DIY bullet CBTO tool

[Ganderite]

In my experience, it's not as picky as some would have us believe.
I experimented with a .243 Tikka a few years ago, increasing "jump" from contact to 0.120" in 0.020" increments. While the rifle did shoot better with the bullets (70 gr Sierra Match Kings) seated close to the lands, there was very little difference between contact and 0.040" jump. Once I got past 0.040", the groups opened up noticeably.
From what I've read, some bullets are more tolerant of the amount of jump than others.

This mirrors my experience. I find 10 to 30 thou accuracy is the same. I usually use 20.

But I also test a rifle/bullet with touching and 10 thou into the rifling, just to make sure which is best. Only once has a rifle/bullet preferred to be in the rifling.

Of course, if it is a hunting rifle, it is always 20 thou off.

 

[Paul_T]

Also there are some throats that are so long that you cannot get near the lands anyway. Tikka T3/T3x are know to be like this.

My T3x in 6.5x55 even with the bullets seated out at/near max on the mag they are nowhere near the lands. The long jump in this rifle does not affect accuracy (it would seem by the groups I have shot with it).

 

[Ganderite]

Remington rifles usually have very long throats. I think the company lawyers designed the chamber. It reduces pressures.

 

[MattE93]

The 8mm socket worked better, contacted about halfway between the 7mm and the cylinder near the cannelure.

My Remington has a very large throat for sure. At .02” off the lands my rounds just clear the front of the magazine so I have no option to go closer