My 350 Legend build is having issues feeding from a magazine. The cartridge makes it part way into the chamber then it hangs up. Winchester FMJs feed the best but still have some hang-ups. 150gr Winchester deer season XP polymer tipped hang up almost every time. The brass shows semi circle gouges after they hang up. Upon inspecting the barrel chamber I noticed there is a small step at the breech end that is collecting brass residue. I'm thinking this step is the source of my problems, but would appreciate your thoughts on the matter.

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One more in case it was unclear where the step is located.

Break the edge with a stone at the area from 5 to 7 o’clock.  Or 11-1 right side up.  It is catching at the top of the chamber?

Don’t go hogging lots of metal.

Had that happen on straight walled cartridges if the chamber mouth bevel where it meets the chamber wall doesn't have a small radius to it and is sharp or the rims are catching on the ejector.  Doesn't take much to remove the sharpness that is snagging the cases and you don't want to remove much metal and leave the case unsupported. You likely could do it with a felt bob and red rouge.    Wouldn't hurt to bevel the ejector while you're working on it.

My buddy's 350 legend was doing the same thing.

It wouldn't close on a round either. The chamber was too short.

He sent it back and they reamed the chamber and it headspaces and will close now, but I don't know if it still kinks the brass like in your photos.

Get a real magazine for the 350 and it will feed fine. Stop wasting your time and $ with mags that don't work.

Yes it seems to be catching at the top of the chamber.

I like the felt bob and rouge idea. I'll see about picking some up today. Pulling the barrel out of the receiver will probably be best to get better access. I agree there should have been a small radius cut on the end of the chamber to help prevent this problem. Hanging on the ejector is something I hadn't considered either, I'll have to take a closer look at that.

I don't think I have any chamber depth issues, once a cartridge makes it in it fires and ejects fine. Out of curiosity is his a CMMG barrel as well?

I have two 350 legend specific mags; one marked CMMG and the other C products Duramag. They are the same besides the floorplates. When compared to the modified pmag they present the cartridges at the same height and angle. If anything the Pmag feeds the best of the bunch and definitely scars the sides of the brass the least. The Pmag has a 5 round limiter installed. I only removed the side ribs down to where the follower stops on the limiter. This leaves enough of the ribs to retain the anti-tilt function of the follower while allowing the 350L cartridges to doublestack.
My feeding problems are not magazine related. The only advantage to the steel mags is they would allow for a longer COAL than Pmags.

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No, it's a smaller boutique brand. I don't remember who made it.

In jam pic the main contact points are the bolt face(most likely ejector), chamber mouth bevel and top of chamber about 1/4"-5/8" in depending on bullet profile. Bolt strips cartridge from mag which rides up the feed ramp until the bullet hits the chamber wall and either jams like in your pic or as it's sliding levers the base of the bullet into position. Friction at one of those 3 spots is stopping the cartridge.   It looks like CMMG cut the large 30ish degree bevel and then broke the sharp corner on it with a 45-60 degree bit or so which is the area that is giving you the issue as it is still sharp.   Polishing that should get you feeding again if the case rim isn't hanging up on the bolt face. The other main friction point is the top of the chamber where the bullet and case are sliding against while trying to lever the cartridge base up.  You certainly don't want to polish your chamber mirror bright and if done correctly it will pretty much look the same, but lightly running a rouge bob over the bottom 1/2"-5/8" of the chamber  will let the bullet and case slide easier and let the magazines job of pushing the cartridge base up easier.

I just wanted to update this thread on where I'm at with my feeding issues. I did one round of polishing, primarily targeting the "ledge" I suspected was the cause of my hang ups. I also lightly hit the chamfer leading into the chamber, the feed ramps, and the top of the chamber. After clean up and re-assembly, there was much improvement in hand cycling cartridges, although live fire showed I'm not done making adjustments. The deep gouges I was getting in brass are now more of a rub mark. I'm waiting on a shipment of ammo to do further testing. I may polish the "ledge" a bit more if problems persist.
I have noticed a potential issue with my 350 Legend specific magazines that might be a contributing factor in my feeding problems. If you load up a mag then slowly strip off a cartridge you will notice the leading edge the of the case catches on the front lip of the magazine. I used a Dremel to lower that lip another 3/32" or so until the case no longer catches. I will report back when my ammo comes in and I get a chance to try everything out. Thank you for all the help I've received so far.

To me, it looks like you have typical m4 feedramps. I'd think something bigger would feed better and initially  not jamb the cartridge up so high to begin with.

Sounds like you're getting very close then.  Do you have a before and after pic/

I would polish the bolt face as well.  Remember, the case head must slide upward across the bolt face in order to get parallel to the bore or it's not going to feed......same with a rough breach face on the slide of a hand gun.

