Now that I have a fairly decent 100 yard zero for the Israeli Mauser, it's time to develop a load that will tighten up the groups. One of the first things I want to do is determine the OAL (overall-length) of a bullet seated into the lands, and then back off of that number by about .020 or .015" or so. So what does this mean, and why might it help the Israeli Mauser shoot more consistently?
OAL (overall-length) is the length of a loaded cartridge, measured from the base of the cartridge to the tip of the bullet. Experience has taught rifle shooters that significant accuracy gains can be had by playing with this measurement. Some say best accuracy arrives from seating the bullets so that they just touch the lands (the part of the chamber where the rifling of the barrel begins). Some say say to back off from that distance .020 to .040 through trial and error. Some get accuracy gains by jamming the bullet slightly into the lands. In any case, .020 off the lands is a good place to start. The reasons for this gain in performance are argued and debated, with several competing theories, although nobody argues that it usually works.
The trick is to measure the OAL for a bullet touching the lands. Every bullet style is different, and every rifle's chamber is cut a little different, and wears a little different over time. There are a number of techniques for arriving at this measurement, but the easiest is to use the Hornady OAL gauge with a modified case. Essentially, on the end of the gauge is a modified case with a wide mouth so that a bullet can slide in and out. At the other end of the gauge is a rod you push to advance the bullet, and knob to lock the rod once you feel the bullet touch the lands. You put the gauge in your rifle's chamber, push the bullet forward until you feel it touch the lands, lock the rod, then measure. Simple. Use several bullets from your lot for a decent sample size, and use a bullet comparator to measure off the ogive, and you get a very good idea of how long you can seat a particular brand of bullet in your rifle in order to touch the lands. All you need do then is adjust your bullet seating die to back off of that number as appropriate, and experiment from there.
(NOTE: this information is for experienced hand loaders...seating bullets into the lands will significantly raise chamber pressures and powder levels must be adjusted accordingly!)
So in the case of 155gr. (Sierra Match Kings) bullets I'm using in the Israeli Mauser, although the bullet manufacturer suggests 2.775 as the OAL, the bullets actually touch the lands at approximately 2.816+/-.001 based on my measurements (2.232+/-.001 with a comparator). Thus, I will try loading at 2.796 (roughly .021 longer than the manufacturer's recommendation).