Hoot brought up some interesting ideas and questions about brass case dimensions in his posts above. Because I had not read the posts in the AR forum about the extractor difficulties, these points were new to me and pretty interesting.
To get first-hand info and insight into what was going on, I dug out three different batches of 450B cases, another batch of 284 Win, and two batches of 7.5x55 Swiss. I measured the diameters of the rims, extractor grooves, and case bodies (just in front of the extractor cut). My method was to select three cases randomly from each lot, and measure the diameters at three different edge locations on each case. I selected an arbitrary starting point, and measured at that point and at rotations of 60 and 120 degrees from the start. I used a pretty good Mitutoyo caliper, reading and recording to 0.0001", and rounding to the nearest 0.001" after finding averages.
All the brass except the 7.5x55 brass by Norma were new, unfired cases or cartridges. The 7.5x55 Norma had been fired 8 or 9 times with mild loads.
Here are the resulting averages:
.
- meas.png (12.83 KiB) Viewed 12559 times
.
There was not a lot of variability around the averages. At most, individual measurements differed from the average by +/- 0.001.
As a point of reference, here are the standards from SAAMI and CIP for these cartridges:
.
- stds.png (10.92 KiB) Viewed 12559 times
.
(Note that the standard max diameter for the head of the 7.5x55 case was given as 0.493 in a drawing in a post I made above. This smaller diameter was taken from a description of an actual cartridge, not the CIP standard drawing. My oopsie.)
Again, I wasn't out to prove or disprove anything with these measurements. I wanted to scratch a curiosity itch, and learn more about the possibilities of converting 7.5x55 brass to 450B.
Interesting Point #1: From the standards table: All the diameter max/min standards for the 450B and the parent 284 Win cartridge are identical, except for the extractor groove minimum. Among SAAMI-approved rimless cartridges, the
450B is unusual in having extractor groove diameter with a minimum of only 0.010 less than the max. Almost all others have a minimum of 0.020 less than max. Even much smaller cartridges like the 223 have the same 0.020 tolerance.
This oddity in tolerance is not tied to having a rebated rim, because the 284, the parent cartridge of the 450B, has a min 0.020 less than the max.
Interesting Point #2: With one exception, the diameters of all the individual cases I measured for the three different cartridges are all within SAAMI/CIP tolereances. The Rim Dia of the 284 has a tolerance of 0.473 max to 0.463 min. The measured average diameter was 0.468, which is exactly in the middle of the tolerance range.
The
exception is the extractor groove diameter for the two lots of Hornady 450B cases. At 0.398 and 0.396, they are 0.001 and 0.003 less than the minimum allowable diameter. This corroborates Hoot's observation of the extractor groove in the Hornady ammo being less than standard.
Interesting Point #3: If the 450B had the same tolerance as the 284 (min of 0.020 less than max), the Hornady 450B cases would be well within the allowable groove diameter tolerance.
Interesting Point #4: The rim diameters of all the cartridges are within tolerance limits.
Interesting Point #5: Relative to Hoot's note about the extractor problems with 284 ammo being used in 450B ARs: Were there any reported difficulties with the use of the 450B cartridges manufactured by Remington?
Interesting Point #6: Measurements of the 7.5x55 case head dimensions indicate that turning down the rim by 0.020 and thinning the rim appropriately should produce cases that are usable in at the hind end. The front end will need relatively more work.
Other interesting points about these measurements will surely occur to readers.
--Bob
- stds.png (10.92 KiB) Viewed 12559 times
