I have a bunch of 404 rounds I loaded up two years ago to take to Zim on a buffalo hunt. They are loaded with 400-grain A-Frames over 92 grains of IMR 4831. When I left for Zim, they were chronoing around 2370 fps, which I figured was plenty for my intended target.

The 404 has been in the shop for a over a year, so when I got it back, I decided to burn up some of this "old" ammo at the range (cold day BTW). Upon the first shot, I knew something was up. It kicked the hell out of me! I looked at the chrono, and it said 2460. Then I proceded to shoot the rest of the box. All rounds chroned above 2450 and accuracy was nowhere near what it had been when I left for Zim with these same rounds (not bad, just not the one-hole accuracy this gun usually produces).

Even wierder is the fact that I shot a hand-full of even older rounds, loaded with the same powder charge, only over 400 grain Woodleigh softs. These chronoed right where they did when I loaded them almost five years ago, 2350 fps.

What's going on here? BTW the A-Frames that made the trip were all loaded at the same time tested, and all test rounds (two years ago) fell in the 2360-2380 range. So I am relatively sure these did not have more powder charge, although I did not bring one back home, break it down and weigh it.

Is it unusual for ammo loaded just two years earlier to produce this kind increased velocity? What about the difference between the old Woodleighs and the A-Frames? Are A-Frames that much of a pressure producer?

Just wondering.

Sounds like maybe they got too hot. A commercial ammunition manufacturer told me that when modern smokeless ammo gets hot for too long, it doesn't "go back", and will produce higher velocity regardless of temperature thereafter.

The A Frames I've shot I worked up with and haven't tried them over the same charge as a Woodleigh, but I can tell you that the shank of an A Frame is much harder than a Woodleigh.
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"Serious rifles have two barrels, everything else just burns gunpowder."

Getting too hot might explain things. These rounds went to Zimbabwe and back, and while there, it got unbearably hot for a few days.

The Woodleighs never went anywhere outside of my loading room.

I've never experienced that phenomenon, and I have used ammo loaded at the same time on three safaris over the course of four years.

Telephone the ballisticians at IMR and ask them what they think it might be. Perhaps the powder clumped up, changing its burn rate.

George

it could be that over a couple years the bullets got a "set" in the case necks. Had thaat happen a few times. Then the peak pressure rises a bit, which would cause the increase in velocity. Either that or you shouldn't have used so much blasto #5

quote:

Originally posted by GeorgeS:
I've never experienced that phenomenon, and I have used ammo loaded at the same time on three safaris over the course of four years.

Telephone the ballisticians at IMR and ask them what they think it might be. Perhaps the powder clumped up, changing its burn rate.

I got that from David Little at Kynoch.
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"Serious rifles have two barrels, everything else just burns gunpowder."