Cadence and the Crack Shot

A few matches ago, I noticed a phenomenon in my shooting.  Every so often, I would notice my shots starting to move.  I’d make a small sight change and immediately see a large motion in the hit location.  I always passed it off as a possible issue with my sight.  Then I was reading a book on high power shooting by G David Tubb.  He talked about maintaining a steady cadence and the idea that heating of the cartridge in the chamber can change the muzzle velocity.  My thought was that if I chamber a cartridge before fiddling with the sights, then that one shot would heat soak in the chamber for longer than all the other shots.  This would throw the shot high, or at least wild.

So, off for a test.  I made up 20 test rounds.  I set up 2 targets side by side.  After shooting a number of rounds to warm the barrel, I then shot at the 2 targets alternating right and left.  Every shot to the left I shot at normal cadence.  Shots to the right I sat on for a measured 30 seconds, then shot at my regular cadence.  My expectations were a similar group size for each style with the slow cadence side grouping higher than the fast cadence.

Wrong.

The variation in elevation of the 2 groups was .06 inches at 200 yards.  Essentially nothing.  However, group sizes were very different.  Fast cadence had a standard deviation of 4.27  while the slow cadence had a standard deviation of 2.99.  This is a big chunk.  Both of these might be a bit higher than normal because I went about 10 shots longer before cleaning than normal.

The next question was what the shortest reasonable wait time would be.  I wired up a cartridge with a thermocouple run in through the primer hole.  I built it up with powder and a bullet, just like the real thing.  Then, after each string at a match, I’d pop the wired shell in and record the temperature rise over time.  

 

The shell is a bit dirty, but I've been pushing it into dirty chambers all day.

The chart below shows the various data strings.  The temperature was rising throughout the day, so the starting point is a bit off for each.  I also was a little slapdash on getting started promptly when the shell went in.  Overall, it was a bitterly cold day.

 Temperatures are degrees F and time in seconds.

Clearly the heating steadies out after about 30 seconds.  My normal cadence would have a cartridge in place for 10-15 seconds, so there was probably a lot of variation at that pace.

I'll probably do a similar test on a hot day, but for now, I'll just be patient when I need to take that Crack Shot.