What is the maximum expansion at the web that wouldn’t concern you. I’m measuring some brass in a 25 CM I built & it’s more than I’m used to seeing. I understand whatever size the case is it has to expand to fit the chamber. On the attached pic you will see 2-1/2 thou. I am measuring not at the .200 line but immediately above the extractor groove? At the .200 line it’s 3 thou
Case expansion at the web after fired 1x. What is acceptable max?
- Thread starter Lynn313
- Start date
The reason for asking is safety concerns. On a new rifle & new brass I always check pre-fire measurements against post fire. I don’t remember having this much.
Murray Brook
Silver $$ Contributor
I don't understand your concern. Could you elaborate. TKS
For any useful comment I'd need to know a great deal more about the techniques used to measure as well as the brass and the load. When I hear web I think of the solid head of the case. Long ago in my folly after reading Bob Hagel and Ken Waters judging pressure by expansion I bought a cute little jig from one of the majors for measuring case expansion to tenths resolving to half a tenth indicated for a guard digit.
In your circumstances I would section a case to see how the case head is laid out and what exactly is being measured. The point of greatest expansion is usually well forward of the solid web with a slight ring (not the bright ring of beginning case separation) as the brass thickens dramatically toward the solid head - except in ballon head cases not seen in the field lo these many years. For the solid head to expand very much is unusual and remarkable but for brass to move around with oval chambers or excessive headspace isn't. Readings can be affected by burrs from the extractor tilting the case in blocks and a number of other things to watch for.
In your shoes I'd likely try measurements of once fired, twice fired and three times fired neck sizing only and as little as possible to get an idea what a case fitted to the chamber is likely to measure. Continued expansion in the numbers mentioned would indeed be a concern and I'd verify load data and as fired.
In your circumstances I would section a case to see how the case head is laid out and what exactly is being measured. The point of greatest expansion is usually well forward of the solid web with a slight ring (not the bright ring of beginning case separation) as the brass thickens dramatically toward the solid head - except in ballon head cases not seen in the field lo these many years. For the solid head to expand very much is unusual and remarkable but for brass to move around with oval chambers or excessive headspace isn't. Readings can be affected by burrs from the extractor tilting the case in blocks and a number of other things to watch for.
In your shoes I'd likely try measurements of once fired, twice fired and three times fired neck sizing only and as little as possible to get an idea what a case fitted to the chamber is likely to measure. Continued expansion in the numbers mentioned would indeed be a concern and I'd verify load data and as fired.
You need to measure the chamber before you have any meaningfull information.
I’ve always read & heard that expansion at the Webb could be the start of case head seperatio. The start of case head sep starts from the inside of case so no visible signs of a problem other than knowing the expansion took place by measuring.
Good to record virgin cases but there only virgin dimensions once, your reamer shows .471 at the .200 your fired cases are at .4695 , that’s about perfect imo’ what’s the resized number just above the .200 ? You need to chamber freely before and after ignition so about .468 -.469 resized should be great.
This was new brass measured before & after the 1st firing.. I usually do this on new brass in a new rifle for 2-3 firings until I think all looks good. I usually see under a .001 thou expansion. I used the thin part of expensive Mitutoyo calipers to measure right at the 1st full diameter above the extractor groove. I also just checked at the .200 line for to measure against the reamer print diagram.
My guess is the next firing will not show the same expansion. Is the webb designed for that kind of expansion on the initial case forming 1st fire?
Thanks for helping!
I didn’t look for it, but I can say nothing obvious. This is the 1st firing of new brass in a new rifle.
A case being resized is unsupported below the .125 mark due to shell holder. An exact .200 measurement isn’t necessary, an easy way is turn your calipers lengthways and measure the fat part. Mine take a couple firings but as I said they never return to the virgin dimensions so all we care about is before and after resized.
Using a caliper to measure case head expansion is pointless.
At the very least you need a blade micrometer that will measure to 0.00005 that is set
up to measure the case at the exact same spot every time.
Another thing to consider is the brass alloy itself, different manufactures brass will
have different results.
In my opinion firing factory ammo, measuring it and then using the same make brass
would possibly provide (maybe the same maybe not) a reasonable expansion measurement.
At the very least you need a blade micrometer that will measure to 0.00005 that is set
up to measure the case at the exact same spot every time.
Another thing to consider is the brass alloy itself, different manufactures brass will
have different results.
In my opinion firing factory ammo, measuring it and then using the same make brass
would possibly provide (maybe the same maybe not) a reasonable expansion measurement.
No. Expansion is the result of pressure inside the case with lack of support outside the case or hammering backwards.
As implied above primer pockets can loosen and in magnum revolvers where there must be enough slack for cylinder rotation the primer pocket can actually be hammered shallower.
Case head separation is caused by the case stretching beyond elastic limits to fill the available space when base to shoulder space - headspace except in belted or rimmed where it is not called headspace but the mechanism is the same and rimmed straight cases can stretch and break as well as cases with shoulders- is excessive. The stretch marks will always show a bright ring outside where the case stretched and separates or will soon separate. Think of it as most of the case from the shoulder back is stuck to the chamber wall and at some point the head and some distance forward of the head moves back to fill the excess headspace. The internal thinning at the depth of the outside bright ring can be felt with a wire probe and seen with a bore scope. I used to keep some sectioned stretched cases around but I haven't seen them in years. Variation in base to shoulder length will show up on Wilson gages or my choice the Redding instant indicator or with attachments on calipers and so it goes.
Dusty Stevens
Shiner
Sounds like you had a significant runout problem when it was chambered or some type of chambering issue. The reamer could be at fault but id point toward the chambering process. Either way, get it measured and fixed before somebody gets hurt.
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