Shoulder bump and comparator readings

[Alex Wheeler]

A lot of guys are having issues with comparators. Measuring excessive shoulder bump to get the brass sized for a smooth bolt close. The main cause is a comparator that contacts at the shoulder/body or neck/shoulder junction. Dies reduce the diameter of the neck and body. When the comparator contacts at these areas you will measure that diameter reduction as shoulder setback, when no shoulder setback has actually happened. Spin the case in the comparator so it leaves a shiny ring on the case so you can see where its touching, you want it in the middle of the shoulder, if its at the shoulder/body or neck/shoulder junction and your having to bump the case more than .001-.002" this may be your issue. The little comparators that come with some dies are a big culprit.

 

[Ccrider]

Neat. So you don’t want it touching where the shoulder breaks to the body. This may explain why I am having some problems with bolt click even though I am setting the break of the shoulder back.

 

[dogdude]

Alex, I'm betting your talking about Harrels, tried four..none left a mark where your talking about, I magic markered them NONE left a mark near where it should be...my Whidden was spot on. The WSM my smith did for me with my reamer @ the edge of the shoulder
Thank You very much for this info Alex

 

[mikeeg02]

Its also likely that the 40* angle that the tool maker made on the reamer may not be the exact 40* on the sizing die, specially on mass produced dies. Since they both have their own tolerances(reamer prints say right on them what their tolerances are).

So in order for the whole shoulder not to touch (smooth bolt close), it may require more bump than where the comparator is touching the case. You're better off checking cases in the chamber than with any measuring device IMO. You can measure it after you sized it to fit the chamber and whatever that measurement is you can shoot for, but somethings just dont have to be measured. They need to fit. (Obviously this does not condone gross over sizing)

 

[Ledd Slinger]

Very true. Good input Alex.
I use the Innovative Technologies headspace gauge. The universal shoulder jaw can be adjusted to make contact on the shoulder of any case anywhere you desire and it only makes a very small amount of contact on the shoulder surface area. I always set it to contact right in the middle. The spring tension of the digital indicator arm always exerts the same amount of pressure on each case as well. Works very good for getting very accurate and repeatable readings on should bump measurements. I spin the case to find the lowest measurement. So repeatable that I can get the same reading on the same case removing and reinstalling it a hundred times.

Dont forget to deprime the cases before measuring!

 

[ridgeway]

Make a stub from the barrel drop. Run the chamber reamer in just shy of body/shoulder junction. Now you have a tool that has the exact shoulder of the chamber.

 

[Ccrider]

@Ledd Slinger, I have and use the same tool. It is surprisingly accurate. My problem is that at the shoulder break, .001 is not enough to get a clean bolt lift. I will take Alex’s advice and measure closer to the middle.

 

[Greyfox]

@Ledd Slinger, I have and use the same tool. It is surprisingly accurate. My problem is that at the shoulder break, .001 is not enough to get a clean bolt lift. I will take Alex’s advice and measure closer to the middle.

Your bolt click is likely coming from the base, not the shoulder. Your dies are probably not sizing the base enough.

Rick

 

[Alex Wheeler]

Make a stub from the barrel drop. Run the chamber reamer in just shy of body/shoulder junction. Now you have a tool that has the exact shoulder of the chamber.

Thats the best way, this advice is for those using store bought comparators.

 

[dolek]

Great info Alex,thanks. I was just wondering about this today .I was using the Hornady comparators and and thought it made sense to use the one that landed in the middle.

 

[JFrank]

Thank you Alex
That’s a great observation, I just had to check both my comparator against the Wilson case gauge indeed the comparator contacts the body shoulder junction whereas my case gauge contact is nearly centered on the shoulder.
J

 

[Alex Wheeler]

Keep in mind if your not having trouble, no reason to change. But if your pushing back the shoulder .006" according to your comparator to get the bolt to drop, you may want to look into this.

 

[memilanuk]

Your bolt click is likely coming from the base, not the shoulder. Your dies are probably not sizing the base enough.

I've ran into this a number of times, especially moving brass from one barrel to another. A small base or ring die can be a very handy tool in the tool-box at those times!

 

[carlsbad]

Mine seems to work just fine but it should be marked better.

 

[Fast14riot]

Well, a cheap and old school method is to use a 9mm case on '08 based carts, will measure close to to middle of the shoulder. The .355" ish dimension will help determine first if your die is too long by cases lengthening during sizing or if you're actually bumping shoulders enough. Either way, it's a cheap double check for shoulder comparators.

 

[tmwinds]

Right or wrong, I've never considered a comparator a measuring tool to determine shoulder bump. I determine the amount of shoulder bump by using a stripped bolt to get the feel I want on closing. The comparator is then used to compare the consistency of my shoulder bump. The number is unimportant as long as I get the same number on all brass.

 

[Joe Salt]

And when you think everything is right! Don't forget to run your cases in the rifle to make sure there all the same feel. Or when you get on the line and some close harder than others you won't like it.

Joe salt

 

[Drolds72]

Would a valid method be to size the case by stripped bolt drop method (Alex's youtube video), and then back into a number with a measuring tool? I never was able to achieve a clean bolt drop- something is catching somewhere, but can def feel where the resistance drops off to practically nothing- and at that point, a number is just a number, right?

 

[mikeeg02]

Would a valid method be to size the case by stripped bolt drop method (Alex's youtube video), and then back into a number with a measuring tool? I never was able to achieve a clean bolt drop- something is catching somewhere, but can def feel where the resistance drops off to practically nothing.

The best advice I was given on this subject was from one of the top shooters at Williamsport (and a hall of fame member). Every time you start sizing cases, check them in the chamber. He checks every case. I check a couple, every now and again youll get one thats a little different. But once the die is set for that one, every case goes through the die set where it is.

 

[Alex Wheeler]

Just be sure there is nothing giving a false reading when checking fit in the chamber. Some actions have extractors that drag on the case and you will never get the bolt to drop. If your pushing the case back much over .002 start looking at why.