Headspace Datum Line?

[Trickymissfit]

Originally Posted by Ludicrous

Thanks Gary. I also base my calculations off fired brass from that specific chamber, if I have 50 fired cases I measure ALL of them and line them up in files, then whichever file has the most cases I base my shoulder setback on those. I also dislike trimming and measure my chamber lengths with the Sinclair gadget in a shortened case.

I'll measure up my fired cases with your OD neck vs. shoulder average and see where that puts the datum line for may cases.

your doing it right! Set up a medium to work off of, and base you settings off that. You'll be OK.
gary

[boomtube]

"I measure ALL of them and line them up in files, then whichever file has the most cases I base my shoulder setback on those."

I'd suggest you not do it that way, it ignores the fact that not all cases springback the same so you'll end up with those cases that are harder won't be sized quite enough. With a bolt rifle it will probably "work" but I believe it's better to use the longest fired case length, not the average, as a goal for resizing. Done that way we only need to restore the shoulders to match the longest fired length without any 'set back' at all; they are a tad shorter than chamber length anyway.

[Ludicrous]

I just have a hard time using the longest measurement. Example ( and it usually is about this way) I fire 50 and have 40 in stack "a", 8 short in stack "b" and 2 long in stack "c". I'll remeasure a few times and even use a second caliper. I can understand how some cases can be shorter but not the "longer than the chamber" ones. Maybe it's because we cam the bolt open quickly and eject them real hot. Who knows. You've shot thousands or hundreds of rounds in your rifle and know that measurement but a few long ones always pop up.

[Trickymissfit]

Originally Posted by Ludicrous

I just have a hard time using the longest measurement. Example ( and it usually is about this way) I fire 50 and have 40 in stack "a", 8 short in stack "b" and 2 long in stack "c". I'll remeasure a few times and even use a second caliper. I can understand how some cases can be shorter but not the "longer than the chamber" ones. Maybe it's because we cam the bolt open quickly and eject them real hot. Who knows. You've shot thousands or hundreds of rounds in your rifle and know that measurement but a few long ones always pop up.

keep this in mind:

* unless every loading is identical right down to barrel fouling; you'll see some difference.

* brass will vary from lot to lot (material not cases). This not just the brand name, but from one lot of ingots to the next lot. Ideally we'd have cases from the same cupola and the same pour, But that's a rare thing.

* assuming your cases are all the same brand, and were from similar lots; then maybe it's time to anneal all the cases.
gary