"If you have not already done so, you now need to acquire a length of 1/4"x24tpi (or 6mm x 1) threaded rod with 2 nuts that is long enough to reach from the muzzle right down to the block face when the action is closed, and still leave a bit sticking out of the muzzle for the two nuts. This is going to be used as a simple, but very effective "depth gauge" for checking head clearance (NOT headspace!) and establishing the seating depth and appropriate load for your cartridges. "
So let's find out how to use this home-made depth gauge! To make it a bit easier, I have gone through some of the steps with one of my own rifles. It is the Boer Mauser, which I selected because a new member on the Mauser forum has a problem that can be narrowed down by using this measurement technique, and I am using a bit of creative laziness and writing all this just once. Place the rifle on its side on a table. Cock the action (i.e. do not fire it - we do not want the firing pin to protrude) Insert the threaded rod until it touches the bolt face (block face for the RB) Run down the first nut until it just touches the muzzle, while keeping the rod in firm contact with the bolt/block face. Use one hand to hold the rod in contact, and run down the nut with the other.
Now, without disturbing the first nut, run down the second nut until it just touches the first. Make a mark with a felt-tipped pen on both nuts. You have now "zeroed" the depth gauge. I know this is a bit of a 3-hand operation, and I had to use one to take the pics. But with some practice you can develop a feel that lets you measure accurately enough for the present purpose. Now open the action (remove the bolt) and insert a bullet into the chamber. Use a rod, pencil or whatever to push the bullet into the chamber until it is touching the transition cone - the throat. This will push the threaded rod back so that the two nuts are now some way from the muzzle.
Now, without disturbing the second nut (further from the muzzle) run up the first nut until it just touches the muzzle again. This requires some practice, as you need to sense when the rod just starts to lift off the tip of the bullet in the throat. The distance that the nut has moved is the overall cartridge length (OAL) that would be required to have a bullet that just touches the lands before being fired. Now withdraw the rod some way, so that you can use calipers to measure the distance between the nuts. Clamp the caliper jaws and use the calipers as a gauge to see how your chosen bullet and cartridge case would match up. The results can be very illuminating!
Firstly, with the Sierra 168 gn boat-tail bullet As you can see, the base of the boat tail will lose contact before the tip of the bullet engages the lands. That means gas blow-by (leading to throat erosion) and the bullet tipping in the free-flight phase, being rammed into the throat and engraved on the skew, with the result that, even if you have a perfect bore and muzzle, it will go way off course and possibly keyhole.on the target.
Secondly, with the Sierra 140 gn flat-base "spitzer" type bullet. In this case, although the bullet is considerably lighter and shorter than the HBPT of the same weight, the fuller nose means that it touches the lands while about 1mm is still in the case neck. It will perform better than the boat-tail (which is why I purchased these bullets) but is still barely adequate (which is why I am going to try some that are even longer). The Norma 170 gn Vulkan, Hornady 154 gn or 175 gn Interlock, or Speer 160/175 gn Spitzers or magTips would be possible choices.
For nitro-powders you can back off this theoretical OAL by about 10 thou, and take that as a first-approximation ideal length. (Do NOT try for a much smaller value, or even zero, as bullet and seating tolerances will lead to a maybe/maybe not contact between bullet and throat, leading to unpredictable pressure variations etc). Of course, in this case it is impossible to get anywhere near the ideal length, as there must be sufficient bullet in the neck of the loaded cartridge (about 1 caliber length) to ensure reliable and consistent seating. For black-powder cartridges, the short, sharp transition cone usually makes it possible to use the ideal length, but you need to back off about 1 mm / 40 thou. This is necessary because fouling is always worst in the throat. If you seat the bullet too close to the lands, then the first shot will be OK, the second will require a good shove to get it to chamber, and the third will need a hammer.
Rolling Blocks are touchy in this respect, as there is no bolt camming action to help you. So give it some space, unless you want to give the bore a complete clean after every shot. The measured values for the
Argentine
chamber and the picture of the throat lead me to expect that the bullet will need to be seated with a good portion within the case neck. I would like to see the results of this measurement before proceeding to the actual loading of the cartridge. That's all for tonight folks! Patrick
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Normally they are at .307 but yours seems to be at the lower side of the manufacturing tolerances. I used to shoot Moly coated bullets which were going a little easier thru the bore.
Originally Posted by Patrick Chadwick
Now i´m a bit nasty!
