Hodgdon's Pressure Monitoring Tip

[Brians356]

I just got this from Hodgdon today. I had not seen it before, but perhaps it's republished.

Excerpt:

Using a blade micrometer that measures in ten thousandths (.0001"), new, unfired cases can be gauged before and after firing to determine reasonably accurate maximum loads. Micrometers measuring in thousandths (.001") are insufficiently accurate to perform these measurements, and should not be used. Previously fired cases cannot be used accurately due to various levels of brass hardening. Measurement is taken just ahead of the extractor groove on the case head and must be taken at the same place on the case before and after firing. By placing a small mark on the case head – entering the cartridge in the chamber with mark at 12 o’clock – a consistently accurate measurement can be taken with each firing.

Lower pressure rounds, like the .30-30 Winchester, usually yield maximum pressures at .0003"-.0004" expansion. Modern cartridges, like the .223 Remington, will show maximum pressure at .0004"-.0005", while .308 Winchester, .270 Winchester, etc., typically yield .0005"-.0006" expansion at max pressure. Magnums, like the .300 Winchester Magnum, show maximums at .0006”-.0007” expansion, and should be measured on the belt.

In conjunction with these measurements, case head signs of pressure should be monitored as well. These signs include very flat primers, slightly cratered primers, ejector marks on the case head, and stiff extraction. All these case head signs indicate high pressure, and loads should be reduced until these signs disappear.

Hodgdon Landing

www.hodgdonreloading.com

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[Laurie]

I read about this method in an old Speer Reloading manual, maybe #10, when I started handloading, This method was used by many bullet and powder companies before the advent of strain gauge technology and cheap powerful computer assistance. Whilst major ammunition companies used pressure barrels and copper crusher technology, investment in a pressure barrel for each cartridge was too great for smaller outfits, at least across the board.

I also read elsewhere that when affordable strain gauge measurement arrived and supplanted case web expansion some companies got a shock when they reran some of their their maximum loads using the new methods and the next editions of their manuals saw significant charge reductions for a number of cartridges. Certainly, the Speer description suggested that this is no simple or easy method and inexperienced micrometer users could easily arrive at incorrect conclusions.

Ken Waters used a simpler variant in his 'Pet Loads' tests which used measurements of the 'pressure ring' diameter - the point on the case at the junction of the solid web and unsupported case walls. He calibrated the results on his handloads by first measuring then firing a small number of factory cartridges and retaking their fired measurements. This assumed that factory ammo was loaded close to SAAMI MAP, so any larger increase in the 'pressure ring' from its unfired state than seen in factory products was probably exceeding maximum allowed pressure.

 

[Uncle Ed]

Does this image make my arse look too big??????

 

[memilanuk]

Might want to read this as well:

PRE, CHE, RIP

 

[mikecr]

The problem you can have with hard criteria(a growth at chosen datum) is that your test assumption about 'new brass' character is likely weak to realities. Also, chamber diameter, chamber clearance, and breech support seems dismissed in this. Can't really do that.

I do a similar test to determine a point where FL sizing would be REQUIRED(regardless of desired), with single shots of new cases. This is measured with a blade mic at what I see as the 'web-line'(which is widest point of the fired case). Seems referred to as PRE here. It's considerably forward of the extraction groove and dependent on brass manufacture.
Every case I've ever used with this test, in a normal clearance chamber with normal loads, grows at least 0.0005" at the web-line. It normally plateaus there as I go up in charge and then suddenly step changes upward. A new yielding.
That point has consistently calc'd out by QuickLoad as just passing SAAMI max for the cartridge. It is in no way unsafe, but stiff and there will be longer term prices for it to the brass.
I log that as MyMax, and load development is held below this.

I could see mentioned CHE measurement point indicating where your brass will pay the same price.
But I doubt you can creep into it with the resolution and consistency of my method.
They didn't consider chamber diameter, chamber clearance, and breech support, which affect case expansion -but I have. My method goes to these things as they actually are(the normal plateau), and THEN challenges them.

 

[riflewoman]

After working with micrometers for Some years, anybody who thinks they can differentiate between .0003 and .0004 with even a ‘tenth reading’ digital micrometer is delusional. Even e perienced toolmakers with better “Mike” sense than I have have difficulty. So the average schmuck reloader certainly doesn’t.

 

[LCazador]

I just got this from Hodgdon today. I had not seen it before, but perhaps it's republished.

