Just got my RRA .458 not long ago. I've put about 30 rounds through it. So far, I can only feed one at a time through my magazine.
I'm using a Lancer L5 mag with the nose modification (1/2" dia trim out).
When I feed the first round in from the mag, the round is very tight. I can get it out, but have to use both hands. I haven't had to mortar a round out.
Once I get it out, I've got the scratches that you see in the picture.
I've read about mopping the chamber, but I don't know what is causing the circumferential scratches. I've got some scratching lengthwise of the case too.

What's going on here?
Thanks in advance for the help.

What ammo are you using?  Do you have a case gauge to check proper case sizing?   The only time I had problems was when I had unburned powder in the chamber.  You should not be having those issues with properly sized rounds.  

The cut away case gauge from Tromix is what I recommend.  They are the experts on the 458Socom.

Eta,  I use the Lancer awf magazines with zero issues.  I have both 20rd and 30rd models.  I have also used the stoner ss mags available from midway.   Neither of the above have given any problems.  I have used some cheaper aluminum and steel mags with mixed results.  Pmags can work with flat nosed bullets but can have issues swelling under the pressure of a full mag.  

If you are not on the 458Socom forum yet I would recommend you do so.  Just follow the directions in the first post regarding registration.

I'm reloading using Starline cases, Lil' Gun Powder, 300 grain Hornady JHP & Acme 300 RNFP bullets.
I measured the cases and they're the length shown on data I've found. Using Hornady dies/Redding crimper for loading.

I'll order the case gauge and join the other forum.

I have had cases that measure fine but will not feed.  The case gauge removes all doubt.  I just did a registration hopefully it was yours.

Yep...that was me. Can't wait to cruise the site.

Those look like BE marks that brass often gets from being banged against the BE lugs during ejection.  It's nothing to worry about and they get less pronounced as the rifle wears in and the bolt and BE lugs self polish.
Lengthwise scratches are usually from the mag or BE lugs that the case drags across during feeding.  If from the mag, polish the under side of the feed lips.  If from the BE, they will reduce as they polish in.

You can also get longitudinal scratches from the underside of the bolt carrier sliding over the top round in the mag.  You can test it by closing the bolt on an empty chamber, then insert a loaded mag and retract the bolt carrier and lock it back.  Pull the mag and look at the top of the round in the mag.  There will be scratches on it.

Tony

Thanks for the info. I'm relatively new to the AR platform. I have a .300 Blackout, but I don't load for that one.
The marks had me worried. Signing up for the other site recommended is a gold mine!

Thanks guys...

For the benefit of those on this site:
Posted on 458SOCOMforums.com

Ok, think I found the problem.
I found the thread in "Equipment" "Tromix Case Length Gauge" where Big Bore talks about adjusting the sizing die to get the case to fit the chamber.
Tried one of my previously sized/trimmed case and couldn't get the bolt to close.
I had to shave .007 off my shell holder, but eventually got the bolt to close with just a very slight push on Forward Assist.
I loaded a few rounds to 2.020 COAL and they stripped of the mag and loaded with no problem. Yeehaaaa!    8)
Obviously, my shoulder wasn't getting pushed back far enough.

My Tromix Case Gauge should be here in a day or two.

Next question, I've got a number of rounds loaded prior to finding you guys and all the great info. They're a pain to get the bullets/powder out and depriming.
Would there be any problem with shooting them?
If not, after depriming, can I use the primers again? Never done it, so I just don't know.

Can't wait to get my Case Gauge to verify everything, get some loaded and get back to the range!

Why would you have to deprime the cases if you didn't fire them?  Throw 'em in a ballistic puller, whack, whack, whack, dump the power back in, reload to correct length using a previously primed case.

Because I have to resize them to push the shoulder back a bit.
Guess I could pull the deprimer out of the die.

Now you're talking.

I forgot how much of a learning curve/ experience it is.

Brings back memories of shattering my first RCBS puller on a 2x4 over concrete in a cold winter garage.

Reeds ammo was the only thing available at the time, 2006.

wouldn't chamber, had to pull em and size em.

Think I still got a box in the back of the safe.

Got my case gauge today...adjusted my sizing die a bit more...all better now!
Can't wait to get to the range. Much easier on the shoulder than my 45-70. Loving it so far. Just had to get out of the bottom of the learning curve.

The feed ramp in my lar-458 upper had some pretty funky edges on the bolt lugs that would scratch my brass up too, I got a little zealous and filed the offending lug edges smooth. On the subject of gauges though, all of my loads feed and cycle fine in my AR but they all stick in my bolt gun's chamber. I've been busy with life for awhile so I never got around to ruling out what I suspect is a headspacing or chamber issue. If I recall correctly even sized brass without a bullet would stick in that chamber. Haven't even gotten around to seeing if the brass needs trimming for that throat or if the shoulder needs to be bumped more.