What would cause this

[Longcarbine]

Took my new to me .300BO AR pistol out for sight in today and it ran flawlessly, no failure to fire, feed or eject but every case mouth had this flat spot on it when ejected. Any ideas as to what could be the cause.

 

[pacificman]

Looks like the case is hitting a small portion of the action after firing...I have mini 14 that does the exact same thing.
It is no big deal to me as they will all get straitened out when resizing.

 

[Richard Coody]

common
hitting the shell deflector
take a small piece of.sticky back velcro and put it.on the shell deflector. that does.help.

 

[SS427]

Soft brass. It will work harden and probably stop doing that after a few firings. At least mine did.

 

[ebb]

What Richard Coody and Pacificman said

 

[dolek]

Easy fix ...here is a visual to explain it.

 

[Rsadams]

+1 on what Richard Coody said , it's common on ARs , your re-sizing die will fix it with no problems.... That's actually not bad at all....

Since it has a short barrel being a pistol , you can do some reading on over/under gassed guns... Different springs and buffers etc....

 

[Longcarbine]

[QUOTE="Since it has a short barrel being a pistol , you can do some reading on over/under gassed guns... Different springs and buffers etc....[/QUOTE]

Well it runs perfect so I dont want to tinker with that, I can deal with a slight case mouth dent.

 

[Rsadams]

[QUOTE="Since it has a short barrel being a pistol , you can do some reading on over/under gassed guns... Different springs and buffers etc....

Well it runs perfect so I dont want to tinker with that, I can deal with a slight case mouth dent.[/QUOTE]
Just leave it alone then , no problems.... Like I said those cases are fine , there hitting the shell deflector , very common... I have seen a whole lot worse.... Your re-sizing die will fix them right up...

 

[dickn52]

HK 91's are notorious for this as they always hit the ejection port.

 

[Richard Coody]

no doubt AR's are hard on brass. i learned a long time ago that it is very disappointing to spend the time on brass prep, like i do my bolt guns, and have an ar beat it up so badly. not only bent necks but look at your case rims after a few roading cycles.

now for AR's it's load it and shoot it. Most i don't even bother to reload for.

 

[K22]

Several of my bolt rifle exhibit the same flat spot on the case necks from the ejection process albeit less pronounced than your examples. There is no reason I can see to do anything about it because the sizing process corrects it.

 

[savagedasher]

Ejection is happening before the case had a opening

 

[Westhunder]

Over gassed. Get a jp silent capture and add some weight and a heavier spring. Will increase dwell time. I had it bad until I tuned it right. Mine will cycle supers and subs, with and without suppressor, perfect brass now. No dings, no nicks. Did it all with the buffer. Eliminate adjustable gas block and control one variable!


Oh and it does this with factory and my loads. Takes some tinkering but it’s worth it.


Yours really isn’t bad, but I’ll be damned if I’m putting a piece of Velcro on my gun when the fix is as easy as a buffer assembly that will change your life anyway. Two wins, and you don’t have to look at a piece of Velcro!

 

[Westhunder]

Here is my before tuneing...

I may have 1 in 50 now that get a little ding

 

[josh shrum]

As @Richard Coody said, your brass is hitting the shell deflector, lots of simple ways to pad it.

As was said by a few here, you can tune your rifle's gas or buffer system to nearly eliminate this.

Adjustable gas blocks, adjustable buffer weights, different springs will all get you there.

I have tuned a few of my AR's, I prefer to use an adjustable gas block. I have found that I only need to mess with the buffer/springs if I am using some type of enhanced/lightweight BCG.