Greg,
Thanks for the info, which explains a lot.
There's no real point in using a concentricity gauge in really accurate rifles (especially custom-chambered with a minimal reamer) when using good dies. I don't bother in my own benchrest rifle, either, because any tiny differences in run-out with neck-turned cases loaded in Redding Competition dies don't result in any discernible accuracy differences. I also use good dies of various sorts, combined with good brass, in my varmint rifles, because I am NOT going to check every damn one of 1000 rounds of prairie dog ammo. But we're talking about differences of maybe .001" or at most .002" in concentricity.
On the other hand, concentricity gauges are VERY useful in many other rifles, especially typical factory big game rifles with looser chambers and barrels, especially when using standard dies. Partly this is because the difference in how much more bullet run-out can result from standard dies.
I started using a gauge because of mysteries in how well big game rifles shot with various kinds of ammo or even bullets. My first was an RCBS Casemaster, and the instructions suggested no more than .005" of bullet runout, measured about .1 inch in front of the case mouth, for big game rifles, and no more than .003" for varmint rifles. Over the years I found those recommendations to be pretty much spot-on, especially with factory rifles--but they're obviously many times what we might encounter with match-type dies and neck-turned brass.
A concentricity gauge not only allows a handloader to figure out which rounds might be causing fliers in his .270 or .338, but to pinpoint the problem in the loading process that causes bullets to have too much runout.
The RCBS Casemaster even revealed why one of my .270's apparently "liked" 150-grain Hornady Spire points so much it would group them in around an inch at 300 yards. This was a Remington 700 ADL with no modifications other than adjusting the trigger, bedding the action, and free-floating the barrel. I never could get similar accuracy out of any other bullet in that rifle, even Sierras, and it turned out the seating die apparently fit the Spire Point ogive closer than tangential ogives in Sierras--so seated the Hornadys much straighter, even in ordinary brass.
Have also used a concentricity gauge to turn ordinary sizing dies into what are essentially Forster dies, by raising the expander ball inside the die. The ball, however, often must be adjusted to a particular point to get the most benefit, and the concentricity gauge reveals that point. Have also used it to make other simple die or loading process modifications that improved accuracy considerably.
Have also used a concentricity gauge to sort factory ammo, with very definite results. Once had a custom 7x57 with a Hart barrel and minimum chamber, and came across a very good deal on a bunch of Hornadys now-discontinued Light Magnum loads with the 139 boattail Spire Point. I sorted them into three batches, one with runout up to .003", another up to .005", and the third over .005". The .003" batch would consistently shoot into .5 or a little more, the .005" batch into around an inch, and the over-.005" batch into 1.5".
As a result of long use of concentricity gauges, most of the time I don't need to measure the run-out on loaded ammo, but do tend to measure it on test loads with new bullets and brass--just to make sure I'm testing the load at the range, and not run-out. But if there are a few rounds with bullets slightly out of line, I use the TruAngle tool to straighten them. Have found through that experimentation that it's pretty much a waste to try to straighten rounds more than .002" over specs, but less than that certainly works. Again, the concentricity gauge revealed that information.
Now, not much of this may be of interest to you, because of the types of dies and rifles you use. But it is of interest to many handloaders who use average ammo in average rifles, where a concenctricity gauge definitely can make a big difference.
