Case forming: case length

[6Dasher]

Hi,

I've been forming my Dasher brass using the Cream-of-Wheat method. I noticed my brass is now, on average, 0.012" longer than before,bullet jammed into the rifling). The load consists of 12.5gr of N320, fill with CoW and use a wax plug to keep everything inside. The brass stays longer even after the second time firing it,with bullet this time).

I noticed the same phenomena on my 6PPC,longer brass when using CoW). Does anyone share this experience?


jim

 

[mikecr]

What do you headspace off. Maybe it's driven forward initially, and then stretches back to the boltface.
I would think the case would get shorter when improving a shoulder.
I know atleast one fellow who always headspaces off the case mouth,jammed .010" into 45deg chamber end) when COW forming wildcats. That is, his chamber neck is cut short .010" and std cases will not chamber without effort,not at all with a bullet seated). This way the case won't move forward on firing. when the shoulder blows out, the neck is pulled back and snaps into perfect chamber end clearance,.005").
Otherwise when improving, many end up way short of the chamber end, and consistant pressures become overly sensitive to neck tension for the life of the brass.

I haven't done this yet. My gunsmith recommended it for a 6.5WSM he's building me, and so I bought a reamer set to do this. It is surprising for me to read of lengthening from COW FF.

 

[6Dasher]

Hi Mike,

I form a false shoulder with a 6.5mm expander plus I turn the neck for about 0.22" length before expanding. I raise the bushing in my die, as on the standard setting the false shoulder will not be enough to keep the case in place.

Maybe the low N320 volume near the bottom of the case pushes out the brass in the rear first and then moves the brass forwards as the pressure expands the front of the case?,sort of a rolling effect of the case walls moving outwards). My headspace is all within 0.0015" or so, so I am not getting anything funny in that area compared to forming with a jammed bullet. I lube the cases lightly with imperial sizing wax.


jim

 

[jb1000br]

that seems like a LOT of powder JAMMING all the COW into the shoulders. Might be creating a ram inside the case, and stretching, then spurting the COW out?

try backing off the charge till it JUST forms the case,or have you already done this)

JB

 

[6Dasher]

Hi JB,

I was thinkning along the same lines. I've tried 10gr before this and the result is that the shoulders to not form properly; round/balloon shoulder with bumps.

I sectioned a case today and didn't find any weak parts in the walls. Nice and regular from top to bottom. No thinning anywhere.


jim

 

[mikecr]

The case can't get any longer than the chamber, and if formed well, it will represent your chamber.
Do you have the chamber/reamer drawing by chance?
Where is the growth located w/resp to chamber?
Maybe it was chambered deep.

 

[6Dasher]

Hi,

The difference is in the neck length. Headspace is idetintical within the region of 0.0015" from each other. It's as if the needed material for the shoulders is coming from the case walls rather than the neck.

I trim them after the second time as well. I noticed when I started measuring for the shortest case to set the trimmer.

Right now I have 20 cases formed with the cow method that have a 0.012" longer neck. I'll measure the cases tonight and post the sizes compared to the 6BR parent.


jim