Shoulder - Case Head Measurement w/Non-Flat Case Head

[Phil3]

I am newish to reloading, but I know enough to know shoulder setback is critical. I use calipers outfitted with a Sinclair clamp-on tool and proper insert to measure. I also added a Davidson caliper clamp-on device that fits the case head, making it easier to hold everything in exact alignment. No more of trying to fit a round case head against a caliper blade. Still, just like with the bare caliper blade, I sometimes have trouble getting consistent shoulder to head measurements. This unnerves me, as I am not confident I am setting the shoulders back the right amount.

After some nosing around, I may have found part of the problem. The case heads are not always square to the case body. I know this because my measurements are better when I put a couple of cases in a Sinclair trimmer case holder, and then put that into my lathe. I faced off the case head until there were no high spots. Depending on case, that took .0015 to .0020". Given shoulders may be set back just .001 - .002, that kind of variation on the case head is significant.

Should I just measure a bunch of cases and take an average and perform shoulder setback from there? These were 223 Remington cases fired in an AR-15, known for being hard on brass. Burrs on case head surface that could interfere with measurements were removed with a fine stone.

Phil

 

[243winxb]

I would guess, the bolt face has to be square to the chamber first. If not square, would the fired brass measure high on one side?

 

[bigedp51]

If one side of the case has thinner case walls when the case is fired the thin side of the case will expand further than the thicker side of the case. This unequal sidewall stretching warps the base of the case and it will no longer be 90 degrees to the axis of the bore.

When this case is full length resized it aggravates this condition because as the case is squeezed in the die the base of the case is pushed and tilted even further.

What you are describing is called a warped banana shaped case. It is more pronounced when firing commercial SAAMI cases in larger military chambers that allow the brass to stretch more in the "longer" and "fatter" military chamber.

Read 1, 2 and 3 below.



If you think your .223 cases are bad then buy a military British .303 Enfield rifle and watch commercial cases really warp when fired.

Your AR15 5.56 chamber is .002 larger in diameter than a .223 chamber and the .223 cases are not made as hard as military 5.56 cases and stretch more in the base. I reload my commercial .223 cases for my ARs to much lower pressures than the 5.56 is rated for to prevent this.





Welcome to the soft brass fat chamber club.

 

[Phil3]

Thanks bigedp51. All makes sense. One note. The rounds fired were 223 Remington, fired in an AR-15, but with a Krieger barrel chambered in 223 Remington. Krieger recommended not to use 5.56 ammo in it, and I don't. I like the Neco gauge, but may not much like all the crummy brass it may find!

Phil