Hodgdon's reloading site recommends a method to determine max pressure for a particular load. It recommends measuring a new case just above the extraction groove to 4 decimal places (.0001). Record the measurement and compare it to the measurement of the case after firing. For cartridges that develop pressures in the 60,000 psi range the expansion should not exceed .0006. Using this method, it appears all my loads are over pressure even though the cases show no other signs of excessive pressure. For instance 6.5 Creedmoor, Laupa brass, 120 Laupa bullet, 42.5 gr. Hodgdon 4350 seated .001 off lands gives me .0009 expansion. Has anyone else tried this method?
Pressure Measurement
- Thread starter FlTarheel
- Start date
No. I would wager that most of us do not posses equipment with enough resolution to measure that accurately. I would also think that it would be difficult to be consistent in measuring location (and being perfectly 'square') without some form of custom jig.
In your case the fact you seated .001” off the lands caused a pressure spike and prolly caused the excess case expansion. Do over with the bullet seated to SAAMI coal.
They are taking a awfully big step.
As we know not all brass mfg are the same and neither are lot numbers from the same mfg.
Not even getting into the equipment being used to measure that difference...when is the last time it was calibrated etc.
As we know not all brass mfg are the same and neither are lot numbers from the same mfg.
Not even getting into the equipment being used to measure that difference...when is the last time it was calibrated etc.
Doesnt take much equipment at all, a simple 50 buck micrometer is capable of it.
Basically it boils down to whether the pressure is forcing the case web beyond its elastic limits. But instead of measuring them I just use bolt lift as an indirect measure to determine if I am pushing beyond the bras's capability to maintain itself inside of the chamber.
Basically it boils down to whether the pressure is forcing the case web beyond its elastic limits. But instead of measuring them I just use bolt lift as an indirect measure to determine if I am pushing beyond the bras's capability to maintain itself inside of the chamber.
Are you going off bolt lift immediately after firing it or are you taking the brass home and checking each piece in the rifle?
Most budget caliper / micrometers are +/- .0001 in accuracy. Just because the product description says it can go to .0000x doesn’t mean it can do it accurately.
So a “measurement” that is .0005 could be .0004-.0006 and the unit will show .0005.
And that range is usually only for a specific range with in the tools maximum measurement range.
Just like scales. Just because it’s 0-300 pounds the accuracy is not the same at 1lb as it is at 150lbs
When trying to measure to .0001 we are already outside the accuracy of the tool.
Deal with this crap on a daily basis in the pharma industry.
Got my ass handed to me years ago by the auditors...never forgot.
This is a terrible method for checking pressure. You'll get different results if you are on the 1st firing vs 2nd. You'll get different results with the same loads and brass in different chambers, you'll likely get different results with the different sizing dies, etc...
Don't try to get fancy. If you have brass flowing into the ejector hole, you are having heavy bolt lift, or you are loosening primer pockets in 2 or 3 firings, then you are over pressure.
Don't try to get fancy. If you have brass flowing into the ejector hole, you are having heavy bolt lift, or you are loosening primer pockets in 2 or 3 firings, then you are over pressure.
As to accuracy of the measuring equipment, I used a RCBS micrometer. The absolute measurement may be "off" one or two 10 thousands but I'm interested in the difference between two measurements. I expect the information obtained from those two difference measurements to be close enough to be useful.
Thank you for posting that article. Great info, glad I didn't get all twisted trying to figure that out! I had read the same thing as OP on Hodgdons website.
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Thanks for that post. I believe that puts this discussion to rest.
Caliper/micrometer is not a thing. A micrometer is a tool used when you need .0001 resolution. A caliper you are lucky to be .001. A piece of brass is no harder to measure than any other round object.
I find it very hard to believe measuring the expansion of the case head is not a good way to measure over pressure. Over pressure without being able to see the signs is exactly why a load shows us no signs "we notice," but stretches out the primer pockets and ruins the brass in a couple loadings.
I’m well aware they are different tools, the slash is there because all budget measuring devices fall into the same bucket. Especially digital ones.
Did not want a uninformed poster to chime in and say...that’s why I use a micrometer not a caliper etc.
They are not consistently capable of accurately measuring to that decimal place. Especially when it counts.
Did not want a uninformed poster to chime in and say...that’s why I use a micrometer not a caliper etc.
They are not consistently capable of accurately measuring to that decimal place. Especially when it counts.
RSI does have some pretty good tech articles... but do bear in mind that they sell a pressure measuring system (PressureTrace). Doesn't mean that the information is wrong, but given the amount of pissing and moaning from the SBA crowd about the articles from AMP... figured I'd mention it.
Many years ago, Ken Waters from Handloader magazine used this method to measure the case web to show when pressures were getting up there. Waters had a monthly featured article called "Pet Loads". I have a binder put out by Peterson publishing in 1980 with all of Ken's articles. It's a great resource when using old powders.
From what I gleaned from his testing and measuring, was the case expansion above the web is showing you that you're getting into dangerous territory.
From what I gleaned from his testing and measuring, was the case expansion above the web is showing you that you're getting into dangerous territory.
This part is about the tool:
But they are more than adequate for what we use them for in reloading.
60 bucks when not on sale. It is not going to be off any appreciable amount in the very limited .0005 range that we are watching for in the before-after comparison. It is more than adequate to see if this relative measurement grew .0001 or .0005 or .0008. If it is off that much in such a short distance then its probably because you ran over it or used it as a hammer. Sure, it may not pass nasa but its more than good enough to tell when something is .0004 or .0008 bigger than it was.
And now Im discussing the method topic:
And yes, this method is really best to compare on new virgin brass. And yes different brass has different hardness and dimensions etc and each time it gets fired those conditions change etc etc so on and so on so its really just a quick reference, not a god damn law. Is it expanding enough for the material to be appreciably altered or not? If it is then you might not know the exact precise pressure number etc but you do know you need to back the heck back down just like with any other pressure sign.
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Just for the sake of argument... does anyone have a lead on an inexpensive *blade* micrometer, like what you'd (ideally) use for CHE measurement?
Regular 0-1" micrometers can be found pretty cheap, both at your local HF and online. Ball micrometers are a little more specialized, but can still be found for decent $$. Blade micrometers... for some reason apparently the Chiwanese copycats haven't decided those are worth the effort
Regular 0-1" micrometers can be found pretty cheap, both at your local HF and online. Ball micrometers are a little more specialized, but can still be found for decent $$. Blade micrometers... for some reason apparently the Chiwanese copycats haven't decided those are worth the effort
