I recently put the Magnetospeed on my Varmint wt Criterion barrel in 223 and shot a ten round group just to watch for any instability in the load, which was running on the warm side with Varget and a 69 gr Lapua.
Starting with a cold (75* F) barrel and never leaving a round chambered for more than 10 seconds before ignition, my velocities climbed with every successive shot until reaching #8, where upon is began to plateau as the barrel was now warm to the touch.
An important point I'd like to make here is that I single load, I do not shoot from a magazine, and when shooting a warmed barrel, I first close the bolt on the new round and then open again full back (I've removed the ejector) to let the round sit on the bolt head while waiting for the right wind condition. Then close the bolt and fire ASAP.
Yes, there was vertical stringing in my groups too, but I think this brings up a critical point about soaking a charge in the chamber.
In my particular case, this (chamber heat) could not have been a significant contributor to the consistent velocity rise. Something else was a work, and I believe that something else is barrel quench, or the process of the barrel drawing out heat and therefore expansion and pressure from the gas charge burning in the barrel.
Barrel quench is never discussed in any forums I've read, yet most shooters will admit that warming up the barrel before shooting for record is pretty much routine.
Anyone else share my theory about barrel quench?
Inquiring minds are always hungry for the juicy details.