OAL variation in 7 mm SAUM

[Al in SC]

using a comparator to reload 160 AB in Rem cases. Length varies from 3.252 to 3.259. Goal is 3.255. Is this excessive variation. What range are some of you getting with your loads? Thanks. Al

 

[Mountain Goat]

I always spin the round and get a few more measurements before I'm satisfied with the final number. Between bullet ogive differnential, runout, and amount of pressure you put on the caliper, your readings don't seem out of line at all. Same goes for when you measure the length to the lands of your rifle. Spin the bullet, measure again and see how much variation there can be. Also, a clean barrel will measure different than a fouled barrel will. When you are trying to achieve consistency down the .001", most equipment available to us just insn't that precice. I think you are doing quite well with your loads. The question is: How do they shoot?

 

[Al in SC]

Thanks MG. I am getting about 3/4" groups. Still striving for 1/2" though.

 

[Darkhorse]

To me, .007 variation is excessive when measured at the ogive.
I seat with a Hornady new dimension seater and rotate the cartridge 90 degrees twice during the seating process.
I also use a primer pocket uniformer that cuts out the tiny radius at the bottom of the pocket which allows the primer to seat solid and consistant and just below the surface of the cartridge head.
I measure with a Starret caliper and a Hornady bullet comparator.
My rounds, after setup, measure less than .0015 variation between all rounds loaded using the Nosler 7mm 150 gr. Ballistic Tip.
Make sure there are no Burrs on the case head and that the top of the ram, and both the bottom and inside the shellholder are clean as slivers of brass and burnt powder can buildup and cause false readings. Also try loosening the set screw so the shell holder can turn freely as a tight screw can tilt the shellholder.
When measuring the round rotate the cartridge a few times and make sure it is perfectly straight (90 degrees against the caliper blades) watch the dial or readout and you will see it settle in and repeat as you work out your thumb pressure and cartridge alignment.
FWIW, I no longer use over all length in rifle cartridges as the condition of the bullet point causes variation in measurement. To me it's all about the distance from the ogive to the rifling.

 

[Mountain Goat]

Thanks MG. I am getting about 3/4" groups. Still striving for 1/2" though.

That's one way to get you back to the shooting range, always trying to better that best group!

 

[skidmark]

I always sort my bullets thru the comparator first by ogive
then load to my ogive seating measurement. I still target a COAL
but as Darkhorse said tip varation changes the COAL every time.

 

[Al in SC]

Thanks fellas. Some great info

Darkhorse, what primer pocket uniformer are you using? Not familiar with that.

 

[Darkhorse]

I have made some from drill bits (ex-toolmaker) and they worked OK, but the ones made by RCBS and Lyman and probably others work much better. My handmade would sometimes enlarge a primer pocket but the factory models don't.
This is the one I use because it was readily available;

http://www.cabelas.com/product/Shooting ... t549388080

Here is a set of case prep tools that looks like a good deal if you don't already have these tools. It includes the uniformer. Neat thing about this set is it handles both large and small primer pockets.

http://www.cabelas.com/product/Shooting ... t549388080

This is a bad photo of the tool I took some months back.

 

[Al in SC]

Thanks darkhorse !!!will have to look into that. Where in Ga are you located. I am in Rock Hill, opposite side of SC from you.

 

[Darkhorse]

I'm about 60 miles due south of Macon.