Do gunsmiths utilize any special gauges or measurement tools to determine throat length in rifle barrels?
I ask because I have tried to determine the distance to the lands in my Ruger m77 in 7mm rem mag to get a seating depth starting point with a cut cartridge neck and bullet in a deprimed case.I seldom get a consistent measurement.

The rifle is going to a smith for some unrelated work.Could he measure this more effectively?

Turnerhunter,

If I need to determine the length of a throat, I will usually do one of two things, either pour a cast and measure that, or use a Stoney point type guage. Both are nondestructive means of measuring a throat.

Malm

Good advice from malm. The reason you're not getting consistant measurements is, that the bullet is, more than likely, getting pulled out of the case slightly when you withdraw the bolt, by being stuck in the rifling. I use a fired case and bend the neck slightly. The amount of drag has to be just right so that the rifling will seat the bullet and then not be able to pull it back out. This is a crush fit, so you'll need to add a little extra depth to the seating to make up for it. In my mind, I'm not looking for scientific measurements, just a reasonable starting point. I suppose if I had a benchrest gun, paying for the measurement tools would be worth it. But I enjoy the trial & error of shooting different loads to find out which is best. It hones my skills, while finding the best load in the process, ie. it's fun. But, a stoney point gauge would give you exact measurements.

Turnerhunter,

Bobby just jogged my memory with his good post. There is one more way to measure the length of the throat which I failed to mention. Seat a "flat base" bullet, nose first, into the neck of a sized case and close the bolt. Theoretically, as you close the bolt, the flat base of the bullet will stop at or near the end of the throat giving you a pretty good idea of where the end of the throat lies and therefore the throat length.

Keep this "dummy", which is a fair representation of the throat length, for future references when determining how far to seat a bullet off the lands. How would you do this you might ask? Simple. Take a bullet you wish to load, and place it "nose first" into the muzzle end of the weapon until it stops. Lightly twist the bullet at the muzzle to make a mark. This mark you just made, represents the bullets "Ogive" and when laid beside the "throat dummy", should provide, by comparing the "ogive" mark on the bullet to the end of the flat base bullet, a quick visual reference for determining how much jump you will have based on how deep you seat that particular bullet.

I hope this makes sense and doesn't confuse you. If so, let me know and I will try and explain it better.

Good luck,

Malm

[ 09-17-2003, 00:35: Message edited by: G.Malmborg ]