Observed Thermal Stability of 5.56MM loadings

[leid]

I just got back from a long-hot shoot over in Mississippi. Used a couple days to do a "front porch" check on the thermal stability of several different powders & 5.56MM factory loads as there is a freezer on the porch for cooling the cartridges right next to the chrono. The handloads were all assembled with full length sized virgin LC 5.56MM brass. The SWA load used new LC brass and the Black Hills MK 262 MOD 1 used new WCC brass. All handload powder charges were weighed. All handloaded 77gr. SMK bullets were seated in Wilson arbor dies to 2.245" COAL. All cartridges were fired in a low mileage 18" SS Krieger barrel with CLE Match 5.56MM chamber. The single base powders showed some extremely small thermal stability numbers. The numbers on Ramshot TAC are about twice as high as Ramshot lead me to believe. So I ran all these tests a 2nd time with no real differences noted. I tried to get an "apple to apple" measurement so all cartridges were heated/cooled as a group for a minimum of 2 hours then (5) cartridges were removed & fired as quickly as I could do it to produce the string. Each cartridge was single loaded in an effort to keep the cartridge temperature constant. NATO pressure is listed to indicate full 5.56MM pressure type handloads (NATO max is 62,350 PSI). Ramshot lists NATO pressure loading data for 77gr SMK over TAC which I used. But AFAIK, no 5.56Mm NATO loading data is publish for 77gr. SMK over Varget or H4895. So I worked it up as best as I could. Brass life is only 2-3 cycles at that pressure. U.S. mil-spec limits 5.56MM cartridges to 58,700 PSI. FWIW

Powder/Pressure/Bullet/Temp/Low/High/Average/ ES/SD/Hot-Cold= FPS Difference

Handload:
Varget/NATO/77gr. SMK/135F/2656/2726/2685/70.0/26.9/2685-2684=1 FPS
Varget/NATO/77gr. SMK/-10F/2658/2714/2684/56.64/20.85

Handload:
H4895/NATO/77gr. SMK/135F/2728/2772/2741/43.75/18.19/ 2741-2737= 4 FPS
H4895/NATO/77gr. SMK/-10F/2692/2768/2737/75.73/34.94

Handload:
TAC/NATO/77gr. SMK/135F/2811/2869/2841/57.97/21.49/2841-2723= 118 FPS
TAC/NATO/77gr. SMK/-10F/2712/2741/2723/28.87/11.22

Factory BLACK HILLS MK 262 MOD 1 SECONDS lot BLH06J123-001
?/BH MK262/77gr. SMK/135F/2834/2898/2875/63.81/28.68/2875-2710= 165 FPS
?/BH MK262/77gr. SMK/-10F/2682/2749/2710/67.01/24.43

Factory SOUTHWESTERN AMMUNITION 5.56MM 77gr. SMK RUN N GUN (no lot number on box)
Hodgdon/SWA/77gr. SMK/135F/2729/2767/2746/38.06/13.85/2746-2728= 18 FPS
Hodgdon/SWA/77gr. SMK/-10F/2711/2739/2728/28.24/11.61