Concentricity Guage - how to and advice

[tailbon3]

I bought the sinclair concentricity gauge that was on sale last week. Thanks for the tip whoever posted that!
I had just loaded 50 308 win so I figured I'd give them a whirl on the gauge.
The brass is once fired military brass that I bought from cabelas. The bag said it was full-length sized, cleaned and ready to load. I did chamfer the case mouth but that was all. I checked the headspace with a wilson cartridge case gauge and it looked normal.
So - to make a short story long, the run-out at the case mouth varies from 2 thou to 9 thou and the bullet is even worse ( measured about where I imagine the ogive would be ).

I then measured some 243 win that I've loaded from new brass and the case mouth looked to be within 1 thou. So that seemed encouraging. I ran a case through my full-length 308 sizer and it reduced run-out from 8 down to 3.5 or so.

I think I'll just shoot them at 100 yards just for trigger time, unless you alls talk me out of it.?.

Also, where do you measure bullet runout? Close to the tip or back by the case mouth?

 

[FearNoWind]

There are quite a few things that can affect run-out. First of all, the thickness of the brass at the neck isn't usually consistent throughout the circumference. Neck turning is often necessary to correct that condition.
Then there's quality of the sizing die, whether it's full length or shoulder bump and how it shapes the neck. A button type sizing die will not normally provide a good, straight neck. A bushing die improves the neck sizing and if it's followed with a neck sizing mandrel to round out the inside of the neck things get even better.
I've found that using a finger to turn the case in the concentricity gauge can create uneven pressures that can introduce slight errors in readings. I use a popsicle stick that has a length of rubber band glued to and use the friction of the rubber band surface to turn the case in the bearings.
Lots of theories on the best place to check bullet run-out. I prefer to take that measurement at the bullet's surface just ahead of the neck. Once you've got all the elementary issues resolved, I find it helps to seat the bullet part way into the neck, turn it about 180 degrees, and finish the seating process for each round. Of course, a good quality and properly shaped seating stem in the seating die helps too.

 

[newmexkid]

FearNowind: I've found that using a finger to turn the case in the concentricity gauge can create uneven pressures that can introduce slight errors in readings.

I find this to be so true. For me anyways! I had fits getting good readings. I have a 21st Century Shooting concentricity tool and I later bought the wheel option for it. What a difference even pressure makes!!

 

[engineer40]

Hey Hey! I posted about the sale. Glad I could help!

I posted about my results here:
http://www.longrangehunting.com/for...s-my-new-sinclair-concentricity-gauge-156920/

Many conversations happened in that post around "where" to measure concentricity on the seated bullet.

If I were you, I don't think I would buy that brass again. It sounds like whoever did the resize on it, did not have good dies.

You should try to start with good brass. Measure the concentricity of the case mouth before you start reloading. Then use your concentricity gauge each step along the process during reloading to determine if your dies are introducing anymore runout onto the ammo.

Once you determine that your personal reloading process and your dies are working as intended; you are good to go!