The fact that the bolt was locking back on the catch (catch in front of the bolt face, and not just under the bottom of the carrier holding it back), tells you that the problem is not a cycling one so long as the bolt is locking back on the last round out of the mag.


So this brings the problem to either the mag not recovering the top round correctly, the mag not help up high enough in the well, or the angle of attack of the round out of the mag, up the feed ramps, and into the chamber with the bolt face behind the rim of the round.

Hence with the bolt lug mark on the case, screaming of a bolt over rim jam, and should have looked like this,



So the quick way to weed out the problem, start with a stripped mag body, and the bolt catch removed from the lower.  Install the bolt until the top of its feed lips max out against the bottom of the carrier then look at the mag catch slot edge on the mag, against the bolt catch channel itself.  The top edge of mags catch slot should be flush to the top of the slot, and not way above it.

Granted that the catch slot is too low on this receiver, the photo at least gives you the idea of where you are looking.



Next, since you already have the mag apart, clean all the parts (including the inside of the mag body) with CLP, then put the mag back together for some testing.
Note here, when you have the mag apart, debur the bottom of the feed lips, make sure that the skirt of follower does not have casting edges, and make sure that the front inside of the mag body top edge does not have a bur that will catch a bullet tip).

On the testing, load 2 rounds, shove the rim side of the top round down, let the stack snap back up, and check to make sure that the entire body of the mag is tight against the entire mag lip.  Do the same at the front of the round, then add in another round, and do the tests both side again.


So the first coil down off the base of the follower is to the right hand side, and if its the back of the follower that needs more pressure, that first coil is bend more open with two set of pliers.  If if the front of the follower that needs more tension instead, then the first coil down off the bottom of the follower is bend more closed instead.

If the mag is having problems getting the round back up fully against the bottom of the entire feed lip, then it the mag spring that needs to be tweaked so the follower is applying tension evenly across the entire round body of the stack.  This mag tweak is done to the first coil wind that drops down from the bottom of the follower.

Now the next check, is to make sure that the mag when locked into the mag well is tight.
Starting with the mag catch, the end of the mag catch threaded section should be flush with the face of the button.  If you end up with either a half wind of the catch short or long of the face of the button, pull the catch, pop the mag catch button out, and rotate it 180 in the receiver slot to solve that problem.  If you are using a bullet button, then adjust the spring tension so the mag will just release with the bullet is pushed all the way in (don't forget to blue loctite the button is place so it will not free rotate back out under rifle recoil).

Now lets take about feed ramps and even back to the mags,


On a standard mag, the side feed guides (just in front of the feed lips on the sides of the inner mag) are guide sections to help guide the case up on the angle of attack.

Granted that you will have to deal with the advertisement, go to the below link, scroll down and take a look between the two different mags.
http://www.shootingtimes.com/ammo/6-5mm-grendel-the-round-the-military-ought-to-have/nggallery/slideshow

On the 6.5 mag, notice that not only are the feed lips slightly shorter, but the front case guides on the front of the mags are much wider as well.
Hence as the round is being angled up out of the mag and into the chamber, if the side lips are too wide for the wider round, then is causes a binding problem of them against the case, and can cause the back of the round to dip way down, and allow the bolt to get past the rim to cause a bolt over rim jam as well.

As for the feed ramps, since you are feeding a slightly larger case up then a 223, then should be slightly wider as well.
Bottom line here, just take a photo of your feed ramps and post it to see what we are working with; hence need to see something like the below.

Barrel slipped when the barrel nut was being tightened, and the barrel ramps are not long correctly indexed with the upper receiver ramps (now indexed towards the 1:oo o'clock position against the feed ramps.

But that's not the really problem, but the low feeds into the receiver face below the feed ramps, and wild drives of the bullet tips into the center lug of the barrel extension instead.

So problems with the mags to start with (maybe even height of retention in the well) and maybe even the wrong mags (case feed guides on the mags) in play as well.

You can see a bullet tip tag way low to  the right and the left of the feed ramps, but the one that I really love is the bullet tag center low of the feed lips isntead.  Hence this one tells me that when the round was being driving out of the mag, and the case guide in the mag was driving the tip of the bullet to the center of the ramps isntead.

As for the barrel extension feed ramps themselves, they look fine, so considerate on the mags and items I covered above, so the angle of attack of the rounds out of the mags are correct into the feed ramps/ chamber to start with, and so the bolt is not pulling a rim over on case drive out of the mag as well.