Down the front of the bullet just a bit.

https://bartleinbarrels.com/calibers/ So this is a link for all of bartleins barrels land and groove dimensions.

We know that a 30 cal bullet is .308 in diameter and that the bearing length is measured at the full diameter of the bullet.

For a typical 30 cal barrel you will have a bore of .300 and a grove of .308. That means there is .008 of interference fit, or .004 tall lands (one on each side). So because the lands engage the bullet before its full diameter the actual place where the bullet hits the lands is actually a tiny bit in front of the actual bearing surface. If you measure the bearing length on the .308 datum we would measure where the lands hit on the .300 datum.

Now what can further complicate the issue is that the lands arent just a flat square 90 degree cut, they are angled in a bit like a ramp (lead angle). So you cant just necessarily add and subtract, there will be a bit of calculus as youre taking a parabolic bullet shape and mating it to what would be a linear angular introduction of the lands. Knowing exactly what the bullet has and exactly what your smiths reamer is would be required. For this reason the dummy bullet is the preferred method; you let the gunsmith do the math and fitting work and dont risk making an incorrect calculation or transfer of data. [IMG2=JSON]{"data-align":"none","data-size":"full","src":"http:\/\/www.navweaps.com\/Weapons\/Gun_Data_grooves_pic.jpg"}[/IMG2]

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http://riflebarrels.com/a-look-at-bu...throat-angles/