Front face of the bolt should stop 1/8" to 1/4" in front of the back ejection port window, and not retract behind it with a full charging handle pull.

If the bolt face is retracting behind the back of port (just by a hair), then chances are the spent cases will hit the back of the ejection port window, and can cause ejection problems with the spent case kicked back into the action to cause a stove pipe. That hair turn into even more behind the window when the buffer bumper compresses slightly on back of stoke compression stop.  

If the bolt face retracts farther back past the ejection port window (buffer just wrong), then now you are chancing that the back of the carrier key will slam off the back of the lower receiver on the back of the stroke (the part where the tube threads into) and you are going to end up with a cracked receiver instead,

Rick_Lind, take a rig, pull the buffer and spring, close the upper, then pull back on the charging handle slowly until you have the B/C all the way back, hence back of the carrier key against the lower receiver threaded section for the gas tube.
Note to get the B/C back forward, just tap the muzzle off carpet ground to walk it back foward so you can shotgun the upper back open to reinstall the spring/buffer.

Now note if the B/C every goes back this far during a normal stroke, the key will be crashing into the lower receiver to crack to crack the lower receiver.
Next take your buffer and drop it bumper side on a hard table or hard ground by about 3' up, and notice that the buffer does bounce back off (normal buffer compressing).

Now by seeing just how shallow that the B/C retracting back behind the ejection port window back edge, and then throw in buffer bumper compression, start to realize just how much you are flirting with disaster by having the bolt face retract back past the back of the ejection port edge.


Next, although the spent case is starting to pivot off the bolt as the bolt is about half way back, it's during the back of stoke end stall that the spent case really fully pivots off the bolt face (if the buffer is causing the B/C to rear stall correctly, and where you get the around 5:00 ejection path from).  If the face of the bolt is behind the back of ejection port window, then instead of the spent case cleanly pivoted off the bolt face, the spent case tags the rear edge of the ejection port window instead.   On slight tags, may be fine, but as the buffer begins to wear and get softer/has more compression/shoot ammo that cycles the B/C harder, this can be the difference of the spent case hitting the back of ejection port window hard enough to stove pipe jam the spent case right back into the action.


As for why some new rifles have the problem with the face of the bolt retracting back past the ejection port back window, something is wrong with the tolerance stacking on the rifle to cause this problem.  The quick band-add, pull the buffer and spring, and start dropping quarters down the tube to shim the tube void shorter so the face of the bolt does stop correctly in front of back edge of ejection port window.   The correct solution, pull the receiver extension, and start facing the front end instead to get the needed correct buffer depth, while at the same time the tube correctly indexing for the stock/ end of the tube just kissing the center post of the buffer retaining pin.

As I wrote you in the IM, a great deal of time, the end of the tube needs to be faced back just for the tube to index correctly for the stock, and kiss the center post of the buffer retainer.

From there, you just working in .065" facings for each more wind that you want to be able to thread the tube in one more wind, and end back up at the same correct butt stock index/ end of the tube kissing the center post of the buffer retainer.

P.S, not grinding needed.   Just make a mandrel to chuck up in the lathe that is longer then the stock inner void, use a live end in the tail stock on the weep hole of the back of the receiver extension to hold the tube tight to the mandrel, and facing the bottom end of the tube bottom only takes a few seconds if you did the math before hand.  If the top end of the end of tube needs to be faces back too (need to wind the tube in more that one wind), then that is mill work instead; but just as quick.  Once done, a touch of aluminum black, and not one is the wiser that any machine was down to begin with.

For process of elimination, since you and your buddy have similar rifles, can you go to the range together?  If so, then try your upper on his lower and vise versa.  Then try swapping bolt carrier assemblies only, and then try swapping just the bolts only.  It could be something as simple as machining tolerances being off just enough on several pieces, that it will not run, this is sometimes referred to as stacking, or it could just be one piece that is machined slightly off like the bolt.  By swapping parts side by side, you may be able to eliminate which parts work, and find which one is causing the issue.

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Did you try swapping parts side by side with your buddy's rifle for process of elimination?  This may be the best way to help narrow down what part or parts work, and what doesn't.

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123