Measuring where lands start?

Thomas Guptill

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I have read in a number of reloading forums with regard to bullet seating depth, COAL , and the effects on accuracy, that some loads/bullets do better with a little "jump" before they engage the rifling. I see figures like -0.003 or -0.006 etc. I get exactly what they are saying so my question is this: how do you measure exactly where the lands/grooves start on a barrel?
 
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Its a controversial subject.
Some long range shooters claim that the bullet needs to engage the rifling so that it starts out inline with the barrel. (concentricity checks first)
Other long range shooters claim they need the "jump" your talking about.
Engaging the rifling can cause some pressure issues, so be aware if you go that way.

Personally, on my long range hunting hunting reloads, I engage the rifling slightly. The way I did this, is to use some Dykem bluing spray to coat the bullet, and start loading different seating lengths until I got rifling marks. Kept playing with it until there was just some slight marks, and locked in the dies.

You don't want to do this with auto loaders. Just bolt guns or single shots.
 
The poor man's version take a sized case and cut a slot or 2 in the neck. Place the bullet you want to use in the case but leave it long. place this in the chamber and carefully close your action. Open the action and carefully remove the case and bullet. it should now be exactly the length so the bullet ogive touches the lands. Measure the COAL and subtract the amount you want the bullet to "jump" and you will have the COAL that you want to load to.If the bullet sticks you have too many slots in the case. You will also need to do this for every different bullet that you use as they all engage the rifling at a different place along their length.
 
threegungeezer said:
You will also need to do this for every different bullet that you use as they all engage the rifling at a different place along their length.

yes, and that brings up twist issue for the bullets.
 
Thanks for the info guys! 3gun that is what I was thinking. Build my own jig and then at least I will know the exact measurement (coal) for that bullet. I don't plan to play with it too much, but my engineering mind just had to know. It's all part of the puzzle in making that 1km shot! :-)
 
The easiest way to measure what you are trying to find is witht he Hornady overall length gauge. You can buy the casings to go with it or drill and tap a spent case. Allows you to measure the distance to the lands and then set your bullets to the depth that you desire. As has been said before its pretty easy to do in a bolt gun but not generally possible in a semi-auto.
 
PapaT said:
99m, it's for a M/N M91/30.
Are you sure it has rifling to engage the bullet?



J/K. I thought you mentioned 3 gun, but it looks like you were using it to abbreviate threegungeezer's name. I thought we were talking about an autoloader.
 
Here is something else you can do -

Take a fired case and size about 1/16" of the case neck. Take a flat base bullet and seat it into the case, point first. Chamber this round and when the base of the bullets base contacts the rifling, the bullet will be pushed deeper into the case. After the case has been fully chambered, remove the round and measure how much bullet is sticking out of the case and this will be the exact distance to the rifling....

David
 
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