What is the method of determining how far from the case head your rifling engagement is?
Not quite as accurate as a bullet comparator, but this method works very well, even though it uses OAL measured from the bullet tip.
This is the easiest and most accurate way I've used and/or read about:
- insert a 30" length of 1/4" or 5/16” so dowel into your muzzle (of course, use 1/8" or 5/32" for .223);
- close and lock bolt;
- push the dowel against bolt face and hold it there while marking the dowel precisely at the muzzle (use a razor blade);
- now remove the bolt from the receiver;
- take another length of 1/4" dowel, say about 12" long, and hot glue your bullet onto the end;
- insert the dowel/bullet through the action, into the chamber, and against the lands;
- holding pressure against the dowel/bullet, push the long length of dowel, from the muzzle, in until it touches the bullet tip;
- holding it there, make another mark on the long dowel with the razor blade;
- measure between the marks. This is your maximum COL (OAL) for that particular make, weight, and type of bullet.
You can repeat this procedure for each bullet make/type/weight that you plan to reload. Then you can deduct .010, .020, .030, or whatever to come off the lands. It takes about 3 minutes per bullet type to get an accurate measurement. I do them all, then mark the inside of the bullet boxes with this maximum COL. Even when I don;t load them even close to that measurement, at least I always know what my maximum is, so when I go to extending the seating depth looking for accuracy, I know how close to the lands I'm getting.
The reason you have to do each different make bullet is because, while the bullet ogive is registering against the lands, you are measuring to the bullet's tip. Again, the bullet comparator is more accurate, because it registers the lands against the ogive, BUT! in order to attain that accuracy, you must, when seating the bullet, check each bullet against the comparator each time. Since the bullet tips will vary, this will be a back and forth trial and error process for each loaded bullet. For me, the variances in the bullet tips don;t give an error factor large enough to justify the time of using the comparator. Now, if you're shooting in a competition match, well.......