Last year my father-in-law are out shooting our .300 Win Mags. He has some Partitions he wants to group so he proceeds to do so. I'm guessing 12 or 15 shots worth. Then he decides to proof his load with Swift Sciroccos. The load typically chronographs around 3150 fps. First shot with the sciroccos gives a reading in excess of 3300 fps . My jaw hits the ground. I go check the chronograph to see if it is spread flat etc. No problems. I looked the brass over and no visible signs of pressure, no hard bolt lift, nothing. Not being to smart , we put another through it and it clocks around the 3150 mark. We did this for several more and they were all fine. I chalked it up to a bad reading. This was in the late summer with temps hovering around 70 degrees.

Skip ahead to last month. I am grouping some Accubonds out of my gun. I shoot 15 accubonds through my gun. I have three of my old load of scirocco bullets left so I decide to burn them up. First one chrono's over 3300 fps . Sounds familiar so after I check the brass I try the next one - 3163, the next one 3158. This is in late winter with the temps hovering around 25 degrees.

I have my own theory on what is happening, but I want to see what some of the other thoughts are before I lay it out there.

Thanks.

PFM?

Not sure what PFM is . Please expound.

I've seen things get squirrily when changing bullet manufacturers and when changing powder. I.e. from standard cup bullets to copper bullets, etc. 150 fps is more than I have seen, but that's where I'll hang my hat. HTH, Dutch.

Another possibility: Barrel temperature is a very strong, and often overlooked pressure driver. So, if you put 15 rounds through your rifle, you warmed the barrel up. Then you shot the other bullet, with the barrel hot, and got high speed. Since this bothered you, you stopped and checked out your system, and the barrel cooled. So when you tested another round, it was normal.

Barrel temperature is roughly 2.5-3x as strong a pressure factor as ammunition temperature.

O.K., My thoughts were right along with what Dutch had to say. We always let our barrels cool for 4 or 5 minutes between shots so I don't think barrel temperature would have been that significant of a factor. In addtion, we don't chamber the round to be fired until we are ready to fire it, so cartridge temperature should be fairly consistent.

So, my thought is we fire the Nosler bullets and foul the barrel with the gilding metal used for thier jackets and then we send a pure copper jacket from Swift down the barrel. The first one must meet more restraint due to the change thus spiking the pressure. Subsequent shots are fired over the pure copper fouling from the first shot and stabilize velocity and pressure.

The only thing I can't rationalize in my mind is why I wouldn't have flattened (or completely blown for that matter) primers, or a bolt I couldn't move with a 180 grain bullet moving in excess of 3300 fps out of a .300 Winchester Magnum.

There is no particular reason why a chronograph reading has to be right. In fact I would trust the gun and not the fragile instrument.

Second best guess is that that shell was not the same as the others.