Question about Case Headspace Measurement

[South Pender]

Two or three weeks ago, I started a thread "Help with Case Headspace Measurements" where I was asking for advice on how to do this. I went ahead and purchased a Sinclair Shoulder Bump Gage insert (the one with the 17° shoulder angle for the .270 Win.) and put it into my Hornady Comparator body. I also picked up a Hornady "anvil," which screws onto the opposite arm of the caliper to steady the base of the case. I haven't done any full-length resizing yet (the main purpose for getting the gauge), but tried it out on one fired, but not resized case and one unfired case just to see how it was going to work. The cases were Sako.

Well, to my amazement, I got a shorter measurement for the once-fired case than I did for the unfired piece of virgin brass. The actual measurements aren't important here because they include the length of the gauge and anvil, but just for completeness they were:

fired/unsized: 3.2115",
unfired: 3.2125" on my caliper.

I rotated the cases several times to make sure I was getting an accurate measurement. So, according to these measurements, the shoulder was actually slightly (.001") further back on the fired case than on the unfired case. (The fired case went very slightly further into the bump gage than did the unfired case.)

How can this be? Am I missing something here? Shouldn't the fired case have the shoulder moved very slightly forward from its unfired position? My result seems to be the opposite of what one would expect.

Help!

 

[ronsatspokane]

Sounds like the chamber is cut at the minimum and maybe just a hair short. With only a couple ten thousands you probably don't even notice a hard bolt close. Does it change when you just close the bolt on an unfired round then extract an measure it.

 

[JFrank]

Two or three weeks ago, I started a thread "Help with Case Headspace Measurements" where I was asking for advice on how to do this. I went ahead and purchased a Sinclair Shoulder Bump Gage insert (the one with the 17° shoulder angle for the .270 Win.) and put it into my Hornady Comparator body. I also picked up a Hornady "anvil," which screws onto the opposite arm of the caliper to steady the base of the case. I haven't done any full-length resizing yet (the main purpose for getting the gauge), but tried it out on one fired, but not resized case and one unfired case just to see how it was going to work. The cases were Sako.

Well, to my amazement, I got a shorter measurement for the once-fired case than I did for the unfired piece of virgin brass. The actual measurements aren't important here because they include the length of the gauge and anvil, but just for completeness they were:

fired/unsized: 3.2115",
unfired: 3.2125" on my caliper.

I rotated the cases several times to make sure I was getting an accurate measurement. So, according to these measurements, the shoulder was actually slightly (.001") further back on the fired case than on the unfired case. (The fired case went very slightly further into the bump gage than did the unfired case.)

How can this be? Am I missing something here? Shouldn't the fired case have the shoulder moved very slightly forward from its unfired position? My result seems to be the opposite of what one would expect.

Help!

It bees that way sometimes...

Just record the measurements and don’t try to bump a shoulder until you have a need, such as hard to chamber .
Interesting also’ that cases will grow during resizing process.
BTW: the case doesn’t have headspace the chamber does.

 

[Bc'z]

Case expansion in width will cause shorter length.
Neck size your 1x fired brass, check for ease of chambering, load and shoot.

 

[South Pender]

Sounds like the chamber is cut at the minimum and maybe just a hair short. With only a couple ten thousands you probably don't even notice a hard bolt close. Does it change when you just close the bolt on an unfired round then extract an measure it.

I would say no. It's hard to get the identical setup with the bump gauge insert and anvil a second time after detaching them and then re-attaching them to the caliper--we're talking ten-thousandths here. I did take a second set of measurements, and they were very slightly different from the first set that I reported, but the same difference occurred, with the measurement of the fired case being slightly less than that of the unfired case. Closing the bolt on both cases showed no resistance at all, and so I don't think that the chamber shoulder is making much contact with either case on bolt-closing.

 

[South Pender]

BTW: the case doesn’t have headspace the chamber does.

Yes, I do understand that. I was just using the same terms that Hornady does in describing their "Hornady Lock-n-Load Headspace Gauge Tool."

