Confused w/OAL measurement - help?

[justpete]

Got a 308 built with an Obermeyer Match chamber and would like to reload for it. First time to reload, too.

I've set up the seater to match FGMM using a Hornady LNL comparator as well as a Sinclair to measure. Ought to be good enough but no idea what the jump really is.

The Sinclair OAL tool returns a number smaller than factory ammo yields with the same comparator. Doesn't make any sense. I used a JGS Go Gage in place of a fired case as I supplied a JGS reamer for the work.

Blacked a live round and chambered it, with the firing pin removed, and there's no rifling contact so the measurement has to be wrong, maybe it's from using the Go Gage.

Thinking the Hornady OAL tool might be worth a try but there's no info on their site about the case dimensions, if they're at zero headspace for instance.

The chamber's supposed to be cut to minimum dimensions but I haven't stripped the bolt and ran cases through the sizing die until the bolt just closes. Maybe that's the best way? Could dremel a slot in the neck and use the bolt's camming to seat a bullet on the lands? Dunno, maybe it'd stick in the throat. Any help would be appreciated.

I've searched through a number of forums here and there and while there're a number of suggestions to determine the right jump dimension I'm still lost as to why the Sinclair tool didn't return a reasonable result even though the measurements were repeatable to a thousandth.

Thanks,
Pete

 

[Obtunded]

While there may be others that find the method crude, you've already touched on what I use... I have a re-sized case that I slice down the neck with a dremel, polish the cuts and close the bolt on that case with a bullet stuck in the end. If you're gentle, it should not stick in the bore.

I do this three or four times and measure the COAL each time to ensure consistency.

My assumption is that you are at the lands. From there, I load develop using that measurment as the basis for seating longer or shorter. When I have the seating depth that works, that is what matters, not necessarily the absolute numbers themselves.

 

[reloader243]

I use the Stoney Point, now Hornady COAL guage, works great for me. Measure any bullet I will use in each rifle and log it. But I agree the spit the case to hold the bullet is a plenty good way as well. I just like to know the actual measuerment of the jump that works in my barrel. Still an old engine builder, lol.
Good luck on your reloading.