350,


223 round,



So starting with the mags, 223 mag is not going to work, since the inside front of it has the ribs to guide the 223 rounds up, as well as side ribs that help guide the 223 case shoulders out of the mag.

Here we can see the front rib indent that allow the side of the 223 round to stay in contact with the side of the body.  So slight ribs on the back of the case to guide the main body, and deeper rib on the front to guide the bullet/reduced neck of the case up the body walls.



So 350 mag, that does not have the inner front deep ribs that would end pushing the front of the round to the center of the mag, but equal slight rib indents for the straight wall case instead.


Next we move to feed ramps of the barrel extension, and since the 350 is using a wider bullet, feed ramps have to be wider as well.
Note, here angle of attack out of the mag, and up the feed ramps has to still have the bullet pointing at the center of chamber.  Hence ramps need to be not only .136" wider, but depth of front of feed ramp .068 deeper front the front, all the while the center line ramp of barre feed ramps still angled center of chamber.  Simply put, you can not pull this off without ramping of the upper receiver, since the deeper barrel ramps are going to require this.

Some 350 barrel manufacturers will try to do slight tricks with the barrel feed ramp angles so it works with a standard ramp-less upper receiver, but in the end, just ends up with the feed ramp angle not correct and the round just being feed upwards too high, until the end of the case is released from the mag lips.  Hence if you put the round on the feed lips angle, and push it forward at the same angle, going to find that the bullet is feed higher than center line of chamber instead. in some cases, so high that the bullet misses the chamber completly half of the time.


So the low down with the 350, is mag has to rise the top round in the stake, so it flush across the feed lip during retention.  If front of the round is not flush against the entire feed lip, but instead of centered via lower rounds pressure, then round is not going to be centered up the feed ramp on the feed.

Next, as the round is begin driving forward while the feed lips are still retaining the back of the case since the bullet is .068 wider, not only do the feed ramps need to the correct width, the correct deep, but angle of attack of the round up the lips correct as well.  So since all of this is based off center line of chamber, this end up with wider than normal M4 ramps in the upper receiver, since the angle of attack of the feed ramps on the barrel extension require such.

So at this point, strip a mag and show me the front inside of it to begin with (if it is a 350, or 223 mag) and then shotgun your upper open and show me the front inside of the upper receiver, as well as the barrel extension ramps as well.
Note want to see the ramps for width, and up the angles of the ramps to see the angle of attack.

My guess, upper with barrel installed, need to be machined to correct the feed ramp widths/depths/angles, including adding feed ramps in the upper receiver to work with the new ramps as well.

From there, quick check on 350 mag heights retention in the well to correct any problems if the mags are being retained low, and depending on the mag type, slight mods to them to get then to feed lip retain correctly,  and feed/release the round correctly on bolt strips and angle of attack as well.
Note, is some cause, chamber entry bevel may need to be slightly more radius chamfering if too sharp to start with.  

Hence case is angled out of the mag up the ramp, then as the mag lips release the rim once the bullet tip is inside the chamber, then rim of the case has to glide up the bolt lugs front surface as the bullet is still begin pushing into the chamber, until the back of the case has slipped up the bolt lugs/case has leveled out into the chamber, so the back of rim can slip into the bolt face from recess.  So in some cause, not does the chamber entry bevel need to be clean up, but bolt face for both the channel and lugs need some work at well.

If the feed lips are clearing the bottom of carrier with the bolt locked home, then not really the mag being retained too high in the well.

As for the feed ramps, lock an empty mag in the well, with the bolt locked back, then use a sharpy to mark the front of mag middle top edge (where the round comes out), to the front of the upper receiver.  Now remove the upper receiver, and take a look that line, compared to the barrel feed ramps.  Hence on a 223, front end of the round is bottle neck, so initial feed out of the mag to the ramps is much higher.  On a straight wall case, initial push out to the mag, requires the the initial feed ramps to be lower instead.  And again, rim of the case can not clear the end of feed lips, until the bullet tip is inside of the chamber.

I bring all this up, since you stated that you have reworked the feed ramps, but may still not be low enough, or the correct angle to feed the round from the mag to the chamber correctly to start with, and what is tearing up the cases.  
As for feed lips on the mag, at best if the mag is formed correctly, may just need to run a file down the bottom of the lips to remove any burs.  
If the bottom of the carrier is marring up the cases as the bolt comes back, then quick polishing of the bottom of the carrier may all that is needed to remove it rough surfaces.

Hence don't start clipping springs, since they are not the problem at hand.

And this is a 350 mag, and not a 223 mag that the inner front side rib is forcing the front of the round inward?

Then just boils down to angle of attack, hence round cleanly feed out the mag, up the ramps without binding, the bullet directed to the center of the chamber, the rim sliding the bolt lugs as the bullet continues to enter the chamber, then case rim snapping into place on the bolt recess as the extractor slips open to allow such.  

So my guess without the rig in hand, ramp depth/width/angles not correct and the case is binding up then on the way out of the mag before the bullet tip reaches the chamber, or bullet not being angle into the chamber, and this is cause the B/C to stop instead.

As for problems with the extractor or extractor spring too strong, this one rears is head with the bolt not fully locking up the last of the way (carrier about a 3/4" off the barrel extension face).

Or, could be crap mags that are not feeding correctly to begin with.