350 Legend Issues

[Dedhed]

This is going to be a long post. I built a 350 Legend this week. Built with Anderson lower, upper, and BCG. A CMMG 16”, carbine length gas system barrel. Aero carbine length buffer tube, and carbine length gas tube pinned to a UTG low pro gas block. Ruger 5 round steel mag. Standard mil spec trigger. Leupold VX-Freedom 350 Legend scope for 150 grain cartridges. Which I’m using the Winchester 150gr XP’s.


Now for the fun. First shots out, bolt goes about half way open with spent case still attached to bolt face, short stroking. Obviously won’t lock back on empty mag. Started out with a Luth carbine buffer spring and an unknown 3.123 once buffer. Bought and installed a Aero carbine buffer weighing 2.989 ounces. Same problem. I had a different unknown buffer spring that seemed lighter, that is black, I put in with that buffer. Broke a piece of wooden cotton swab, stuck it in the gas hole slid the gas block on. Which was dimpled from the factory. Turned upper upside down and the swab fell into the gas block, so everything is lined up. Has no visible gas leaking from around the gas block, or tube. Still nothing.

Measured my gas port on barrel, it measured .0995. Drilled it out to .116. Now with the Aero carbine buffer and unknown black buffer spring after drilling it ejected but didn’t lock back. Never checked to see if it would pick up another round.

Next steps were to drop a weight out of the buffer, weight of buffer is now 2.230. Now it locks back about 35-50% of the time. After making progress, now, I cut 3 coils off of the unknown buffer spring, it locks back 100% of the time now.

I did put that upper on a 300 blkout lower I have with an H2 buffer just to see what happens. Well it ejected but, barely, brass fell at my feet.

It has never had any issues feeding, from an open bolt or slingshotting a round in.

All that being said,what am I missing, or what else should I check to run an uncut spring and regular weight carbine buffer? Am I just bound to run it the way it is now? Ejection pattern is about 4-5 o’clock. Thanks!!

[titsonritz]

. Built with Anderson lower, upper, and BCG.

I think I found the problem.

[Dedhed]

I could agree with you if you give me an example why. Yes this is the cheapest build I’ve done to date. To be honest the BCG is very well machined, I would say just as good, if not better than other high dollar BCG’s I have. The only downside I can actually fault Anderson for at this point, is the play between the upper and lower.

[Disciple]

To be honest the BCG is very well machined, I would say just as good, if not better than other high dollar BCG’s I have.

There are lots of shiny carriers that are pretty on the outside, but with rough and out of spec inner bores or with improperly seated and secured gas keys. After confirming gas port alignment the very next thing you should do is get a known good BCG and swap it in, before you start drilling, trimming, etc.

[georgeib]

There are lots of shiny carriers that are pretty on the outside, but with rough and out of spec inner bores or with improperly seated and secured gas keys. After confirming gas port alignment the very next thing you should do is get a known good BCG and swap it in, before you start drilling, trimming, etc.

Exactly this. First thing to do is try a known good BCG after insuring the gas block & tube are installed correctly and lined up with the gas port. Lube everything liberally also, and add a little lube to the entire buffer spring while you're at it, OP. This is a new build, and friction will play a factor until the parts mate.

[HKGuns]

Even if you use known quality components, it can sometimes be difficult to get these odd caliber rifles to feed, eject and cycle reliably. You can ask my 6.5 Grendel if you don't believe me.

What you've built, as you are finding out, is unlikely to run properly EVER. You've literally picked the academy award winners of what components to avoid when building a rifle you expect to run properly.

UTG gas block, seriously? You can get a reputable gas block for less than $30. Take it off, trash it and replace it with something of known quality.
Anderson BCG? Take it off and trash it for a reputable BCG like SIONICS - Colt - etc......

You're already drilling the gas port too large, so I'd trash the barrel as well and buy a midlength gassed barrel with the correct size gas port for the cartridge.

Why you jumped right into gas port drilling and spring cutting is beyond puzzling. Its likely you have and still have a gas problem either from poorly aligned gas system or crap components.

ETA: After you get your components replaced, buy yourself big bore followers for the mags you plan to use and label them 350 Legend. No, regular AR15 followers are unlikely to ever feed reliably.

BIG BORE Followers

[mig1nc]

If you have other builds laying around, try swapping to a known good BCG.


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[Disciple]

ETA: After you get your components replaced, buy yourself big bore followers for the mags you plan to use and label them 350 Legend. No, regular AR15 followers are unlikely to ever feed reliably.

Doesn't 350 Legend require its own magazines, not just a follower? The case is wider and will double-stack wider than a 5.56 case. Unlike 458 SOCOM, 50 Beowulf, etc. it will not approximate a single-stack. This is surely a recipe for severe binding.

[HKGuns]

Doesn't 350 Legend require its own magazines, not just a follower? The case is wider and will double-stack wider than a 5.56 case. Unlike 458 SOCOM, 50 Beowulf, etc. it will not approximate a single-stack. This is surely a recipe for severe binding.

Never built one, you may be correct. I was going from experience with other cartridges in the platform.

[hotrodder636]

Even if you use known quality components, it can sometimes be difficult to get these odd caliber rifles to feed, eject and cycle reliably. You can ask my 6.5 Grendel if you don't believe me.

What you've built, as you are finding out, is unlikely to run properly EVER. You've literally picked the academy award winners of what components to avoid when building a rifle you expect to run properly.

UTG gas block, seriously? You can get a reputable gas block for less than $30. Take it off, trash it and replace it with something of known quality.
Anderson BCG? Take it off and trash it for a reputable BCG like SIONICS - Colt - etc......

You're already drilling the gas port too large, so I'd trash the barrel as well and buy a midlength gassed barrel with the correct size gas port for the cartridge.

Why you jumped right into gas port drilling and spring cutting is beyond puzzling. Its likely you have and still have a gas problem either from poorly aligned gas system or crap components.

ETA: After you get your components replaced, buy yourself big bore followers for the mags you plan to use and label them 350 Legend. No, regular AR15 followers are unlikely to ever feed reliably.

BIG BORE Followers

Where is the "like" button?