Saturday, July 09, 2011

Jumped or Jammed?

Jumped or jammed? That's the question that handloaders often ask, and while there are no steadfast rules, it's a question that every rifle will answer if you let it. The sad truth is that each rifle is different, even two made side-by-side on the assembly line. Each bullet is different, with varying lengths and ogive shapes, so there are no steadfast rules.

SAAMI, the Sporting Arms and Ammunition Manufacturers Institute, realizes this and have published standard specifications for firearms and ammunition. It's a one-size-fits-all approach and guarantees that the ammunition you buy will fit the firearm you have and will be safe when used properly. However, that ammunition is a standard length and your rifle might not be standard length. If you set the bullet out a little bit, you might get better accuracy, but you won't know until you take some measurements and shoot some ammo.

There are all kinds of ways to spend money in this hobby. You can make chamber casts and you can buy comparators, indeed, there are plenty of ways to make the measurements you need. We frugal types only need to know how far it is from the bolt face to the point where the bullet is jammed into the rifling. You've got your rifle, and an empty case or two, and hopefully a box of bullets.

Find a case where the bullet is a tight slip fit and insert a bullet by hand into the cartridge mouth. Then carefully chamber that dummy cartridge. You should feel resistance as the bullet hits the lands of the barrel. Lock the action, then carefully extract the cartridge. Sometimes the bullet will stick in the lands, but most often it will come out along with the cartridge. If the bullet sticks in the lands, tap it our with a cleaning rod from the muzzle. Most often, the bullet will simply be in the end of the casing, and you can then measure it with a caliper. Of course, you can click on the picture to enlarge it.

In my case, I had some 150 grain Bornady bullets and wanted to find the length for my Remington 700 in .308. If you look at the picture above, the dummy cartridge on the left is the jammed cartridge. The dummy cartridge on the right is the SAAMI length cartridge. As it turns out, my Remington must have the longest lede in the history of loose chambers. That cartridge measures 2.966, much longer than the SAAMI spec 2.800.

Indeed, with this particular bullet, there is very little in the case mouth, so little that I plucked it out with my fingers after the photo was taken. If I want the finished cartridge close to the lands, I'm going to have to use a heavier bullet, which is longer than the lighter bullet. Bullets in the 165-180 grain weights might be better for this particular rifle, as I can seat them closer to the rifling with enough bullet in the case mouth for good tension.

Decisions, decision. I've got a couple of boxes of 165 Gamekings on the shelf and my son's .30-06 really likes 150 grain bullets. I might save this box of Hornadys for his rifle.

1 comment:

Old NFO said...

That IS a long way to the lands... :-) You might need a 200gr just to get there!!!