Tony

Found this, which is a review of a 450/458 bcg on Brownells:

This BCG caused fail-to-feed and fail-to-extract issues in my .458 SOCOM. Rounds were jamming nose-up half way into the chamber and I noticed small brass shavings coming from the case rim. The bolt would often close but not go fully into battery, and rounds would be difficult to extract. I learned these are known issues with aftermarket bolts for this caliber, and fortunately there is an easy fix thanks to Tony of Tromix (who developed the original .458 SOCOM). The issue is the extractor. Specifically, too much spring tension. As the round is stripped from the mag moving at an upward angle along the feed ramp, the rear of the case needs to lift into a horizontal position and smoothly center into the bolt face so it will chamber properly. The extractor needs to snap onto the rim easily for reliable feeding as well as extracting. If it doesn't, try this simple modification. 1. Strip the bolt and remove the extractor. 2. Remove the rubber o-ring. 3. Remove the extractor spring from the extractor. 4. Remove the rubber extractor spring buffer from the extractor spring. 5. Invert the extractor spring and reinstall it UPSIDE DOWN. It will no longer seat firmly into the recess in the extractor, so be sure to keep it aligned as you reattach the extractor to the bolt. 6. You're done! Reassemble the BCG, and save that tiny rubber buffer and o-ring in your spare parts collection. You should now be able to wiggle your extractor with a finger. I'm giving this BCG four stars because despite this little hiccup it seems to be a quality part at a good price. My rifle is now cycling reliably. Hope this helps out a frustrated shooter or two!

This happened to mine at first. The bullet case was the catching on the lip of the mag and kicking the bullet up. I filed down the lip a little and beveled. Also made a little wider. This was on a moded pmag

Does anyone make 30 round magazines for it?

Not that I've found, but I don't see much different other than the follower (which looks just like my Grendel one without mic'ing it out)

I opened the mouth on mine as I have don't on other metal magazines that might have caused some feeding issues on some odd-ball catridges.

That didn't help my powerpoint loading problems that are the same as above. I would think that swapping the follower on a regular metal 30rd mag might work which I'm about to try out once I get it running on something other than the FMJ's

I am having similar issues, but not exactly. On mine, about one in ten are mis-feeding and getting crimped by the bolt lugs. So much so that I don't want to fire them. I don't think it's even reaching the feed ramps so I don't think that is my problem. I am using Winchester FMJ ammo and C-Products mags.

Any ideas?
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Seems that some barrel manufacturers re using 300 blk extensions instead of the standard M4

If you look closely he has ground off the center lug of the extension. This cartridge is really picky on the crimp the round has. Too much and it causes the round to not be able to headspace right and creates high pressure. Too little and the mouth of the case grabs every sharp edge on the way it feeds.

That's what mine has 300 blk ramps. I had to file my upper receiver (m4 ramps) to match.

Issues here too, faxon barrel, three different ammo types two different mags. In contact with faxon waiting for a call back. Got to be something small on the feedramp mag or something tweaked just a little... first time i had a build not run correctly. Not sure what to think.

Check your extractors ,, seen problems like this

I guess I’ll be the turd in the punch bowl
WHY?
What is the purpose of this 10 round mag specific .357 Mag rifle cartridge ?
The 6.5 Grendel and the 6.8 are the absolute best upper options to a AR lower
The Blackout is another story but I keep asking why?
I just don’t get it
I live in TX I guess everyone is going to say you have to have this .30 cal cartridge to live up north and shoot a deer
Well buy a bolt gun
??

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Update! Trying different things because i have seen slight tolerance differences and what some call tolerance stacking. Swapped in and out different parts one at a time. Slid in an older more used bolt out of a colt and magically it feeds and functions fine! No scars or aparent abuse of bullets or brass. Gonna try to get out and try and fire it tomorrow.

Got my bear Creek barrelled 350 legend built.

Bear Creek upper (Primary arms BF)
12" PSA light weight rail
Started with a PSA premium BCG.

Using that BCG, the bolt didn't close very fluid. Release with bolt release it'd close with no ammo, letting it ride forward it took the forward assist to close.

Loaded mag, CMMG 10 round, it'd always need the assist to close.

First shot fired, ejected, fed next round but needed assist again. Tried to fire but a light click. Eject and repeat 4 more times(had 5 rounds loaded in magazine). 4 unfired rounds had very slight firing pin mark on primer.

Took out the PSA premium BCG and put in one of my PSA nitride bcgs. Very fluid bolt lock up. Didn't need forward assist riding it forward. Shot 39 rounds flawlessly. One 9 shot group had two shots separated by about an inch and a half, seven shots directly in between that 1.5" spread making one big hole about the size of a quarter, with those two outer shots close to a 1/4" inch to the right and left of the quarter sized hole.

Not sure what the issue with the premium PSA bcg is, but I'll disassemble, clean and lube, and try it out in a 223 and see how it goes. Never had a feeding issue getting the ammo chambered except the premium BCG needing the forward assist to completely close it.

You sound like a fudd democrat there Gramps.

I've been chambering for the 350 since last April.  All straight wall and big bore cartridges have issue with feeding if things are not running smoothly across the whole rifle.  I fix these issues for a living. Biggest issue is bolt nose, extractor and ejector. Next is extension feed ramps, feed cone and chamber surface finish.