Excerpt:

Using a blade micrometer that measures in ten thousandths (.0001"), new, unfired cases can be gauged before and after firing to determine reasonably accurate maximum loads. Micrometers measuring in thousandths (.001") are insufficiently accurate to perform these measurements, and should not be used. Previously fired cases cannot be used accurately due to various levels of brass hardening. Measurement is taken just ahead of the extractor groove on the case head and must be taken at the same place on the case before and after firing. By placing a small mark on the case head – entering the cartridge in the chamber with mark at 12 o’clock – a consistently accurate measurement can be taken with each firing.

Lower pressure rounds, like the .30-30 Winchester, usually yield maximum pressures at .0003"-.0004" expansion. Modern cartridges, like the .223 Remington, will show maximum pressure at .0004"-.0005", while .308 Winchester, .270 Winchester, etc., typically yield .0005"-.0006" expansion at max pressure. Magnums, like the .300 Winchester Magnum, show maximums at .0006”-.0007” expansion, and should be measured on the belt.

In conjunction with these measurements, case head signs of pressure should be monitored as well. These signs include very flat primers, slightly cratered primers, ejector marks on the case head, and stiff extraction. All these case head signs indicate high pressure, and loads should be reduced until these signs disappear.

Hodgdon Landing

www.hodgdonreloading.com

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I've been measuring my brass case heads for quite some time now so this is not new. I can't remember where I got the information but it's pretty much what it says here. One thing is for sure when you get near that .001" measurement you will notice loose primer pockets. Frankly any of the signs of high pressure listed here are not a very accurate but certainly are good indicators to measure the case head. I use a pointed angle micrometer the reads to .00001" and mark the case as suggested here. That one case is a base line case or 'control case' as the load progresses. I have a book for each cartridge to record case head measurement, as well as round count, loads that have been reviewed as well as headspace and die adjustments and much more. Good record keeping is a must. I absolutely can't rely on memory for anything anymore! Great post!

 

[riflewoman]

A micrometer that reads to 1/100,000 of an inch? Make &model?

 

[Clark]

I have my own system of measuring extractor grove changes all the way around, only needing 0.001" of repeatability. I back off 6% with non temp compensated powder and 3% with temp compensated powder.

I want nothing to do with PRE or CHE or stain gauges...... but I did use a strain gauge to measure torque when designing a jet engine starter generator. They have their place, but not on my rifles.

PRE CHE strain gauge talk was all over the internet 15 years ago and I seem to always be arguing with other electrical engineers.

My father had an electrical engineer degree but spent his life designing guns [artillery and machine guns]. He had a strain gauge on the barrel to see if the bullet stayed jammed in the lands of fell back out.

 

[jelenko]

A micrometer that reads to 1/100,000 of an inch? Make &model?

I have a Mitutoyo that does show 5 digits to the right of the decimal point. I've been assuming that there's no 'meaning' to that 5th digit.

 

[Clark]

It is my contention that in rifles stronger than the brass, the brass is the weakest link.
Loads should be designed for those rifles that have adequate safety margin so the brass has long life.

a) Looking at Mauser case heads with large Boxer primer pockets we see cartridges like 22-250, 243, 6mm Rem, 250 Savage, 257 Roberts, 25-06, 260 Rem, 6.5x55 [US brass], 270, 7mm-08, 7x57mm, 280, 308, 30-06, 8x57mm, 338F, 358, and 35W.
While the 8x57 may be registered at 35,000 psi and the 270 at 65,000 psi, I have overloaded both until the primer pocket gets loose, and it is at the same predicted pressure in Quickload, of over 70kpsi and less than 80kpsi.

b) The belted magnum case heads can take more pressure than the Mauser case heads, but they are never registered at more than 65,000 psi.

c) The 223 case heads can take more pressure than the belted magnums before they get a loose primer pocket, but are somehow always registered at less than the Mauser and belted case heads.

d) 6mmBR case head and rimmed brass are not the weak link, but rather the primer piercing is. The exceptions include 1) brass flowing into a poor fitting extractor, 2) a 357 mag exceeding elastic deformation of the brass but not the cylinder steel resulting in stuck cases for finger ejection and 3) Norma rimmed rifle brass that is soft.

Working up to lose primer pockets [primer falling out or feeling easy to deprime ] is erratic and not very repeatable. This is because the pockets do not expand uniformly.
The problem with this is that the pocket may expand in places, but the spent primer is still captured by other parts of the pocket.

While it might be possible to check the pocket with a "D" shaped gauge, I have found a convenient predictor is any growth in the extractor groove as measured with dial caliper measuring with 180 degrees of rotation to check 360 degrees. This is reproducible to within a fraction of a grain of powder.


What does it all mean? Answering the engineering question, "What are you trying to do?" If the problem is loose primer pockets, measure the threshold of loose primer pockets with a sensitive instrument. Do not measure pressure.