Shown below is the information I sent to sfchemist’s questions:
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resulting in the round head spacing off the lands instead of the cartridge case mouth
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In a pure definition of headspace my above statement may open to debate about whether it is technically correct. In simple terms, headspace is the distance from the face of the bolt to the chamber datum which prevents further forward movement of the cartridge. In a rimless pistol, such as 9mm, 40S&W or 45ACP, the datum is supposed to be the edge of the chamber counter bore on which the mouth of the cartridge case will come to rest. Technically speaking, as I understand it, your pistol headspace would be the distance between the pistol bolt (or we could say slide) face and the internal “lip” which stops round.
My problem was simply that the bullet was coming into contact with lands (or some may say grooves) in the barrel just beyond the chamber. The cartridge case mouths were not in contact with chamber counter bore, as they should be, since the bullet contacted the lands first. I didn’t have this problem with round nose bullets, only with the 124 grain flat point bullets.
I don’t own one, but I think the Wilson Pistol Max gauge will measure case length, max chamber dimensions, and max length of loaded round. I don’t think it would help to determine whether a particular bullet profile will come in contact with the lands of a particular barrel. For example, the Berry’s 124 grain HBFP bullet, loaded to a COAL of 1.120 inches, was contacting the lands in my Springfield XDm barrel but did not contact the “lands” in my Kahr polygonal barrel; therefore, I could build longer cartridges for use in the Kahr. The Wilson gauge would tell me if I exceeded the 9mm maximum of 1.169 inches, but wouldn’t help since I had started at 1.120 inches.
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Do you insert a cartridge into the barrel's breech?? And then what??
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My country boy method is to take several empty (fired) cases and measure their length. I will make sure a bullet can be inserted yet will be held in place by case tension. I push the bullet and case into the barrel with thumb pressure until the case stops moving. Then I remove them and measure the overall length. I normally use a little overkill in this procedure, by using 5 cases, 5 bullets and 5 insertions per case/bullet combination for a total of 25 measurements. Care must be taken because sometimes the bullet can bind in the lands, yielding an over length error. Or sometimes the bullet can be pushed farther into the case resulting in an under length error. Ultimately, the OAL measurements made with the shortest case will become the control that I use to calculate the target final length. I decide how much I want the bullet spaced off the lands, generally in the range of 0.005 to 0.015 inches, and subtract that amount from the average OAL length measurements made with the shortest case.
I can also use the measurement data to calculate the average length of bullet projecting from the case mouth to the bullet tip. That bullet projection distance would be reduced by the amount I wanted to move the bullet back from the lands. Cartridges could then be built around using the bullet projection as the control point; however, doing this would make it necessary to trim pistol cases to obtain consistent OAL. I have no desire to trim pistol cases.
All of the above discussion relates to rimless pistol cases. It does not apply to rifle cases…they are a whole different kettle of fish.