Confused by case expansion numbers...

[daved20319]

Forgive the new guy and all the questions, but the 10 is new to me, and a higher pressure round than I'm used to dealing with in a handgun, so my tendency is to err on the side of caution, if I must err.

So I was doing some research, came across some good posts about case expansion, one here, one somewhere else.  The first confusion came from the somewhere else, I think an old post over on Glock Talk.  Someone cited a well respected source saying .0005" was max acceptable expansion in new brass with max loads.  Based on other reading I've done, and the fact that most of us are only using calipers rather than micrometers, shouldn't that be .005"?

But then I started measuring my own ammo and brass.  Here's the results, using Federal AE 180 gr. FMJ, small primer brass, and reloading that same brass.

New ammo, .4195"
Fired, .425"
Resized, Redding carbide die set, .424"
Second firing, .429", test loads with charges from 9.1-10.6 gr.  No appreciable difference in case expansion with charge weight.

Of note, a significant number of fired cases are actually a little oval, numbers above are approx. averages, I should probably slug my barrel.  But here's where it gets a little weird.  I also fired more of the factory ammo, only this time, they also measured .429".  Same day, same gun, different box of ammo, but the previous box was shot under very similar conditions.  Don't know if both boxes came from the same lot, I didn't check.  So about .009" expansion, but no smiles or bulges. Also worth noting, this box chronographed higher than spec, 1090's vs. the 1030 claimed on the box.  Didn't chronograph the first box.

So if I'm interpreting the data properly, I'm getting about .005" expansion with both new and reloaded ammo.  Based on my research to date, that seems high.  BTW, pistol in question is a pretty new Witness Full Size Steel, total round count to date is only about 200.  Would appreciate any thoughts, and maybe a little hand holding, based on my research so far, these numbers are a little alarming  .  Thanks.

Dave 

[Graybeard]

Don't get too hung up on case head expansion. 5 ten thousandths is probably a typo. If it isn't, it should be. 5 thousandths isn't a big deal either. The quality of the brass, chamber size, and the amount of case head support are all variables in the amount of case head expansion.

If your concern is case head failure, check out some of the Glock guppy belly pics online. The brass is no longer reloadable, but the cases still didn't fail.

Stick with ladder work ups and don't go over the highest book max powder charges and you'll be fine. If you want to go after more speed than the Blue Dot loads you mentioned in other posts, give Power Pistol and Longshot a try. Neither have given me any surprises, like pressure spikes, and both like to be run pretty hot.

[sqlbullet]

Hey Dave.  No worries about questions.

You should get a micrometer if you are going after case head expansion data.  They aren't that much and while gross expansion of 0.005" or more is common in my guns too, the difference between in-spec and out-of-spec expansion is indeed in tenths of a thou'.  I don't know for certain the post you were referring too, but I suspect that poster was trying to say that the differences in fired brass between start charge pressure and max charge pressure can be as little as 0.0005".

Here is the chunk of the puzzle you are missing.  When using case head expansion to indicate pressure, we don't really care much about the gross pre to post firing difference.  My glocks turn in heads in the 0.432" range with practice loads.  Those would have started out at 0.424" just like your brass.  All that tells me is that the chamber in that gun in generous.  Same ammo in my Para P16-10mm will measure 0.427" cause it is on the tight side.

This also means you can't really use someone else pre/post data here.

What you are looking for is a variation, for that one gun, in the amount of post firing expansion.  The only way I would get to 0.432" fired heads in the Para is with ammo that was actually stretching the steel of the barrel.  And honestly I would never see that number out of a chamber that tight as the gun would spontaneously disassemble first.

The real data you are after requires a micrometer.  If you graph the average case head expansion numbers for your Blue Dot loads from 9.1 to 10.6 in tenths of a thousandth, you will see that they are going up a tenth or so at each interval.  Something like .4288, .4290, .4291, .4292, .4293 (just an example, not saying those will be your numbers).  You aren't going to get those measurements from a set of calipers.  And from a 10% reduced start charge to max I would guess a spread of 0.0005" to probably be about right.

There are lots of other factors here too.  Brass gets harder as it works, so all the brass in the test needs to be fired the same number of times.  I prefer to do this type of test with once fired starline brass.  New brass can be on the soft side, resulting in wider variances.  Once-fired brass I consider normalized and ready to give consistent data.  Also, when possible I use brass from the same lot, and weigh and measure the cases as best I can to select as much uniformity as possible. Starline again helps here as they tend to be very consistent.

Hope that helps clear things up.

[The_Shadow]

sqlbullet has made a very valid post!   
I think that you sizing dies are not doing you justice or you measurements are incorrect @ 0.4195.  Why do I say this?
Most factory ammo I have measured run 0.4215" to 0.4225" near the start of the extractor cut.
The case when resized should be near or less than 0.423" and possibly slightly less.

Of my 10mm locks I see a max expansion of 0.434" (smooth rounded bulge) anything over a distinct lie will start to form. (SMILE)
My S&W10mm the chamber sees a max of 0.4290"  Only ever had one exceed that with a SMILE line, Underwood and it also had a loose primer pocket after firing.

See pass through sizing: https://10mm-firearms.com/reloading-10mm-ammo/pass-thru-sizing-using-lee-fcd/

When the brass gets too stiff you might find it not to hold the resized dimensions and loose case to bullet tension.  That can lead to bullet setback issues.

[Kenk]

Never gave this much thought. With my primary shooting spot being a hay field, I'm lucky if I can reload my new Stareline brass more then twice : (

[daved20319]

Thanks, guys, I think that clears things up for me nicely.  I'm still going to hold off on any more testing until the new Starline brass shows up.  And yes, I do have a micrometer, micrometers actually, my old airgun hobby morphed into a home machining hobby, so I'm pretty well set for measuring tools  .

Shadow, you mixed up some of my numbers, the .4195" was new Federal AE factory ammo, that resized at .424" from my Redding die.  Was looking at the SAAMI spec drawings for the 10, it calls out a max case diameter just ahead of the extraction groove of .425, but a minimum chamber is .4281 at approx. the same location, so it's looking like I'm probably in pretty good shape, it would seem my pistol has a fairly tight chamber, and my resizing die won't have me overworking my brass too badly.  Anyway, thanks again, guys, I really appreciate the time and info.  Later.

Dave

[The_Shadow]

Here are SAAMI Specs for 10mm



A good case gauge is a helpful tool to insure fit.

[daved20319]

Same drawing I was looking at, only yours is missing the notation at the top that says maximum cartridge spec and minimum chamber spec.  Interesting info, and I'm a little ashamed to admit the 10mm is the first time I've actually paid much attention too.  As to case gauge, I'll get one when/if I get a second 10mm, but in the meantime, a barrel plunk test is sufficient unto my needs  .  My new Starline brass showed up yesterday, so I'll start working on getting some of that loaded up soon.  But now I'm going to need to source more bullets  .  Always something, isn't it?  Later.

Dave