Interesting

[GunnyMack]

Recently I have invested in some new reloading tools, Frankford universal seating die, and the Hornady OAL gauge. I also got the modified cases for the cartridges ( most of them anyway) I reload.
Finally a chance to tinker with the OAL gauge today.
Each of the modified cases is in a plastic blister pack and yes I fumbled the 308 case upon opening it and dented the mouth!
Reading the directions you stick a bullet into the case, slide into the chamber and push the bullet to contact the lands and lock it down. Got it. Bullet won't go in to case cuz some dummy dropped the case and then ran it into the seating die! D'oh!!
Oh what to do now? Light bulb( dim that it is) - take the ID of a fired case. Oh look, its. .312" or 5/16s. So I scrounge for a piece of 5/16s rod, into drill press and file a slight taper on the end and polish with 400grit at 3200rpm. It started into the case and a few taps of a hammer and it ironed the neck out.
So here is the interesting thing I found;
The jamb method, sized case with bullet just started into case, put into gun and close the bolt produced 2.948" length. The OAL gauge gave me 2.940".
The jamb method gives exactly where the lands are, gauge is off by .008... if I want to be .015, .020, .025 whatever off the lands the jamb method will be more accurate in theory because that is THE point ( until throat erodes more) of contact.
So if you use one of these gauges, check your jamb vs gauge lengths .
Of course I have a bit more fiddling to do with this gauge but I'm sure the jamb method is where I'll stake my claim.

[Walt]

I have found that the Hornady tool allows too much movement of the bullet; the neck doesn't have quite enough tension and so the bullet stops as it enters the leade instead of moving on to the rifling. If you insert the dummy Hornady round enough times you will find that the bullet will sometimes go all the way to the lands though most of the time it will be stopped by the leade. My method is to insert the bullet just short of the point that I need to tap it out with a cleaning rod. A dozen or so insertions should give a measurement that's very close and one that you can rely on to give an optimal COAL with a given bullet.