I would like to know what is happening as a hull is reloaded many times. Do you know of an article reporting the results of scientific tests, with reports on variance, etc?
Also, do you have suggestions on relatively inexpensive chronographs? I note that Chrony offers a basic unit for under $100. How do they work?
What is a “diffuser”?
Waddayamean “scientific tests”! Are you imputing that Technoidal tests are less than that? Well, if you are- you are right as rain of course. My testing procedures tend to be somewhat haphazard, but they may be helpful. I can’t put my hands on any exact article at the moment, but after reloading for 35 years I have a pretty good idea of what happens to shells as they become oft reloaded. My experience agrees with what the other writers say. Fortunate for them.
As a hull ages, two main things and a couple of minor things happen.
First of all, when the hull gets quite old, the body of the hull can split in two ways. AA 410s can separate completely just above the brass. The shell just shoots the front off and you get a “whistler”. 28 gauge Remingtons tend to split vertically just above the brass and vent the powder gas into the chamber. Other hulls of different gauges do the same thing, but these two seem to self destruct the same way each time so I use them as examples.
Secondly, badly used hulls degrade in the crimp area. This can take the form of softening of the crimp folds, splitting between the crimp folds or actually shooting off one or more of the crimp fold “ears”. Any way you look at it this weakens the crimp and affects ballistic performance. A nice tight crimp is required to hold the shotshell together for a moment after primer ignition to ensure that the powder ignition is properly begun. Weak crimps generally produce lower and more erratic velocities due to variations in powder burn. I have seen velocity drops in the 50 feet per second area for old hulls, but the main thing is that they are erratic.
As hulls age, the inside of the plastic gets rougher with each firing. This also affects wad seal, but I don’t really know to what extent. Additionally, some oft reloaded hulls start to stretch the primer pocket. This seems to be more of a problem when switching brands of primers from the original one, but it can lead to loose primers.
As to inexpensive chronographs, I have used a Chrony and it was OK, but I bought the ProChrono instead. They are both somewhere around $100. The only real testing that I have done on my ProChrono was to run Eley Match ammo over it as a benchmark. Eley match is extraordinarily reliable and uniform. The ProChrono was dead on. I also tested various lots of standard factory shotgun ammo and they were in the right ball park.
A diffuser is an translucent light screen placed over the light receptors of the chronograph in difficult light conditions. The chronographs need decent light to function correctly and the diffusers seem to help this if the light is strong but not exactly “aimed” right.
Because light conditions vary so much, I always “set” my chronograph using test loads of known velocity. When my chrono is up and running, I use a couple of these loads through it to “calibrate” and make sure that all is well- then I get into my testing. It isn’t very scientific, but it is practical. The ProChrono has indicators which tell you whether or not you are getting a “good” read.
Shotgun Report’s Technoid
(Often in error, never in doubt.)