Regarding Brithunter and CSl's back-and-forth:
One is relative, and one is absolute. Relative can "work", but requires more knowledge and skill by the user. Absolute is 'easier' to use, but usually costs more money. It is not difficult to have the 'comparator' on a "Lock'n'Load" type device be EXACTLY the diameter of your specific rifle:
1) Slug the bore,
2) Ream the "comparator" to the dimension of the bore.
I do it all the time, and I have individual comparators for most of my individual rifles.
I'm always taken aback by reloaders of rifle cartridges that want to do things "fast" or take shortcuts because "that takes too long". Really? What's the hurry? (Don't start in with pistol and shotgun ammo. That's NOT what we're discussing here.)
Regarding Muir and other's magazine length comments:
Magazine length is not in my experience the MOST COMMON limiting factor to cartridge length, chamber length is. Of course there are exceptions; the most common ones are in semi-automatic and automatic rifles. In my experience, and certainly when I cut a chamber myself, the cartridge length limit is ALMOST always the chamber length. Again, there are exceptions, but there is NO reason to start with the assumption that magazine length will be the limiting dimension. Measure the chamber and measure the magazine. The shorter value determines the 'starting point.
With respect to "on the lands" for "accuracy" (precision actually):
The benchrest/competitive shooters have caused this approach to be far too widely accepted as generally "good". After years and years of fiddling with this concept, the reality is that selecting a seating depth that puts a bullet "on the lands" is far more likely to present problems than it is to improve precision. CAN it improve precision? Certainly. WILL it improve precision in a over-the-counter stalking rifle? More likely than not, "no". There are many reasons and I'm not going to go into them here. More important is the potential for problems caused by trying to set seating depth "on the lands".
There are very real problems with trying to get your COLs to be "on the lands". The most significant of which is that SMALL errors, caused by component variation or lack of attention to detail by the reloader, can lead to serious problems, the most significant of which are 1) "jamming" the bullet into the lands causing SERIOUSLY increased chamber pressures, and 2) having a SLIGHTLY elongated neck get 'squeezed' in the leade, resulting in SERIOUSLY increased chamber pressures. The potential for "bad" usually exceeds the TRUE potential for "good" when trying to get the COL to be "on the lands".
With the advent of the "on the lands" concept for jacketed bullets, ballyhooed by ignorant or dishonest gunwriters (ptooey), 'regular Joes' forgot (or were mislead) that MANY rifles and cartridges did "just fine" when the bullets weren't seated to be on the lands. In point of fact, for those that are paying attention, there is a COL "sweet spot" that occurs somewhere between 0.030" and 0.150" OFF the lands. That 0.120" window is pretty large, and leaves LOTS of room to experiment.
All of the above noted, it should also be noted that the "INTO the lands" concept is one promoted by CAST BULLET shooters for more than a century. BUT... cast bullets are NOT jacketed bullets, and the differences are almost 'night and day'. Practices and concepts between the two do not overlap except at the most general levels.
Finally - Here is a general step-by-step sequence for locating your rifle's precision "sweet spot" for a given bullet.
1) Determine the MAX length your rifle can "handle" - be that limited by magazine or chamber.
. . . . .a) If limited by the magazine, determine how far back from the lands the "magazine-limited" length is,
. . . . .b) If chamber-length limited, note the actual chamber length,
2) Starting at about 15 thousandths from the actual chamber length, (or as close as you can get if magazine-limited), load 3 or 4 or 5 rounds at progressively decreasing lengths in increments of 0.030" until you get to 0.150" short of the actual chamber length.
3) I promise you that barring some truly extraordinary circumstances, ONE of those loads will produce a smaller group than the others.
4) Turn your focus to 'fine tuning' THAT load by shortening the COL increments to 0.010" increments (0.005" if you really think your equipment will allow you that level of REPRODUCABLE precision). Once you have found THE precision "sweetspot", you can, if you must, fiddle with charge. I would recommend against doing that unless your best groups are still larger than you are comfortable with in a stalking rifle.
This method has 'worked' for me for decades, and it will 'work' for you if you use it.
This particular element of reloading is, to me, identical to the element of reloading called "sighting your 'scope in". There is a very simple, very straight-forward procedure for sighting in 'scopes that WORKS and works EVERY TIME if followed correctly. Yet, I still see people wasting boxes and boxes of ammunition because they either don't know the procedure, or they are in too much of a hurry to perform it properly. If the procedure outlined above is employed, it is the VERY odd 'scoped hunting rifle that will not shoot 3 or 4 shots into a 4-cm circle at 100m.
Best of luck,
Paul