First round super, the rest sub

[RobRat]

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?

[N310toN170]

I'm reaching here and bordering on a guess...

The first round is cold parts and pushing air that is cold and therefore speed of sound is lower, the following shots are pushing a warm air mass in a warmer surrounding for the initial flight of 10' or so and allowing a gradual introduction.

Either that or you were cleaning between groups and sliding on the CLP acting as a lubricant.

[rsilvers]

Anyway, 11.4 is your load, but even that may be too fast. I published 11.2 as my recommended load at that OAL.

[rumlover]

Interesting, your results are exactly opposite of what I typically get with sub loads of all calibers. I typically get a slower velocity on the cold bore, then higher on the subsequest shots.

Not sure what the issue is, but I recommend delcaring it doesn't matter. If you are going to hunt or try to punch little groups and need the dope, drop to 11.2-11.4 gr and then test with a cold bore & clean barrel, testing a single shot at a time allowing the barrel to cool for at least 5 min with the chamber open. When you get a good sample of velocities to establish an average, move on to testing the variations on follow-up shots.

[buffetdestroyer]

It may be that the powder is positioned differently in the case from the recoil of the first shot.

I would still lower the load slightly so you are not affected by temperature & conditions.