Measuring bump

T

tenring

Guest
I know how to bump shoulders, and how the bolt closure should feel with firing pin removed. But I have had problems getting accurate readings with Harrel's "thingy" and with a custom made chamber guage. What is the best possible method to measure the amount of bump?
 
tenring said:
I know how to bump shoulders, and how the bolt closure should feel with firing pin removed. But I have had problems getting accurate readings with Harrel's "thingy" and with a custom made chamber guage. What is the best possible method to measure the amount of bump?
Click to expand...
We just had this discussion. I can't remember the thread title, but the discussion was lively.

My opinion is that using the actual case, the actual chamber, and the actual bolt (stripped) is still the best method.

Think of the purpose of shoulder bumping. The purpose is to safely resize the brass to where the cartridge base is just contacting the bolt face with the cartridge case fully seated in the chamber and the bolt fully closed.

If the fit is too tight, the bolt will not close or will be hard enough to close as to disturb the gun in the bags. If the fit is too loose, way too loose, say 0.015" or more, there could be a safety issue primarily caused by the cartridge case being stretched near its base and over time allowing the case to separate during firing and releasing hot gases into the shooters eyes.
 
What problems are you having with the Harrell's measuring sleeve ? I find it totally repeatable if I make sure the primer [ or a crater ] is not high and I make sure to center the measuring sleeve and the case on the jaws of the caliper. I use a dial caliper.
 
Thingy

I agree with crb. I use the "thingy". I bump until i get a measurable difference - usually .001 using dial calipers.

You have to kind of wiggle the case as you measure to get a good square fit - with a little practice it becomes pretty easy.

You can do it with a stripped bolt ...but if you don't know what you are doing this can be way dangerous. If you have a FL die that doesn't size the base enough, and you keep running the die down until it closes easy, you may have bumped too far. And if you use the "thingy" it is still a good idea to double check with a stripped bolt afterwards.

so....if you wiggle your thingy...your shoulders will be bumped.


scott
 
Just make sure that your reference brass is deprimed and that it has been fired about three times in the chamber with a good stout load, so that it has reached the full size of the chamber. If you are trying to use the same die setting to size mixed brass, and you measure each case. You will notice variations that are caused by the differences in hardness of the cases. Differences in lube applied to each case will also cause variation, as well as dwell time at the top of the stroke.

The problem with the whole use the chamber idea is that it supposes that the die is small enough at the base to size that area without the die having to be adjusted so that the shoulder is bumped too far. If you have a die that is correct for your chamber, my point may be hard to fathom, but it is not a good assumption that the die of a questioner is correct for his chamber. I have friends that took a while to get that if the die is too large in the back, you cannot adjust your way out of it. You need another die.
 
Bump

As with all precision measuring procedures, it takes a certain amount of skill, (aquired through practice), to arrive at good readings.

If you are getting erratic readings, it is probably due to a flaw in your using of your tools. Afterall, thousands of shooters do this on a regular basis.

As was said, be sure the case shoulder is indeed seated square in the tool, 9thingy), and that your calipers are seated square as well.........jackie
 
I don't like to measure things like this with a round object just because of the issues when there's an irregularity in the shoulder surface.

However, with that said, what I find the easiest and simplest (read, cheap/reliable) is, get a drill bushing that has a diameter roughly 1/2 way between the shoulder and neck diameter. These will be hardened steel, and will have a pretty nice radius on the inside of one end. They're pretty straight and the ends are reasonably close for parallelizm. Drop it over the shoulder, and measure base to end of bushing with a caliper.

Using this method, you really see shoulder bump, and not change in the radius at the neck or shoulder junctions instead.
 
Tenring, for the most part I make my gages using tools that exist, the tools are not commercial and are cheap. I do not 'BUMP', I leave the shoulder where it is or control the length of the case from the shoulder to head of the case by adjusting the gap between the shell holder and bottom of the die with a feeler gage. If the die is adjusted down to and contacting the shell holder with the ram up, the die is adjusted to full length size cases that are -.005 below a go-gage, for a rifle that (just)closes on a go-gage. If I know the head space, I can adjust the gap with a feeler gage to control moving the shoulder back on cases that have been fired to conform to the chamber.
Purchasing once fired cases from a range gives me cases that have been fired in many different chambers, another home make tool aids in sorting through cases of different length (shoulder to head of case). The 'DATUM' on a 30/06 is .375, the datum is not a line but a hole that measures 3/8 of an inch in diameter,

I have an Eddystone M 1917 that stretches cases .016, I am told to fix or get it fixed, when I size the cases for that rifle, I adjust the gap between the shell holder and die with a gap of .015, life would have been more simple had the threads on the die been cut 10 to the inch, that way 360 degree turn of the die would be .100, but 14 threads per inch makes it difficult to make a degree wheel where each turn equals .071428571, so I use a feeler gage I believe Hood uses a degree index.

Using a feeler gage is repeatable, I have used dies/shell holders that came from other hand loaders, some have been ground to to increase the amount of sizing for short chambers, with a feeler gage, that is not necessary and the method preserves the integrity of the die/shell holder.

F. Guffey
 
shoulder bump

I always deprime prior to measuring. Dwell time at top of the stroke is a new one. Not sure how this affects bump, excepting the amount of pressure exerted at top of the stroke. I try not to wiggle my thingy, but still find it difficult to make repeatable readings with my Mitutoyo caliper. As Jackie suggests, it probably does take practice to do this accurately, and something I need to work on. I usually make two or three readings on the same shoulder, and frequently, there are variances. I really like Guffy's idea of a feeler guage against the shellholder. However, before and after that procedure, I would like to measure the amount of bump, to know how far back I have pushed it. I just think there is not a good tool out there to make the measurement reliably. This would be a good opportunity area for one of the great benchrest machinists who are out there. Thanks to all.
 
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