Hoot wrote:Something's not right, either with Hornady's SAAMI spec sheet or their manufacturing tolerances. From four different brass sources, both used and brand new, the grooves are running .393-.400, not .409 and rims are running .366-.368 not .473.
On the Winchester .284 brass I bought today, grooves are running .408-.410 and rims are running .472-.473. Seems Hornady is drifting downward from the SAAMI spec for some reason.
What's equally as odd is the Bushmaster extractors are favoring the out of tolerance Hornady brass. So, is the dog wagging the tail or vice versa?
Anybody got the inside scoop on this?
Hoot
Before modding anything, I'd want to know whether the extractor is riding high on the groove or the rim.
Here's an
exaggerated diagram of what I mean:
To test this, I took a fresh .284 case with a groove diameter of .410 and rim diameter of .473.
First, I trimmed the groove down to .400 like new Hornady brass
I tried hand-cycling it and felt the same resistance as an un-modded .284 case.
Then, using the same case, I trimmed the rim down to .468 like the same piece of new Hornady brass
I repeated the process and suddenly, the bolt was easily slid into lock and out like the new Hornady brass
control round.
So, my perception of this issue and I'll be tyhe first to admit it's anecdotal to my rifle and brass lot, is that the issue lies in the .284 brass having a rim diameter that is to spec for Hornady's SAAMI drawing as opposed to their own brass diameter which appears to be low, again to make a point
to their own spec.
Before I took metal off of my extractor with unknown consequences to how it would behave with the smaller Hornady brass, I'd consider just turning down the .284 rims a little.
This whole conundrum with the specs versus what is being delivered leaves me at a loss. If I was grasping at straws of explanation, I guess that Bushmaster started out with a custom extractor spec'd to accommodate Hornady's SAAMI brass drawing and when demand exceeded their ability to produce or contract out that custom extractor, the switched to a 7.62x39 extractor which was more available and this is a reach, got Hornady to reduce their brass diameter spec for the groove and rim to match. That is a lot of speculation and there is a plethora of other explanations.
Hoot
edit: I was composing this while gunnut was posting his reply so excuse the gap