Gunsmithing HELP !! Marks on Brass

[WillAdams]

Friend bought an Armalite AR10T 24" after having had one of their AR10 carbines. He was up here the other weekend and we went shooting using some never fired LC brass I had. Loaded up 50 - 168 Amax and IMR4895 powder. Rifle shoots very accurate and he was extremely happy to get out to 800 (his home range is only 200).
Anyway, we get home and start prepping brass for reloading & I notice two marks on the neck and when checking all the other pieces ... they are the same. Do a check and ALL the brass has the same marks. Looks like rifling, equidistant, parallel and fairly deep. Not good, Using a light I can't see anything in the neck but we do a overnight soak and in the AM we run 2 pieces of never fired brass and they have the same marks.
Chuck calls Armalite and they tell him "Shoot it, won't hurt". Gunsmith (at Armalite)even goes so far as to tell Chuck, "You'll only get 5 reloads." This is virgin LC brass I had tucked away.
So ... anyone got a connection with Armalite so we can talk with someone who "has a clue"? Rifle has less than 100 rds down tube. 50 we shot plus 39 from earlier. Chuck found that brass (FGMM) and it has the same marks. I don't own a gas gun, but this doesn't look or sound like an "Operator error".
Ideas ?

Thanks

 

[bigwheeler]

Re: HELP !! Marks on Brass

It's not uncommon, sounds like the ramps are scraping it on it's way in. Hand cycle
some dud rounds through the mag and load one by hand into the chamber and you
will know if it's happening on entry or ejection. 5 rounds is about it on brass if you
load hot at all. If you keep your pressures down and no extraction marks you may
get more. Gas guns are a lot harder on brass than bolts. Remember you are full length sizing every time.

 

[JRose]

Re: HELP !! Marks on Brass

Will, my DPMS LR308 beats up brass as well...

 

[Pat M]

Re: HELP !! Marks on Brass

double mark on neck is typical, happens on my DPMS 308 and 260

 

[338LM]

Re: HELP !! Marks on Brass

My assumption is that the 2 marks on the neck is the brass moving sideways toward the ejection port from ejector pressure and extractor pull. The neck of the brass is riding between 2 splines of the barrel extension that have sharp edges. Take a new (empty unprimed) case and insert it into the chamber by hand. Close the bolt. Pull the charging handle back slowly and hold the brass in from flying out. You will see the 2 lines in the neck on the ejection port side when you have pulled the bolt back far enough. I locate the sharp edges on the splines of the barrel extension that are leaving the marks and stone them out. The more overgassed a rifle is the deeper and more noticeable these 2 lines will be.
Does your brass have a dent in it from hitting the brass deflector?

 

[WillAdams]

Re: HELP !! Marks on Brass

Well it is new to Chuck, he said he hasn't seen or had this problem in the carbine & that is why he bought the 10(T). I know gas guns are rougher than bolt or lever or drop action or ???? That is why I used LC "plain jane" brass that had never been fired. That way it would form just to that chamber.

Looks like either 338LM or Bigwheeler nailed it. It was 2 sharp areas on the ramps/barrel extension (??), which is "fixable" as compared to being a problem inside the neck/chamber. No ejection dings , I recall . I thought I had hand loaded an empty case into the chamber THEN dropped the bolt. That was the other week after a late night & early AM. Rifle is about 500 something miles away from me right now & Chuck still hasn't sent the pictures he promised. Oh well, it is somewhat solved. He said he could feel the rough area ...

Thanks guys,