 

[bozo699]

Did you remove the primer on fired case before measuring?
Wayne

 

[bozo699]

Never mind Pender I was thinking backwards
Wayne

 

[cw308]

My fired cases would also get shorter from expansion . How I found the length of my chamber is I but up the base on my Go Gauge until the stripped bolt closed with a slight resistance . That is your zero headspace measurement . Setting up your full length die to size the neck without bumping the shoulder , your cases will get longer, by testing in your chamber with a stripped bolt you'll be able to size from zero on down. Check out the stripped bolt method videos. Hope I Helped in some way.

Chris

 

[K22]

A few things to consider:

1. One of the reasons I went to a single piece cartridge specific bump gauge (Widden Gun Works) was to avoid the interfaces common with other gauges w/ interchangeable parts. Not saying that's the source of the variation in your case - just something to contemplate.

2. Don't know the precision of your electronic caliper but the one I had wasn't repeatable on a consistent basis. I switched to a dial caliper which gave me more consistent readings. I check with length standards over the range use. Again not saying this is the source of your variation just something else to think about. How reliable is your caliper? When you get beyond .001" your getting into "rare air" for a conventional caliper.

3. Suggest you read the article entitled "Dealing with Headspace" by Todd Spotti on the Redding-Reloading web site.

4. Bottom Line - the goal is to obtained a sized case the will chamber easily but with a slight resistance to bolt closing. One of the reasons I do not like neck sizing is that it does not address the radial expansion of a case when fired. With a full length sizing you can address both, radial and longitudinal expansion.

 

[gman47564]

alot of times when your setting up your fl sizing die the shoulder will move forward a couple thousandths before you get the die adjust to where it starts moving the shoulder back... let it move forward and see if it chambers easy... if not bump it back a .001 and try chambering it again... if it chambers easy at this point you know your headspace is ok... if it doesnt chamber easy bump it back another thousandth at a time till it does...

 

[ronsatspokane]

I would say no. It's hard to get the identical setup with the bump gauge insert and anvil a second time after detaching them and then re-attaching them to the caliper--we're talking ten-thousandths here. I did take a second set of measurements, and they were very slightly different from the first set that I reported, but the same difference occurred, with the measurement of the fired case being slightly less than that of the unfired case. Closing the bolt on both cases showed no resistance at all, and so I don't think that the chamber shoulder is making much contact with either case on bolt-closing.

I've got a 223 that will not close on a go gauge but will close on a round. New factory and reloaded rounds work. Not willing to trim the go gauge to find out how many thousands the chamber is short but it can't be that much.

 

[Coyotefurharvester]

ronsat try the spent primer thumb seated in a fired case and a sized case, let the bolt seat the primer when you close it. Measure the case length change(no primer to bolt seated primer) these measurements should give you a good idea of case to chamber fit and how much you are sizing.

 

[ronsatspokane]

That's a good idea. Not to worried about it as everything works fine in the rifle. Still, it is a short chamber without a doubt.

 

[INTJ]

This is not a big deal. If you want to find where the brass went, measure both the shoulder diameter and head diameter of both new and unfired cases. Chances are they are larger on the fired cases.

Regardless of what the new brass reads, the best approach is to take the shoulder length measurement of fired cases, then set up your sizer die to bump the shoulder about .002" each time it is fired. That will give you the most consistency and best accuracy, and that is how competitors in high accuracy shooting competitions size their cases.

 

[Willie]

Just a thought, you can get your actual base to datum length by simply zeroing the calipers, when gauge and base are touching with light pressure (without the case).
Add the case, that's the number you need.

 

[JFrank]

@K22
Dude’
Radial Expansion ? Never heard that

 

[Uncle Ed]

I've got a 223 that will not close on a go gauge but will close on a round. New factory and reloaded rounds work. Not willing to trim the go gauge to find out how many thousands the chamber is short but it can't be that much.

Do not pass GO and do not collect $200.00

What type/make .223 rifle do you have and does it have a barrel nut.

 

[INTJ]

@K22
Dude’
Radial Expansion ? Never heard that

It's a common idea. A new case usually expands in diameter to fill the chamber. It takes a FL die to squeeze the diameter back down and avoid sticky extraction.

 

[JFrank]

It's a common idea. A new case usually expands in diameter to fill the chamber. It takes a FL die to squeeze the diameter back down and avoid sticky extraction.

I know what it is I just haven’t heard that term so it caught my eye.
Hell I even wrote it down