clearance question / jump-jam

[Ned Ludd]

Regardless of the method used, our initial measurements of the distance to "touching" the lands for a loaded round are going to be relative values. That is, they may vary from the true value by as much as +/- .003" (or more). Nonetheless, the value we obtain serves as a good reference or starting point for seating depth.

From that point on, we determine seating depth using cartridge base-to-ogive measurements taken directly from the loaded rounds we will use to empirically determine optimal seating depth. Direct measurements of loaded round CBTO is markedly more accurate/precise than measurements of the distance to "touching". Therefore, the seating depth of loaded rounds can also be reproduced with very good accuracy/precision.

So, the seating depth of loaded rounds has been measured directly, and those rounds are then used to empirically determine optimal seating depth on the target. The only way that movement of a loaded round inside the chamber could impact our seating depth measurements would be if the movement was non-uniform from shot to shot AND was of a magnitude equal to, or greater than, the increment we used during seating depth testing. For example, I typically move shoulders back by about .0015" during resizing. I test seating depth in .003" increments. Even if case movement inside the chamber was non-uniform from shot-to-shot, the movement should generally be much less than a single seating depth increment, meaning it is unlikely you would ever see it unless you happened to be right on the very edge of an optimal seating depth window. I try not to do that.

 

[cw308]

When finding my closed bolt to ogive measurement using the Wheeler method I use a case with zero headspace , there is no movement in the case shoulder to the ogive is the as close to exact as possible , no wiggle room , from there you can get a exact jump , jam or touch measurement .

 

[Dusty Stevens]

And if you come back the next day and get the exact same measurement id say its a pretty good method. Benchresters have been using this method for many years and its even repeatable on other barrels if your gunsmith is good

 

[JMayo]

I may be off here but I'm thinking about the half to 1k difference in sizing ....
neck/shoulder junction stays constant. Mid Shoulder (lacking a way to describe it) blows out to chamber size, this is what I'm decreasing when I'm bumping shoulder back along with body of the case.
The measurement from base to neck/shoulder junction stays the same.
Thus my in or out of the lands measurement is correct.

The dirty neck line also tells me this.

 

[Steve Ladino]

If you do the stripped bolt method (Wheeler) of seating depth measurement be sure to lightly hold the bolt rearward on the extraction so not to be getting a false reading because you are pushing it forward into the lands.. If you fall out of the chair do it four more times.. Find a measurement that repeats and record it, tune from there to what the gun wants.. Record that measurement and call it a day..

Ray

Your method includes whatever the bump is. I will wear a seat belt while i'm at it, thanks

 

[searcher]

I believe the general consensus is that depth DOES matter to attain the best sweet spot for a given load. To your question - take a .010" shoulder clearance - like a lot of AR-15's have, for example. When reloading accurate ammo - drawing that clearance down to .002" to .003" will, in almost all A/R's - make ANY load you are shooting behave more consistently. That might be consistently good or bad - but more consistent. Target rifles like this and more. Using Raythemanroe's cited "stripped bolt" method for attaining proper brass clearance will help you better judge the brass fitting the chamber. With the stripped bolt - the brass fits when you can feel just the slightest resistance just before the bolt closes all the way - then adjust your sizing or bump die down ever so slightly (.001" or less increments) until the resistance disappears. Eliminating inconsistencies is the name of the game - whether it is powder volume, length, primer seating, bullet weight, trim length or whatever. They all matter most of the time.

 

[WIB3]

I will clarify, in 1000 yd benchrest the goal is to create 10 record rounds that exit the barrel at the same FPS. Theoretically they should all hit at the exact same place. We all know that that is impossible. I measure my headspace on a granite stand using a Wilson case gauge and a .0001 dial indicator. The bullet seating due is also capable of this accuracy. My record rounds are determined by my hydro seating press that is capable of reading ounces. Bullets are weighted and so are primers on a scale capable of reading.0001 gr. Bushings are purchased and polished to measure the pressure ring on every lot of Bullets. All of this affects the velocity of the bullet. The more anal you are the tighter your groups will be.

 

[Dusty Stevens]

I will clarify, in 1000 yd benchrest the goal is to create 10 record rounds that exit the barrel at the same FPS. Theoretically they should all hit at the exact same place. We all know that that is impossible. I measure my headspace on a granite stand using a Wilson case gauge and a .0001 dial indicator. The bullet seating due is also capable of this accuracy. My record rounds are determined by my hydro seating press that is capable of reading ounces. Bullets are weighted and so are primers on a scale capable of reading.0001 gr. Bushings are purchased and polished to measure the pressure ring on every lot of Bullets. All of this affects the velocity of the bullet. The more anal you are the tighter your groups will be.

If they all leave the barrel at the same fps but your scope shifted, your firing pin dragged or your sand shifted causing weird vibrations, or anything else caused anything out of the ordinary such as a wind shift at 400yds then that perfect es doesnt matter much.

 

[KenOath]

In benchrest shooting ,you need to be able to measure two things accurately. One being seating depth and the second is headspace. When I say measure I mean to the .0001 of an inch. That should be you goal. Get 10 rounds as close to that measurement as possible. You will see your groups shrink.

Wow .0001” of an inch is a very difficult number for me to measure with any repeatable accuracy...

 

[JMayo]

Do it all, try it all ...keep shooting.
Every dog has his day !