Need subsonic load, since will bank that gunpowder being used, has too fast of burn rate, and not generating enough gas pressure to full cycle (maybe).

I say maybe since problem may not be the powder, but the mag in play.  300BK has a wider bullet, and when used in 223 mags, the internal ribs on the front of the mag will cause stack rise problems, and since the distants from rim to shoulder is shorter, can cause problem with the tip of the bullet not being raise high enough to feed cleanly into the chamber from front section side lips of mag.
Also to point out. Mk262 with it longer ogive bullet can have the same problem of not coming up faster enough during the feed, and why the M-4 has extended free ramps in the upper receiver/feed ramps in the barrel extension are lower.



As for why super sonic may not be having same problem, higher buffer impact of buffer off back of receiver extension, causing more jolt to rifle and mag, which allows mag to recover battery instead.

So step one, is to use 300blk out mags, since they do not have the narrower front inside ribs that can cause the stake rise cleaning to bottom of mag lips fully problems to begin with.

Step two, make dam sure that the threaded end of the mag release piece if flush with the face of the receiver button, so the mags are correctly retained in the mag well to start with. And don't use mag body for either hand hold, or for bypod on the table, since both will end up with the mag cocked front end downward during the strip, and will induce the low feed.

Step three, back to the subsonic load, since a slightly slower powder will add a touch more gas pulse to the gas system, or may need to open the gas port one letter side bit to add slight more gas to the action.  I say may, since if bolt is locking back with subsonic load on last round out of mag, then cycle rate is fine, and its coming back to mag feed problems so long as you have widened M-4 ramps for 300 bullet in play.
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Too add, since have not stated load, and rig does seem to need more gas pressure at port, may want to cast/powder coat, and load up some Lee TL-309-230 bullets.  Hence at subsonic speeds, any 30 caliber bullet is not going to expand, so the concept it to lob as heavy of a bullet that you can at subsonic speeds in the first place.
https://youtu.be/OBvK7f7S5aI

And truth be told, 300blk out never made sense to me, even with it  was still the  Whisper, since can achieve better performance with a 45acp round at sub sonic speeds, and something like a 45 cal 230tc bullet is still going to have the same knock down at subsonic speeds/your dealing with a blow back action that you don't have to worry about the gas pulse to any gas port as well.  
Hence subsonic 45acp with flying ashtray 230gr HP bullet, have a chance of some expansion of the bullet on impact, while with a 30 cal bullet, even if varmit HP bullet, still going to just punch straight through.

To add, with ammo being 1-3/4" long,  leaving 6-1/4" bore in the barrel ,is our super sonic ammo even getting to super sonic speeds since it loaded with slower burning powder that will take much more barrel bore to get a full burn?

Need a few  things,

Pull the upper receiver off the lower, and photo of where the tips of the bullet at tagging the upper below the ramps.

Second is to strip a mag down to the body, remove the mag catch from the rig, shove a mag into the well all the way up until it bottoms out on the bottom of the carrier, then take a photo through the non ejection side of the mag catch channel, so I can see just how high the mag  catch slot, is above the bottom of the receiver's catch slot.

Last one, when putting the mag back together, clp clean the parts, and leave a slight lube to the parts.
Now load two rounds in to the mag, shove the back of top round down, let the two rounds spring back up in the mag, and is the top round all they way back up against the entire bottom of feed lip.  If yes, add another round and do again.

If no, and top round is not snapping back up so it it seated all the way across the top of mag lip, then need to tweak the cleaned mag.

Top right, first coil drop of the mag spring, needs to tweak that coil spring in the direction that you need to add more tension to the follower, so when it does push the mag stack up, it pushes the top round to be tight against the entire bottom of the feed lip.  Hence top round pushed from center, or from rim side down, needs to snap back up its own so top round is fully seated tight to entire bottom of mag lip.  The other one while mag is apart, may need to break out a exacto knife  to trim any casting flaws off the follower side skirts, that may be causing the follower to bind in the mag well on the way up.

Again, if bolt is locking back on last round out of the mag (bolt catch on the face of the bolt and not just catching the bottom of carrier), then rig is cycling fine, and it a problem with either mag not recovering correctly fast enough with standard jolting of buffer off the back of the receiver extension, or mag not held high enough in the mag well to cause the low feed below the ramps.

Bank that old carrier has key leaking between it and top of carrier.

Hence use new bolt with old carrier, same/same with old bolt in new carrrier,  and if problem comes back with old carrier and new bolt, its the carrier key that is leaking at the top of the old carrrier.

To test, just hold bolt into carrier, CLP around base of key to top of carrier, then 100lb compressed air out of the compressor to see if back of key is leaking.

If yes, get new key bolts, remove key and lap it bottom on lapping plate, then lap key to top of carrier, use sealer between two (loctite will work fine), install new key bolts to 37 in lbs, stake the solfter key metal into side spines of harder key bolts, and all should be fine with the carrier in the end.

Myself, when ordering parts kits, will include all the parts of the B/C to be stripped as well.
Reason why, have had seen enough leaking keys/over torqued key bolts (to higher new specs, but bolts are not grade 8 to torque to that spec) to cause problems, long enough in the tooth to have seen bad keys that it base gas port did not align with the carrier gas port, and even rings that have been a mess installed. On the rings, they are stamp cut, so one side rounded edges, the other side sharp edges, and when installed, all the sharp edges should be facing the bolt tail.
Not only that, but bugle sections of the tail end of the bolts not milled correctly to leave ridges that will bind up in the tail of the carrier channel, and enough bent firing pins, or firing pins with too large of stop collar for Semi carries that leaves the edge of it proud of the bottom of the ramp, or just less then clean bolt faces or extractor that you still have to go through to clean up, so they are not scrapping brass to leave a gold min in the chamber in the first few hundred rounds.  

So short of true military parts like colt or FN that have been tested/QC'd no less then 3 times, end up tearing B/C's apart to go through then to clean up the defects left behind, so don't have problems with then down the road.

And don't get me started on FCG's , since lots of cleaning and polishing on those parts, before they are even ready to begin to be fitted to the receiver in the end.