I ended up taking .1 out of my data in the afternoon. And the last stage I was .3 or so high. I found out while shooting and had to correct on the fly. Cost me two or three targets. I still placed 1st and didn't bother chrono'ing after the match to see how much I sped up. Just didn't care that much.
The point is it can be misleading to just load ammo, drive to the range, and chrono. Those long hot days where you can't escape from the sun and you and your ammo are exposed to increasing temps as the day wears on is where you really see it. And I BELIEVE that you see it in not just higher velocities but increased ES and pressure spikes.
StaBall is temp stable enough to use for matches and in a lot of scenarios you may not see the sensitivity but it is sensitive. Had I subjected it to the sun and heat at 6000ft ASL like I did 4350 last weekend I would have really seen some increased velocity and weird shit. And I can also take it to a match with no wild envirommental temp swings and it acts stable. It's not everytime in all conditions. It's the extremes.