How much case web thinning is too much?

[Pyrocat]

I'm pretty sure this must have been discussed in the past but I wasn't able to track threads on that.

I'm reloading for 308 win and I'm monitoring case web thinning using RCBS tool. I have noticed some "ditches" (circulars) that are up to 0.003". I decided to shoot the brass anyway and did not notice much change. I repeated the experiment over again and after 5 reloads what was 0.002-0.003" is now 0.003-0.004". There is no bright ring on any of the case and I always measure after thorough cleaning and FL sizing of the case.

My question: on much thinning is too much in your experience? Using an objective measurement like the rcbs tool obviously; so not through visual observation.

I guess the ditches existed in the initial ammo I shot and that brass is slowly fooling after each reload. I would like to know "when to stop" to avoid the bad surprise of a big cloud of gaz in my face :-/

 

[912173]

I would think there has to be some kind of minimum web spec out there required to contain max sammi chamber pressure in a maximum headspace chamber. Different brands of brass have different internal volume and I think the case head and web is probably where most of the difference is. Which brand of brass are you using? It wouldn't surprise me if your 5x reloaded brass isn't as thin at the web as some brands of new brass. Of course, this is just me thinking out loud, don't blow your face off because of me. I hope somebody has an actual number figure. Most people will toss brass if there is an inside groove detectable with a paperclip. If it's not that bad yet, I would keep measuring after each firing and if your primer pockets go out first, you will have your own confirmed safe web thickness number for future use.

 

[straightshooter1]

I would think there has to be some kind of minimum web spec out there required to contain max sammi chamber pressure in a maximum headspace chamber. Different brands of brass have different internal volume and I think the case head and web is probably where most of the difference is. Which brand of brass are you using? It wouldn't surprise me if your 5x reloaded brass isn't as thin at the web as some brands of new brass. Of course, this is just me thinking out loud, don't blow your face off because of me. I hope somebody has an actual number figure. Most people will toss brass if there is an inside groove detectable with a paperclip. If it's not that bad yet, I would keep measuring after each firing and if your primer pockets go out first, you will have your own confirmed safe web thickness number for future use.

I use the paperclip method to detect an "inside groove" and I use it to test after every firing. I'll detect a groove on the inside before I'll be able to see that tell tale sign on the outside (that light off color ring that will develop). If I see a hint of that ring on the outside, is when I'll get rid of the cases, though the groove on the inside tells me in advance that I'm about to see that. So, though I'll feel the groove on the inside, I'll still fire the case until the faint outside ring is observed.

If the inside groove develops early (lets say like in 5 firings), that tells me that my head spacing set up is not correct. When I have the correct head spacing and not bumping my neck shoulders back too far, I've not experience any of that issue and got a large number of firings out of a batch of brass when loose primer pockets become the issue.

 

[B y r o n]

The groove that is detected is the first sign of case head separation. Why would you risk reloading a case that could have a case separation? In years gone by, cases were rejected for reloading at the first sign of the ring. I know component prices have increased but why risk damage when it can be avoided?

 

[hlee]

Yeah, 20 years ago I was paying about $0.90/piece for new Lapua brass. Today they are $1.10/piece. Loading brass showing measurable signs of case head failure is “penny wise and pound foolish.” If you can feel a ring inside the case, time to dump it.

 

[FLIGHT762]

O/P, how are you measuring the shoulder bump on your fired cases? How much are you setting F/L sizing shoulder bump on your fired cases?

I haven't had a case head separation since controlling my F/L sizing shoulder bump and properly measuring fired case datum length and using a minimal bump on F/L sized cases. My preferred method is using a comparator showing precise numbers.

 

[straightshooter1]

The groove that is detected is the first sign of case head separation. Why would you risk reloading a case that could have a case separation? In years gone by, cases were rejected for reloading at the first sign of the ring. I know component prices have increased but why risk damage when it can be avoided?

Because I detected the first sign of a groove on the interior and nothing on the exterior, I was curious as to how long until something showed on the exterior. On my .308 cases, it took 3-4 more firings from when when I first detected interior groove until I could see the exterior sign. Then on a second firing after that, a crack was detected. This was Federal brass used in a factory barrel of a bolt gun. Repeated the test again with the same components and got the same results. My conclusion is that first sign of a groove on the interior is not a sign of imminent failure, but the exterior sign sure is.

BTW: On inspection of the chambers surface after a crack was detected, I found no apparent damage. The cracks were apparently too small to leak much into the chamber.

 

[Pyrocat]

O/P, how are you measuring the shoulder bump on your fired cases? How much are you setting F/L sizing shoulder bump on your fired cases?

I haven't had a case head separation since controlling my F/L sizing shoulder bump and properly measuring fired case datum length and using a minimal bump on F/L sized cases. My preferred method is using a comparator showing precise numbers.

I use redding competition shell holders with hornady tool to measure the shoulder bump. I have set it to 0.003" below (stable) fired cases. I targeted 0.002" but for some reason I either get no sizing or 0.003".

To stabilize the measure I shot them 3 times doing neck sizing only. I measured 50 cases and they are all within 0.001".

I know about the paper clip method but I'm curious to know about how much it translates in thousands. I don't know if you can feel extremely low values with it.

I did not toss the brass because then I should discard all the cases I have while experimentation actually show that they do work fine. However, as many mentioned, no one wants to end up with case head separation either...

 

[Pyrocat]

Ok so I tried feeling with my fingers and the RCBS needle-thing on a case that has a 0.001" ditch and one that has 0.003" and I don't feel anything at all.

On the other hand, I already cut open a case that had 0.003" and you do see something with your eyes.

One thing that I could try is unmount 5 factory ammo of the same brand and see if I spot anything in unshot brass.

It bugs me to have this measurement but don't know what to do with it.

 

[iceng]

Shoot and drop it at the range during a PRS comp and flex on poors with it. THAT is the best use for it, and gives you a laugh.

Ask for your brass from a stage officer, and randomly keep leaving it around. Drives people nuts.

 

[Gohring65]

Unless you jam the bullets on the first firing, you’ll get an initial stretch. Even jamming the bullet will allow a slight stretch. It could reduce thickness too .003 or even more, depends on brass brand and the size of the new brass compared your chamber and how hot the load is. It most of the time isn’t fully formed to your chamber on the first firing. Some guys will make the mistake of setting up their fl die to bump back .002” on the first firing and further exasperate the problem. You’ll see post on guys getting case head separation on the second or third firing and this is most likely the culprit. I don’t set up my die until the third firing, I’ll neck size or partial fl size til then. It’ll continue to stretch throughout the case life. How much depends on your die set up.

 

[Doom]

As @Gohring65 stated the predominate source of case stretch is headspace clearance. On initial firing the case stretches to fill the chamber. On resizing the case diameter is reduce which pushes the shoulder up and the die then tapers body into the shoulder pushing shoulder into the neck. So the case grows longer. on each subsequent firing the process is repeated when Full Length Sizing. The source of the brass is normally from the web junction.

One advantage to neck sizing is that case stretch is virtually eliminated. With full length sizing minimizing the headspace dimension will minimize but not eliminate it.