First shot always slower...

[too_many_hobbies]

I fired 5-shot groups in the 90 degree heat today with 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, 10.1, 10.2, and 10.3 grains of Vihtavouri N110. This was my first time to shoot groups through a new CED M2 chronograph and a new AAC 762-SD suppressor mounted on my AR-15 in 300 Fireball. The first shot of each 5-shot group had a substantially lower muzzle velocity. The following example velocities (fps) are listed in the order they were shot:

9.9 grains - 967, 1040, 1049, 1040, 1048
10.0 grains - 999, 1065, 1057, 1068, 1039
10.1 grains - 1004, 1068, 1066, 1070, 1068
10.2 grains - 1019, 1066, 1081, 1084, 1079

I cleaned the barrel and then fired one 9.3 grain fouling round before I started shooting 5-shot groups. I did not clean the barrel during the rest of the session. I normally walk to the target and mark the hole after each shot to let the barrel cool, but today I was having trouble keeping cool so I fired each shot with only about a 30 second pause and only walked to the target after each 5-shot group. That resulted in about 10 minutes between groups during which time I removed the AR from direct sunlight by placing it in the shade of my pickup cab with the windows rolled down.

I was using a Leupold 3.5-10x-50mm VX-III scope and could usually see the bullet holes in the target at 100 yards. The first lower velocity round was hitting the target on the average about 2 inches low. Ignoring the first shot, I was getting from 1.25 to 2.5 inch groups at 100 yards.

I would like to hear the reasoning of others as to why the first round always had a substantially lower velocity. Also, will velocities I am getting now in hot weather be reduced in cooler weather? Should I wait for cooler weather to work up my subsonic loads? Will I have to go through this exercise again when the weather is cooler to come about a subsonic round? I would assume that if the velocity changes with a corresponding change in the barrel harmonics, that the load that showed the best group (10.1 grains) may not be the best load when it is cooler? Any other suggestions?