nighthawk and Ken...
At first blush, that sounds like a more plausible explanation. It certainly requires fewer assumptions.
With all the extra space, the powder would burn more as it does in open air at first. Also, a small amount of oxygen would be available for the first little while.
In Brownell's writings, you can clearly see numerous very fast spikes riding on the normal pressure wave when using small charges. They appear to be fairly regular in frequency, but very irregular in amplitude.
