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Measuring throat length / seating depth.

AManWearingAHat

Sergeant of the Hide
Full Member
Minuteman
Oct 10, 2019
271
255
What tools are yall using for measuring the throats in your rifles? I have a procedure outlined below that is from my days as a poor student that has worked very well for me so far, but has some slop built into it unfortunately. Specifically I want to be able to set seating depth more precisely.

What I have done previously.

1. Run a closely fitting cleaning rod down with a special flat threaded brass head that I fabricated down the barrel of the unloaded rifle to the bolt face. Place a piece of masking tape on the rod such that it straddles the plane of the muzzle. Use a razor blade to score a line parallel to the end of the muzzle.

2. Take bullet of choice, drop it in the chamber and jam it lightly into the rifling such that it sticks. Repeat the process above with the flat brass jig on the tip of the bullet.

3. Measure between these two lines. This gives me the maximum theoretical OAL from tip to base.

4. Subtract whatever jump I want (typically .015) from this theoretical maximum OAL seat the same bullet in a case at that OAL, measure with a comparator from Ogive to base, use that measurement to set seating dies in the future. Function check, check for magazine length, etc.


I recognize there is error in scoring the lines, measuring between the lines, and the headspace of the rifle, amount of jam etc. However this has produced good results for me, but I would like to move to something more repeatable and precise. What tool(s) would I need?
 
If you have the ability to spin a barrel off I have been using Mark Gordon's method and found it to be extremely repeatable. There is a video on YouTube Ill see if I can find it.
 
Either a stoney point/hornady tool, or this method



I actually used to do this but I would cut a split into the neck. I suppose the method I described above is actually similar to this, but with a bit of feel involved.

I will have to check out the Stony Point or Hornady tool.
 
There is this method as well. The other video shared is also interesting. I will probably do both just to compare results.

 
A friend of mine has been trying get me to measure the throat depth of my rifles for a more precise reloading measurement. I originally did not use this method due to lack of confidence in the measuring technique used. For example. With the Hornady tool, my .308 Winchester and and AMAX bullet the OAL come out to 2.9555. The reloading books have that round at a max C.O.L. of 2.800. A similar scenario occurred with my 77grn SMK for my AR so I stopped and packed up the tool. Is this normal to have larger throat measurements? Lastly, is it safe to reload and fire those rounds?
Thanks!
 
A friend of mine has been trying get me to measure the throat depth of my rifles for a more precise reloading measurement. I originally did not use this method due to lack of confidence in the measuring technique used. For example. With the Hornady tool, my .308 Winchester and and AMAX bullet the OAL come out to 2.9555. The reloading books have that round at a max C.O.L. of 2.800. A similar scenario occurred with my 77grn SMK for my AR so I stopped and packed up the tool. Is this normal to have larger throat measurements? Lastly, is it safe to reload and fire those rounds?
Thanks!
Yes.
Lots of chamber designs have the OAL a good bit shorter than the length where bullets jam into the throat.

jamming bullets is a great way to cause issues for target and hunting rifles.
Jump is good, it is your friend!
Embrace it!

Use the tool to get consistent bullet position and increments if doing a jump test.
 
I take a fully fire formed case that will hold a bullet but will move easily when inserted in the case neck. Run a tiny bead of super glue around inside of the neck. Set the bullet long and gently chamber. Taking no chance to let the glue get out of the case. Remove and measure after set up.