I shortened the 22" barrel on my 30-06 in increments. At each step I fired one jacketed validation load and two 180 gr. cast loads. The cast loads are known to have consistent mega-spikes.
Unfortunately, I overwrote the trace for the 22" barrel. Ooops. However, there is plenty of data for the 22" barrel from previous tests, showing the cast load spikes almost always redline the PT at 105,000+ psi.
The graphs are self explanatory. Note that the cast loads do not trigger consistently, making it difficult to pinpoint the exact time when the spike begins.
I would have like to continued cutting but with my luck a BATF agent would have shown up at the range just as I turned my rifle into a pistol.
As you can see despite the erratic triggering, the spike did not go away, but it did move closer to the breech.
No disrespect to Charlie Sisk, whose work I admire, but this contradicts his experience. Charlie's spikes went away after he blew the muzzle off.
If the spike is due to the powder "catching up" to bullet's base, how can we explain the spike moving as the barrel is shortened?
Another theory goes like this: the air being pushed out the barrel ahead of the bullet forms a supersonic shock wave (verified by shadowgraphs). When the shock wave hits the muzzle, part of it reflects backwards and starts traveling backwards (pure speculation). The spike is caused by the reflected shock wave colliding with the bullet. Actually, that's not a bad theory, because reflected shock waves are known to be very destructive, but if the spike is caused by a reflected shock wave, then why do Winchester powders greatly reduce the spike? After all, no matter what powder is used, the air in front of the bullet is still going to act pretty much the same, right?