Bullet runout gauge

[gnoahhh]

Here's a question that's been nagging at me. With gauges that spin a loaded cartridge on v-blocks and use a dial indicator to check bullet runout, what or who's to say that the brass cases are perfectly round at the points they are bearing on? Random sampling of virgin and used brass shows a bit of variation in the circumference measured at different points, which I've always known existed but pretty much ignored. Should I think that variation would skew the results with the dial indicator? Would one be behooved to check the concentricity of the case and make notes at various points on it, and calculate the effect of that data on detected bullet runout? I obviously don't use one, but am considering building one with a spare set of Starret v-blocks and a dial indicator but won't if I'll just be pissing in the wind.

I'm not a benchrest competitor, nor am I particularly anal about striving for tiny groups. I'm merely curious from an experimenter's viewpoint.

[Baron von Trollwhack]

Don't worry about runout checking on new brass. You are going to fireform to chamber dimensions and carefully reload if you are accuracy serious to start with. If the case ain't right, the cartridge won't be either, but I ignore the base buldge in commercial chanbers. One has to.

You already know what dimensions you are changing in the process. Of course with a marlin quality chamber, you better select your balance points carefully, although neck mid-points are defined. Selection is easier with a world class chamber though. Wipe the boolit clean of LLA before measuring for sure!

BvT

[Char-Gar]

You are correct about your assessment of new brass. The Baron is also correct about using fire formed cases. It needs to be noted that a rifle also has to have a chamber that is dead straight with the bore and a bolt face that is perfectly flat and at an even 90 degree angle to the bore. Straight ammo isn't much help in a crooked rifle.

I have yet to find a perfectly round cast bullet nose. Like you all of this bench rest stuff in interesting, but does not scratch where I itch.

[gee-gaw]

I think that on factory chambers, you don't worry about concentricity, you just mark your cases and index them in your chamber and sizing and seating dies. Custom chambers are a different story. If you have a good, straight chamber, then you can concentrate on maintaining that alignment with you neck sizer and seater. I use hand dies made by LE Wilson for my benchrest ammo and find that the run-out is always under .001. There are systems that will help you, like the Redding S bushing die And forester's floating chamber seater die. Then all that hinges on good uniform brass. If there is a lot of variation in the case wall thickness, the indexing method is the only way to go. Even if you turn necks, the case walls will eventually cause your brass to bend somewhat as a result of different expansion characteristics on the thin side versus the thick side. Did that make any sense?
Just my .02 worth,
Wayne

[felix]

Wayne, indexing will wear the chamber out-of-round. Load randomly daily, and use indexing for match shooting where winnings can purchase a good re-chambering. ... felix