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SINCLAIR BULLET SEATING DEPTH TOOL | Sinclair Intl
I like this tool and you will need to watch their video. It is very helpful.
The main goal is safety. The measurement COAL is very deceiving due to the curvature of the bullets we use in rifles. Since we can not climb in the chamber and measure from the bolt face to the point of the rifling that touches a curved surface of the bullet, we must use a COMPARATIVE measurement.
This can be done with the above instrument, with a Hornady OAL gauge, a RCBS CASE MICROMETER ( they don't advertise a .260) or the old fashioned way of using a bullet just seated into the neck of a resized case.
All of the above will need to be compared to SAAMI specs and carefully approached.
Hopefully this will point you in the right direction to get more info. Buy a reloading manual and read it and then you can get a more experienced reloader to check you out in person with measuring the length to the rifling.
I like to start with a box of factory bullets and use a bullet comparator to measure the ogive and then keep ten to compare to your reloads. I usually shoot the other ten factory loads and then measure the used cases to determine the headspace and use them to set up my full length resizer.
All of this will require some practice and supervision. With a little help you can do it.