Longshot,
They show up in strain gauge measurements of pressure (see images below). BL-C(2) is popular, and its fine with heavier bullet weights for sure, but it is 7.62 powder. The version the military likes for 5.56 is sold as H335. Same powder, but faster burn rate due to lower surface deterrent concentration.
Whether it has caused your guns any issue or not depends, in part, on bullet weight. Bore scope examination is the only way I know to look for a problem positively. You can ring a barrel a little and never notice it on the target for the same reason some pitted barrels can still shoot pretty well.
Bluetopper,
Set down the hack saw. Pop the cap off a bottle of brew. Relax. Don't worry. There's no need for anything as drastic as a muzzlectomy.
These spikes reliably disappear if you either increase bullet weight or change to a powder with a faster burn rate for the cartridge and bullet weight.
"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity."
Homer Powley
"Perhaps there is no more persistent crank than the Rifle Crank. He has more theories to the square inch than there are hairs on a dogs back."
Theodore Roosevelt III
603Country,
Well, you've gone and opened a can of worms. With kind permission to hot link from Jim Ristow at
shootingsoftware.com, take a look at the two misbehaving .223 loads below. One has a muzzle velocity of about 3100 fps, and the other about 3500 fps, but notice the secondary spikes start and peak in about the same number of milliseconds. You can bet the two different velocity bullets are not the same distance down the barrel when these peaks occur. As near as I can tell by tweaking QuickLOAD for pressure match, it works out the 40 grain bullet's pressure peak occurs right about the time it would exit a 20” barrel, but the 55 grain bullet is only about 17.5” down the barrel. Assuming it took some time for the barrel distortion to travel to the gauge at the chamber, the actual bullet positions would be closer to the breech. The peak is additive to the chamber pressure curve, further confusing matters.
And then, just to add insult to injury, a load that I measured at a snail's 2792 fps @ 15 ft:
The tiny secondary spike starts late, but peaks at about the same time as the others. The bullet would have been about 12.5 inches down the bore at the time of the peak. Indeed, if you look through RSI's stuff, only their 7 mm STW load has different timing; a peak at around 1.3 ms.
So, by now you should be questioning everything you thought you knew affected where the bullet was when these secondary “pressure” readings occur. That coincidental peak timing is exactly why I thought it was some kind of instrumentation anomaly when I first saw the effect. It doesn't seem to matter what the chambering is, or what final velocity the bullet is, much less any differences in barrel condition or friction. That first plot is for a moly-coated bullet, demonstrating that reduced friction doesn’t affect the peak timing.
But the temptation to think it’s just the instrumentation is roundly knocked down by the fact you can make the phenomenon stop with heavier bullets or faster powder. Plus, there is the 800 lb gorilla in the room, Charlie Sisk's blown-off muzzles. These both say, no, unless you believe you are living in the Matrix, it's not just a computer illusion.
It dawned on me eventually that what the two .223's and the '03 might have in common was they were all guns with 24” barrels. The 7 mm STW probably had a 26” barrel, as a lot of longer case magnum sporting rifles do. So what the similar timing proves, I think, is a barrel disturbance or wave that strains the chamber area enough to look like a high pressure. Indeed, you can't strain steel enough to look like a high pressure without some serious stress, though whether that is all due to pressure directly or not, I can't say without more data than we have. Characterizing the strain better looks like it would require putting gauges around the barrel to catch different axes, and several hundred dollars worth of strain gauges up and down the length of the barrel, also on different axes, then a data logger fast enough to capture it all and show the relative timing. And, of course, we'd want to know where it looks like the bullet actually is each step of the way, which multiple gauges would reveal.
The spike does not appear to be a resonance, as the curve decays on the back side, same as a pressure curve.
Varmint Al, a former Lawrence Livermore National Laboratories engineer, did a lot of modeling of firearm events on their fancy FEA software that you can see on his site. He concluded:
Vamint Al said:
Maybe the "consensus" was that a rifle barrel vibrated in one or more of the mode shapes when fired. That was because the mode shapes and frequencies were easy to calculate and they did seem to answer some of the questions. From these FEA dynamic pressure calculations, it appears that the recoil and forced deformations are much more important than the natural vibration modes in determining where a barrel is pointing when the bullet exits the muzzle.
So, what are we seeing exactly? I don’t know. My current working theory is that when the powder mass catches up with the bullet, the bullet is upset outward during the attempt at sudden momentum transfer. If that stretches the bore radially even a little it will greatly increase the apparent friction. At that point pressure will abruptly straighten and push forward on the bullet base and bore. There are a couple of kinds of elastic event you can imagine occurring from that. One would be like Al’s Mode 6 on the harmonics page linked to above. Looking at how a long barrel deflects in his rifle animation, you can imagine that something pulling the barrel straight would crack a deflecting muzzle like a whip, which would be the other form. The problem is, I would expect an elastic event like that to ring, and not just stop and be damped out, the way the spike is. Though, of course, the stock bedding may do some of that damping, as might the receiver mass. Still, I’d expect some amount of reflection and ringing.
That’s about as far as I’ve got with it. Expensive measurements appear to be needed to get further in a definitive way.
Unclenick would be the person most likely to have the best info, but I'm trying to understand as much as I can on the issue. Being an engineer, I always want to understand things rather than just hear it and believe. Doesn't always work that way though. My plan going forward is to keep using the H335 as my primary powder behind the 40 gr Noslers and the BL-C(2) behind the 64 gr Nosler BSB bullets. I've used a lot of AA2230, but always behind the 65 gr Sierra GK. 
