Ran across something last night that I hadn't been tracking as I started using my new Giruad for the first time. I was noticing that my cases were coming out varying case lengths after trimming. That got me to exploring my shoulder bump's to see if I was standard across all the cases. What I found was that I had varying shoulder bumps on some of the cases that had ejector swipe marks on them (hotter load). After rechecking my Micron sizing die in my Co-ax I had all the non-swipped brass consistently at 1.6080 on my comparator. Regardless of how many times I did the sizing die on some of the ejector swiped brass I could not get the comparator to read less than 1.610. Thought that was really strange and then I started looking at the bottom of the brass and noting that the distance between the case rim and the bottom was wider on the ejector swiped brass. Nothing I could do on the Co-ax could get these to standardize with the other cases. Assuming that is because of the way the jaws grip the case in the ejector groove. Has anyone experienced this issue. I am culling those brass out as they are not at the same case length, but wanted to see if this is common or a way to be able to reform to get common sizing.
Shoulder Bump Inconsistency on Co-Ax
- Thread starter Stratofisher
- Start date
jelenko
Gold $$ Contributor
All? How many pieces?
1-2 thousandths across 25 pieces is pretty good.
FWIW, when I measure headspace on a piece of brass, it can vary 1/2 thousandth just by turning the case a bit. I can also get it to vary a bit by just measuring again.
For the most part the it is +/- a couple of thousandth's on the non-pressured ejector marked brass. Probably just a good idea to cull out those pressured pieces for consistent/safety. Just an interesting discovery trying to keep everything to good tolerances
Lots of threads on here about this. Few different schools of thought.
Ejector impressions on the case head and hard bolt life are almost always sure signs that the pressure is too high. I would be more focused on that then your bump numbers.
With that said, as stated in post # 3, it takes some skill to obtain consistent readings using a bump gauge and caliper. Developing a technique with practice can help in obtaining consistent results. In addition, the slighted spec of debris on the interior of the jaws of the caliper can change a reading by .001". Keeping the jaws free of debris with a lint free cloth can help.
One of the steps I took to improve consistency was I purchased a Whidden single piece caliber group specific bump gauge which has no inserts. I mounted it permanently on a caliper. This improved my consistency. Also, I have seen those anvils cause variation if they are not cut properly. In addition, each time you remove, and re-insert can create inconsistency. I do not use them for this reason.
While I do not anneal, those who do report improve sizing consistency.
With that said, as stated in post # 3, it takes some skill to obtain consistent readings using a bump gauge and caliper. Developing a technique with practice can help in obtaining consistent results. In addition, the slighted spec of debris on the interior of the jaws of the caliper can change a reading by .001". Keeping the jaws free of debris with a lint free cloth can help.
One of the steps I took to improve consistency was I purchased a Whidden single piece caliber group specific bump gauge which has no inserts. I mounted it permanently on a caliper. This improved my consistency. Also, I have seen those anvils cause variation if they are not cut properly. In addition, each time you remove, and re-insert can create inconsistency. I do not use them for this reason.
While I do not anneal, those who do report improve sizing consistency.
I do have the Whidden and SAC comparators. Also run my Burstfire induction annealer every cycle through. Doing what I can for consistency
Murray Brook
Silver $$ Contributor
Is the Co-Ax the only press you have. Maybe trying to size on a different press may give you some insight.
jds holler
Gold $$ Contributor
I've had cases that show much greater tension when sizing, and sometimes even heavy bolt tension after sizing. After being used a time or two with the same load that I use for general shooting, those cases even out, and become like the others.
I try not to use a shell like that when I'm doing serious load testing, or trying for a "real group". jd
I try not to use a shell like that when I'm doing serious load testing, or trying for a "real group". jd
I have lee that I could give it a try on to see.
D-4297
Gold $$ Contributor
I use a RCBS Single Stage press to do all of my re-sizing on , after annealing every firing .
