Case extractor groove variations

[Siringo]

There has been some posts here lately about cases chambering hard, poor extraction, broken extractors, ect. The weekend I was shooting some loads in prepared 284 cases and noticed something that had not been apparent before when using 450B cases. The extractor on my rifle did not have any significant wear marks prior to this weekend -- extremely minor. When cleaning I had noticed substantial wear on the sides of the extractor where it would passes thru the locking lugs on the barrel extension.

I checked the extractor groove dimension on the 284 cases and mine measured .407 inches. Saami drawing I have says .4091. New 450B cases measure .400. Could this be enough to push the extractor out to rub on the barrel extension locking lugs????

[Hoot]

Your hypothesis has merit John. Interestingly, after 350 rounds, my extractor is barely showing wear enough to even polish the oxide coating on it. I'll take that as a good thing. actually, the only wear I'm seeing on my bolt and carrier is an effacement of the oxide in a small streak near the gas key.

Some of my 450b brass have grooves as narrow as .393 inch. That's getting close to the 7.62x39 spec.

Hoot

[wildcatter]

There has been some posts here lately about cases chambering hard, poor extraction, broken extractors, ect. The weekend I was shooting some loads in prepared 284 cases and noticed something that had not been apparent before when using 450B cases. The extractor on my rifle did not have any significant wear marks prior to this weekend -- extremely minor. When cleaning I had noticed substantial wear on the sides of the extractor where it would passes thru the locking lugs on the barrel extension.

I checked the extractor groove dimension on the 284 cases and mine measured .407 inches. Saami drawing I have says .4091. New 450B cases measure .400. Could this be enough to push the extractor out to rub on the barrel extension locking lugs????

Siringo 'ol Buddy, you may need to dust the backside of the extractor a few thou. Keep in mind that this conversion is like owning an ZR1 Corvette (needing tweaking from time to time) and is right out to near the end of the capabilities curve. With the +- standards being what they are, well, just keep the extractor and the corresponding, barrel extension lug/grove clean. If you did start to get short strokes, from the backside of the extractor rubbing in the groove or on a lug and cleaning it becomes excessive, then dust off a few thou with some emery cloth..t

[Siringo]

This is not the best photo (from my phone), but you get the "picture". No more 284 cases until I can get the groove reduced. Another added step/operation for 284 case use!

Also note that after the first firing of the 284 cases, I had difficulty getting the cases into my shell holder on my Lee Priming Tool -- fresh cases were OK . I also suspect that this area (the groove) can grow in diameter and shorten over repeated firings due to case thrust. I ran into a post somewhere that the 50B was having this very problem. Since the 284 case is thicker in the walls, at 35K psi it does not grab the chamber walls as easily as the thinner 450B case. No matter what load I use (within Hornady data), I get sooting in the extractor groove and this does not happen with the 450B cases.

[Siringo]

T -- I get what your saying.

[Hoot]

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

[gunnut]

This may be related. viewtopic.php?f=5&t=65&start=0

[Hoot]

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

[BD1]

It would be interesting to see what different extractors mic at, and compare that to the production dates of the uppers. If I get a chance this evening i'll take a close look at mine which was produced winter of 2009, or before.
BD

[Siringo]

As a matter of interest -- all my new factory rounds (with my cheapo caliper) measure .469 to .470. I measure a 100 of them. The used brass is hard to measure because they are bunged up quite a bit. The 284 brass I have measures .473.

Seems we need a cutter tool to true up the 284 brass.