Short Chambe:rM305

Sluggo_m14

Sluggo_m14

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I have noticed when checking my brass from my M305, that the neck appears a little "rounded" after firing. I measured a couple of cases and they were around 1.998-1.999" before resizing, I haven't had to trim the brass after resizing but the case length will come out at around 2.001, .The SAMMI chamfer diagram shows the chamfer at the throat to be located at 2.025"from the breech face, the rounded portion of the case looks suspiciously like the 35 degree chamfer. I have'nt really noticed this on other cases from other rifles, maybe this is more common than I realize.
I was wondering if anyone had any thoughts on this?

308M14firedcase_zpsc152f12b.jpg


The case on the right is fired showing the "rounding". The case on the left is resized.
 
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I've never noticed the 'rounding' but I have noticed that line just down from the mouth on the resized cases. Not all cases have it and I just keep reloading. If you hadn't mentioned it I wouldn't have remembered it.
 
get some cerro-safe chamber casting medium and cast the chamber. pretty easy to do and will show you every detail of whats inside there
 
When the gasses is pushing the op rod to eject the case brass is still not out of the chamber. During this time the case is trying to form to the chamber. For the m14, since the rod is already pulling out the forming case in the chamber, it is therefore not chambered formed. The case is actually being deformed in the process. Thus the rounded shoulders. This is an issue for those that need the brass for hand loading. Otherwise it's perfectly normal in a military battle rifle. BTW, this is where powder burn timing is crucial in timing the op rod. This one reason 4895 has the right burn rate for the m14.
 
308M14firedcasewithnote_zpsab875bb5.jpg
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ton45-II -Thanks -Sorry-my original post could have been a little clearer,I was asking more about the forward edge of neck opening rather than the shoulder,and it seemed to me that my cases were maybe pinching at the throat a tiny bit maybe.
 
Still might be a case of case deformation during the ejection process. It's too bad there is no way of locking the bolt in place so one can truly know the chamber form when fired. The best next thing is to shut your gas valve and see if you still have the same result.
 
so just to clarify....... yer talkin about the rounding at the lip of the case mouth?
it's a good pic but still hard to see on my laptop.
but judging by the circles and arrows you drew, it looks like that area
that wouldn't be caused by extraction/ejection i don't think as it looks slightly and evenly rolled inwards...... or i'm blind and out to lunch LOL
another vote for a chamber cast ;)
 
Yes I am referring to the rounded lip at the case mouth and there is no crimp.The left hand cartridge in the picture is fresh from resizing. It is a very uniform feature on the fired case. I have noticed the same thing with factory ammo.
 
I religiously inspect my fired brass and the test fire brass from every rifle I've built and I have not seen that as far as I can remember anyways.
 
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