Shoulder bump issues for .223

[Forensics Doc]

I've been reloading for a CZ527 Varmint in .223 for several years now using a Dillon 550B and Dillon dies.  The gun is incredibly accurate, but like most of us I want to wring out every bit of accuracy it is capable of.  The other day I wanted to recheck the shoulder bump so I took some fired cases and measured them from the base of the case to the shoulder using digital calipers and Sinclair Bump Gauge Inserts.  The brass is twice-fired Lake City brass that has always worked well in the gun.  Yeah, I know, it's not Lapua but it works well.  The brass was not resized prior to measuring.  The primers are seated below the base, so that's not an issue.

 

I expected to see the lengths within no more than about 0.002, but I was surprised to see the lengths were all over the map, varying from .002 to as much as 0.011.  Given that they were all fired in the same gun and would be expected to expand to fit the chamber, I am stumped as to why they vary so much.  I thought maybe the gunk on the neck and shoulder could account for the variations, so I cleaned them slightly with 000 steel wool  It had no effect. And with this much variation, I can't see any way to set the sizing die to set shoulder bump back about 0.002.  

 

Does anybody out there have any idea what's going on?  I don't. 

[bandw1dth]

Through trial and error I’ve learned that I get the most consistent shoulder bumps on my progressive when I set the press to have just a bit of camover. Typically I use an empty decapping die screwed in till it starts touching the shell plate. And then adjust from there for the right amount of camover. Hope this helps.


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[Powder Finger]

I would start by getting a few that represent the spread you are seeing and knock the primers out and re measure just to be sure. 

[bigedp51]

The variation is caused by the flex in the press and because the die that does not make hard contact with the shell holder.

 

I bought my Rockchucker press in 1973 and it has a little flex that caused variations in shoulder location, and I solved the problem by using Redding competition shell holders.

 

These shell holders allow you to adjust the amount of shoulder bump without touching the resizing die.

 

These shell holders allow the die to make hard contact with the die and have the press cam over and remove any slop in the press.

 

And if buy bulk once fired brass all you need to do is pause at the top of the ram stroke for 4 or 5 seconds. The pausing greatly reduces brass spring back for more constant shoulder location.

 

Bottom line, you will get more constant shoulder location with a single stage press with the die making hard contact with the shell holder and no press flexing.