trim length

[kenny s]

Ok while the house is full of turkey smells..(Happy T Day all) and I have been shooed out of the kitchen,,,,

I made another chamber cast of my 40 70 Shiloh because I'm working on 100 new shells. sized, primer hole and primer pocket trimmed, annealed, and fire forming now.

question for the experts. The cast measures 2.53 end of shell to front.
I'm trimming my 40 70SS shells to 2.515
what do you think?

and would you expect to trim AFTER fire forming?

Ken...

[SchuetzenDave]

A .40-70 SS case has a maximum length of 2.500" and most manufacturers trim cases to .005" less than the maximum to allow for stretch.

Remember there is a transition between the end of the case and the rifling which gets narrower and can crimp case mouths tight on cases that are too long.
Your chamber cast of 2.530" most likely includes the narrowing transition.
To prevent your case crimping down on the bullet; which can result in excessive pressures occurring: it is recommended to trim cases to 2.495".

That is the Proper, Safe way of trimming cases.

Unfortunately there are those who recommend pushing in the longest case possible when they do not understand the safety ramifications that can occur.

[Tomklinger]

When I fire form with black powder, my cases tend to shorten a bit in overall length as they expand to fill the chamber. I don't understand why one would trim cases short Before they are fire formed??? Black powder pressures are not extreme. Now smokeless is another story regarding case over length
Tom Klinger

[John Bly]

In another post several months ago Kirk said his 40/70 SS chambers are 2.504" long. I have a new Shiloh in 40/70 and my chamber is really close to his numbers. I trimmed some full length sized cases to 2.510 for fireforming and after firing there was a slight turn in right on the end that looked like a crimp looks. I trimmed them to 2.500 and they now are correct for my chamber. Double check your chamber depth.

[kenny s]

thanks guys. all good advice. I think that trimming after fire forming, will be at 2.495. better safe etc. Ken

[SchuetzenDave]

Although Black Powder does not exert the pressures of smokeless it still requires not exceeding the maximum case length.

Many Black Powder shooters frequently try duplex loads, BP substitutes, or smokeless powders suitable for BP cases and there have been catastrophic failures as a result.
And when they experiment with these alternate loads these use the BP cases already being used with straight BP.
Over length cases contributes to these failures; from crimping due to the reduced/tapered diameter of the barrel leade, leading to the jamb of the bullet in the case so that pressures build up behind in the case.

[kenny s]

I agree Dave. just trimmed the first batch to 2.498. checked the casting and I think the lead was added into the 2.53 measurement I took.
better safe than sorry....thanks..Ken

[Kurt]

If you trim a case that is an exact fit to the chamber, in other words tight, you better take a chamber cast and measure from the front of the rim, not from the head stamp, for that proper fit. And trim the case that has been shot in the chamber unsized especially if your chamber has a 4 degree transition chamber end.
The rims on cases vary in thickness changing the case length with a lot of case trimmers that hold the cast head stamp tight against the trimmer and they are all the same length from the head stamp top case mouth but they are not from the front of the rim at the base where it seats against the rim recess of the chamber and it gets pushed forward when the firing pin hits the primer or when the shell is seated. This is we're leaving a case short .005" or .006" comes into play to make up the rim thickness variances. The last thing you want is letting the cast get pulled past the 45 degree chamber end.

[kenny s]

interesting Kurt. the cases I'm doing are all from the same lot, 100 in all, and that is all I'll use.
I'm measuring from the case ind to the front, and not the 4 degree lead.

the case cast measures 2.53 and I'm cutting the cases , after firing, to 2.498. That should give me the proper length.
I plan to watch them closely and trim again if necessary.
good advice....thanks..Ken