Headspace Vs Datum Line ????

[CatShooter]

Is that true and if so why wouldn't one want to purchase the Sinclair tool instead of the Hornady? Would appreciate your thoughts on this.

Because the Sinclair does not use any standard "datum" line to measure - it just measures in "it's own line". So the measurements have meaning to you - you can tell if your cases grew or the shoulders got set back 0.00X" but those numbers have no meaning to anyone else.

Which for most people, it is just fine - all most people need is relative measurements to what they are doing, and what changes are affected, not absolute numbers that will be sent to a gunsmith.

 

[BoydAllen]

If you measure using a tool that makes contact across the width of the shoulder, with an insert face that is angled to match the shoulder. The angles should match, to get a reliable measurement. on the other hand, with an insert that makes line contact, there is no such requirement. Besides, I bought my Stoney Point kit a long time before Sinclair came out with their tool ;-) It works, I like it, so I don't need to buy another.

 

[bigedp51]

Topic: Headspace Vs Datum Line ????

So who's datum do we use, the "standard" SAAMI datum or the "standard" Army datum? Could this mean all datums are "relative" to who decides where X marks the spot is.

All that matters is measuring your fired case at point "X" and how much shoulder bump at point "X" you need during sizing and not having excess head clearance.

Point "X" for the SAAMI 30-06 isn't the same point "X" that the military used for the M1 Garand.





Below is excess headspace created by a unscrupulous bartender moving the datum point of the glass. :

 

[BoydAllen]

If you do before and after sizing measurements with the same tool, it's all good. As to the selection of the insert, as long as the same one is used for both measurements, and it lands on the flat of the shoulder, not too near the point or neck, you should be fine.

 

[bigedp51]

All that matters is measuring your fired case at point "X" and how much shoulder bump at point "X" you need during sizing and not having excess head clearance.

If you do before and after sizing measurements with the same tool, it's all good.

One of us is being redundant, redundant by four minutes