I then check 100% for the "need" to Trim . Often the cases are within my parameters and I don't Trim until they need doing so .
I use a RCBS Trim-Pro II with a triple cutter head , and it holds within .001 trim length , so I'm good with it . One thing to mention is that the Trim Pro locates the case off of the Base , so you will get a consistent Overall Length . Bump should be within .001 under chamber spec. , regardless if you are locating off of the shoulder .
I then check 100% for the "need" to Trim . Often the cases are within my parameters and I don't Trim until they need doing so .
I use a RCBS Trim-Pro II with a triple cutter head , and it holds within .001 trim length , so I'm good with it . One thing to mention is that the Trim Pro locates the case off of the Base , so you will get a consistent Overall Length . Bump should be within .001 under chamber spec. , regardless if you are locating off of the shoulder .
I find those bump gauges that attach to a caliper very rickety. I get more consistent results from the RCBS Precision Mic. You screw a calibrated drum down over the case and the measurements are very repeatable.
Murray Brook
Silver $$ Contributor
If it is a decent Lee press like the Lee CC then it would be worth a try. Try to isolate things to see where the bump differences are coming from. JMO
Nothing personal but we dont know you or your abilities. Hell, I scratch my noggin & question myself sometimes. Hoping all brass is the same maker & lot#. Are you sure the die is set up for the proper amount of "bump"? Please expand on your method of adjustment. Is the rifle/shooter capable of shooting the difference?
While reading the OP a couple things popped into my brain. Do the offending long brass chamber in your rifle without difficulty? Do they shoot into the group with the other brass? Could the ejector mark be raising a small burr, of say .002" on the case head? Judicious light polishing of the case base on a piece of fine (400 or 600 grit?) sandpaper on a flat surface will fix the burr. Oh... the way the Co-Ax shellholder jaws grip the case has nothing to do with base to shoulder measurement. The bottom is the bottom. Jaws slide above that.
The cases with ejector marks have apparently** been run at a higher pressure than the others. If so the brass has also likely expanded more at the .200 line. When resized back to nominal die diameter, where does that extra diameter go? It seems to always move brass upward into the shoulder/neck.
**New brass with minimum or less headspace fired in a generous to maximum chamber (excesses of .010" or more are not all that uncommon) can & often does leave false pressure positives in the form of ejector marks. If so check for case stretch, thinning inside the case, above the extractor groove. Impending case separations are more of a safety issue than brass of slightly differing lengths.
The die has been set to a .002 shoulder bump. This in my custom rifle I built that I am normally running single digit ES and SD on reloads. I had worked a ladder up with some N565,H1000, and N570 in an attempt to find the best powder combo in my 6.5 PRC. Have settled on N565 at a mid level as it offers the best accuracy at less than .3 MOA groups on most 5 shot groups. I probably should just remove the pressure sign brass and move on as the other all size appropriately. On these pressure sign brass the distance from the .200 line to case base is larger than the non-pressure brass. I can size all the non-pressure brass and the bump is exactly on with a .002 bump. These pressure signed brass will not size down with the same die setting in the Co-Ax. I hypothesize that is is something to do with the the way the jaws grip the case. I am getting a Co-Ax shell plate holder to try out to see if that is part of why this occurs. These brass on on their 4th reloads. Just something interesting I have found and was curious if anyone else had seen this happening.
Anneal your brass if you are not doing it already.
I am annealing with a burstfire induction annealer every reload. Used the templique 750 deg paint to verify temps
Coyotefurharvester
Silver $$ Contributor
See if someone with a roll die will run your "pressure" brass for you, than try sizing again.
Back off on the powder. Shoot them and see if there is a difference. What's the rifle used for? I don't know if you can measure cases to 1 thou. with a simple gauge. Some machinist can comment on measuring with a simple piece of metal with a hole in it as a gauge and calipers to measure within 1 thou.
jelenko
Gold $$ Contributor
This.
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