The "Manuals" are a good starting point, but as I posted in an earlier entry, even the manuals occasionally jam things into the lands.
The dice are loaded in your favor to be sure using "the books", but you still roll them vis-a-vis what you actual rifle's throat might be cut to.
I have always been a bit leery of the slip`n slide method shown in the U-Tube film (although it's
a pretty slick production
). Reason being that I could not repeatedly control either
(a) how much the bullet impinged against the lands, nor ...
(b) how much the cartridge extraction might pull the bullet out before it freed itself from the lands.
For a long time I used the "smoked bullet" technique, which was "successful" to my youthful eyes
but still left things open to interpretation. Then I got the Honady/Stoney Point gauge and never
looked back.
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But
lacking the Hornady/Stoneypoint gauge, there is another way to approximate your maximum
cartridge/bullet length. It uses a cleaning rod and is only moderately less precise. It's used this way:
1. Take a wooden dowel and gently press a bullet (only) into the chamber until it stops up against the rifling. Insert a plain-base cleaning rod (no jag and with any socket securely plugged/taped) down the barrel until it meets bullet.
2. Rock the bullet back & forth a bit with one hand on the rod and one on the dowel until you're comfortable it's gently seated but not 'jammed' into the rifling
3. With a razor blade, mark the cleaning rod exactly at barrel's end. This is your reference point for zero clearance.
4. Pull the rod out and mark another reference point 30 thousandths further out on the rod. (In case you're wondering, that's about the diameter of a standard office 1-1/4" paper clip wire.)
http://www.whimsie.com/gauge wire.html
5. Seat a dummy cartridge/bullet (OAL 3.30" to start) in the chamber and use the cleaning rod to see where the bullet tip is relative to the mark. If the rod is pushed out past the zero-clearance mark, the bullet is jammed into the rifling.
6. (Assuming the cartridge OAL is too long) gradually seat the bullet further into/out of the case until the rod's second (paper-clip) mark is at the muzzle. You've now got 30 thousandths (approximately) standoff from the lands. Measure the cartridge OAL and that's where you stay (for that bullet) until you get a Hornady gauge (and the modified case it requires).