What cause this flat spot at the end of the neck?

[jlow]

This is a piece of Lapua .308 brass after it was fired from an Accuracy International AT bolt gun with an OEM barrel. The case has been trimmed/chamfered/deburred to a length of 2.011" and verified with a caliper so under SAAMI spec for .308. Bullet was a 175gr SMK and power was 43.3gr of Varget so not a hot load. Bullet was seated to obtain a jump of 12 thousands. Good accurate load with no pressure signs.

What is interesting are these swipe marks across the ends of the case neck. Swipes are never across the complete ends of the neck but in distinct spots, sometimes one, sometimes two. All cases have this. Not visible in photo is the fact that the swiped area are bright signifying a recent event i.e. from the firing/extraction. I have tried cases with slightly longer neck i.e. 2.015" i.e. right up against SAAMI specs and I get the same thing i.e. never swipes across the whole neck end, just one or more small spot like this.

I am just curious as to what would cause this?

 

[T 700]

I bet it's hitting something as it is extracted/ejected.

 

[Sniper338]

well you can rule one or the other out if you fire a new piece of brass, then slowly take it out and dont let it touch the side of the action and remove it by hand.... to check and see if its happen upon firing, or upon extraction..

thatd be the first thing Id do, narrow it down as to when... after that call in the experts cuz im not one.

following though because ive never had that exact thing happen.. curious...

 

[ireload2]

Plunger ejectors do that. I first found that with a Rem 700 about 50 years ago.

If you pull the bolt open slowly the case snaps sideways in the action as it is pulled from the chamber.

If the bolt is pulled open quickly the case comes out in a flat spin and hits the stock, the receiver or the scope.

To stop it cut a coil or 2 off of the ejector spring.

 

[Toejoe12]

Most likely hitting your action just behind the opening of the port due to a strong ejector spring. I fixed mine by cutting out a piece of moleskin (or any soft piece of adhesive felt would work) found at any Walgreens or CVS in the blister/foot products section. I've had even small dents on the necks from one of my GAP builds. Moleskin piece fixed the issue.

Look closely right behind the port opening. If there are very light small marks, That's the issue. My gun was cerakoted so it was easy to see.

 

[jlow]

I have now put in 3 different fired but resized brass and trimmed to 2.011" length into the chamber, close the bolt and ejected them carefully so they are not hitting anything.

All these cases have been recently trimmed/chamfered/deburred with a Giraud trimmer and have good clean ends. What I find is when I chamber and ejected them in this manner, they all have mushed sections at the end of the neck.

I am now convinced that what is happening is the necks are still too long (despite the fact that SAAMI spec is 2.015" and I am at 2.011"), and the necks are hitting the second shoulder.

My plan is to take a piece and trim it down gradually with the Giraud and chamber it after each trim and see if there is a length when this does not happen.

 

[Toejoe12]

You may have found the issue. My Giraud is set to trim my 308 cases to 2.005. I've always trimmed 308 to this length

 

[jlow]

That is the length that most reloading manual use.

 

[hrlincoln]

Can you tell if the 'spots' look scraped (i.e.. loss of material) or just deformed?

 

[dminn1]

I have now put in 3 different fired but resized brass and trimmed to 2.011" length into the chamber, close the bolt and ejected them carefully so they are not hitting anything.

All these cases have been recently trimmed/chamfered/deburred with a Giraud trimmer and have good clean ends. What I find is when I chamber and ejected them in this manner, they all have mushed sections at the end of the neck.

I am now convinced that what is happening is the necks are still too long (despite the fact that SAAMI spec is 2.015" and I am at 2.011"), and the necks are hitting the second shoulder.

My plan is to take a piece and trim it down gradually with the Giraud and chamber it after each trim and see if there is a length when this does not happen.

It would be interesting to know if the mark appears in the same area on each case with respect to how it was oriented when it entered the chamber. If they do it might indicate a burr or problem with the chamber in that area. If they are showing up in random areas then it could indicate a problem with your trimmer not cutting the case mouth square. Just a thought.

 

[mr45man]

I have a 308,coned breach and bolt.
I can feel the cartridge hit the chamber edge on.
Loading.
Check that out.

 

[jlow]

Can you tell if the 'spots' look scraped (i.e.. loss of material) or just deformed?

The spots looks like they are flattened. My guess is the trimmer cannot trim the ends of the neck 100% flat and it is the high spots that gets mushed down first.

 

[Bart B.]

Plunger ejectors do that. I first found that with a Rem 700 about 50 years ago.

That's why I removed that ejector from two Win 70 and one Paramount bolts. No more empties flung out and onto crud. Pull 'em out with fingers then put 'em in the box without dirt, sand, or other crud. Powder residue is not a problem.

While those spring loaded ejectors push chambered rounds into chamber shoulders perfectly centering their neck and bullet into the chamber neck and bore, firing pins do the same thing without the spring loaded ejector.

I've "tuned" M1 Garand ejectors to fling fired cases out at 1 to 2 o'clock instead of into eyes, foreheads and clothing plus onto op rods and nearby shooters. Cut off one turn of the ejector spring at a time, then shoot a clip of ammo.

Some box magazines, receiver cartridge guides and barrel breech shapes are prone to catching chambering case mouths.

 

[Toejoe12]

That is the length that most reloading manual use.

Yep. And AI is known to be well within SAMI specs (similar to custom work) and not be on the min and max thresholds as some commercial firearms mfg. so in this scenario it would be good to try the listed norm. My point is their chamber cut areas are gonna be similar to custom cut chambers in many firearms and not out to the 2.015 region.

 

[Comrade Terry]

despite the fact that SAAMI spec is 2.015" and I am at 2.011"

I'm not sure that's going to solve it. I trim my .308 cases to 2.010" with a Giraud, and have had no issues. Of course, your chamber may be shorter than mine.

 

[Aceball]

Make certain the cases are 10-12 mils shorter than the published max case length. Feed by hand an FL sized empty case in the chamber, close the bolt completely and carefully extract. Does it have the mark? If so, its something in the chamber neck. If not, and you're feeding from the magazine, then the chamber most likely has a sharp edge catching the mouth as it being ramped in. One other thing, marked the case head and note the orientation of the mark.

 

[jlow]

I'm not sure that's going to solve it. I trim my .308 cases to 2.010" with a Giraud, and have had no issues. Of course, your chamber may be shorter than mine.

Well looks like you have the magic number... I sequentially trim three different pieces one thousands at a time, and they all had that problem until I reached 2.010" at which point no problem. So it looks like the chamber is a bit short at the second shoulder but I am now good to go. Just glad that I have the Giroud so trimming all my brass a bit more is not a hassle.

 

[dmoran]

@jlow
Another aspect to consider is: carbon build up at chamber end, and/or carbon rings, can be a culprit to the issue that you are experiencing, that can greatly effect case end to chamber clearance.
Donovan

 

[jlow]

@jlow
Another aspect to consider is: carbon build up at chamber end and/or carbon rings can be a culprit to the issue that you are experiencing, that can greatly effect case end to chamber clearance.
Donovan

Agreed, but this does not appear to be the case as I inspected my chamber today with a borescope.

 

[Steve Ladino]

It would be interesting to know if the mark appears in the same area on each case with respect to how it was oriented when it entered the chamber. If they do it might indicate a burr or problem with the chamber in that area. If they are showing up in random areas then it could indicate a problem with your trimmer not cutting the case mouth square. Just a thought.

I agree that the orientation would be very helpful and marking with a magic marker should enable this process.