Relationship of Bumping Shoulders (Headspace) to Land Seating

[dariof]

Last night a buddy and I spent a couple hours discussing this, and we are not sure of the correct answer. SO, once again I am turning to the folks here for some help.

I understand how to figure out bullet seating/ogive in relationship to the lands. I have a comparator, and finally know how to use it.

BUT, here is my question.

1. Do you use an unsized cartridge fired from your rifle (place a projectile into the neck of the cartridge and push the cartridge into the chamber to measure to the lands) OR

2. do you first size the fired cartridge (bump the shoulder for correct headspace) then place a projectile into the neck of the cartridge and push the cartridge into the chamber to measure to the lands.

So, basically, to bump first or not.........


Keep in mind with either method I know there has to be some tension on the neck to hold the bullet and to put a couple slits into the neck so the bullet can slide once it touches the lands.

 

[Graham]

Relationship of Bumping Shoulders (Headspace) to Land Seating

The dimension you want is case head to ogive.

I take an initial recorded measurement (for the rifle) using a once-fired case, but then measure the resized rounds with a new primer (for the batch of loaded ammo).

Your datum measurement only tells you where the shoulder is, which - one would think - shouldn't affect case head to ogive OAL unless you don't bump enough and your cases are too long for the chamber.

 

[thrusty]

I use the Hornady LnL OAL measuring tool...which requires a special case for your caliber. So, I guess to answer your question, I don't think it matters whether you first bump or not because you are taking your measurement for OAL from the case head to the bullet tip OR the ogive.

In other words....an inch is an inch whether you are measuring a fresh turd or a polished turd.

 

[shoot4fun]

It makes no difference if the case is bumped or not. I get the best measurements with the Hornady LNL measurement tool and modified case. Usually you will get more variation in seating from two sources, variation in bullet ogive length and inconsistent loading technique, than other factor.

 

[colscar]

i think you would need to resize it as it need to be checked as it were a new round... i would thing that a fired case might have inconsistancies when measuring like the brass expanding more on some than others then again this is my understanding... check it as if you were about to fire it or get the hornady tool some of the guys have mantioned.. which i think the modified case is a sized case aswell... it must be so it can fit diffrent rifles...

 

[Killswitch Engage]

im odd man i suppose. i use a piece of fired brass from my gun and drill and tap for my own chamber specific modified case.

 

[shoot4fun]

im odd man i suppose. i use a piece of fired brass from my gun and drill and tap for my own chamber specific modified case.

You are odd but not odd in this respect
I have a few cases that were modified at home too.

 

[MitchAlsup]

With each of my rifles, I keep three cases. All three are within 0.00015 in base-to-shoulder. One closes with no pressure on the bolt, one closes with some pressure on the bolt, one won't close without unreasonable force on the bolt.

I use the shortest one when setting up the body die, and use the mid-sized one when setting up the RCBE case micrometer (relative zero-point). This allows me to control base-to-shoulder to about 0.002" and base-to-ogive to about 0.003".

So to answer you question: you get better accuracy if you carefully select the case used a) mesure bolt-to-lands in the chamber, and then b) to set base-to-ogive on the seater.

 

[ptaylortx]

I make my own modified case. I sacrifice a sized, ready to load case and split the neck with a cut off dremel wheel, then drill and tap the primer pocket. This gives it enough neck tension to hold the bullet exactly to the lands without slipping, but allows you to easily press it in with the bolt. I push it out with a piece of 1/4-20 all thread and the primer pocket.

 

[colscar]

I make my own modified case. I sacrifice a sized, ready to load case and split the neck with a cut off dremel wheel, then drill and tap the primer pocket. This gives it enough neck tension to hold the bullet exactly to the lands without slipping, but allows you to easily press it in with the bolt. I push it out with a piece of 1/4-20 all thread and the primer pocket.....

What drill and tap did you use to make that case

 

[ptaylortx]

Number 7 drill and a 1/4-20 tap

 

[bruddah]

With each of my rifles, I keep three cases. All three are within 0.00015 in base-to-shoulder.

You sure about that measurement? Maybe you mean 0.0015" Unless you have a gage that will measure out to ten thousandths of an inch.

 

[ShtrRdy]

I don't know if this has anything to do with what you're asking but your question got me thinkin'. So, if the shoulder is pushed back by different amounts, it seems that the bullet jump would end up at different amounts assuming the case was located in the chamber against the taper in the cartridge.

 

[MitchAlsup]

You sure about that measurement? Maybe you mean 0.0015" Unless you have a gage that will measure out to ten thousandths of an inch.

Your are correct, one to many zeros.....argh

 

[dariof]

I don't know if this has anything to do with what you're asking but your question got me thinkin'. So, if the shoulder is pushed back by different amounts, it seems that the bullet jump would end up at different amounts assuming the case was located in the chamber against the taper in the cartridge.

Yes.....that's another way of asking the question......maybe more answers to follow.

 

[bruddah]

Yes.....that's another way of asking the question......maybe more answers to follow.

It is the bolt face that pushes the cartridge into place.