That is true, its the point where the fattest part of the bullet (called ogive) first touches the lands in the bore. Jump means the distance the bullet goes before hitting the bore, and jam means how far touching the bore the bullet is seated.
Bearing surface: The length of the bullet jacket which actually is engraved as the bullet travels thru the bore.
So you can't really use a bullet comparator to measure this distance? The reason I ask is because I'm trying to get the 180 grain Ballistic Tips to shoot in my .308 Win. I so far haven't had any luck. I thought I might try using a bullet comparator and set it to the same distance as my 175 smk loads (which shoot great). I haven't had any luck so far with varying the powder load.
If this were true you could not easily 'Jam' a tangent ogive with a common 1.5deg leade angle, as these ogives contact nearest the groove(secants contact nearest the bore). In otherwords, anyone would have to fully engrave a tangent ogive to call it jamed, and I have never heard of anyone doing this(it would take huge neck tension to engrave rifling all the way to bore point here). Seems like I'm misunderstanding something here...
Ed, you could use a single comparator to compare them -if the ogives were the same type and radius. But SMKs won't likely match your ballistic tips. You really have to start over with new seating tests.