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First round super, the rest sub
I had 1/2 and our to shoot on Satuday, and wanted to shoot some loads, to see where they went subsonic. I was shooting a 300/221, 16-1/2" AR15 upper, topped with a YHM 762 Phantom suppressor. I didn't have a lot of time and the range was covered with ice and snow, so I was just loading 3 in a mag, aiming at a rock on the 100 yard berm, and pulling the trigger when the rock was in the crosshairs- -5-10 seconds between shots. It was about 34 degrees, and slighly foggy.
The load is Fed 205 primer, 220gr SMK bullet, seated to 2.090" OAL. The powder was AA1680. Starting at 12.6, then 12.4, 12.2, 12.0, 11.8, 11.6, 11.4gr.
12.6-12.0 were all supersonic, definite crack. When shooting the 11.8 and 11.6 loads, the first round would crack, then the next 2 would be subsonic. They did that both times I tried them.
The 11.4gr loads were all subsonic- -pull the trigger, hear the buffer moving, followed by "thud" coming back from the berm. No "crack".
Is it common for the first round to be faster than the rest in a string? What would cause that?
I don't feel the gun was getting hot enough to heat the first round hotter than the rest, but thats the only think I can think of. Would the suppressor have anything to do with it?
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