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I finally got more time to get to the bottom of it and happy to report that the problem is now resolved. TL;DR I have 5 round mag tube and I replaced the spring with Carrier comp one AND shortened it to 20'' (most important part). Additionally, I polished the carrier latch with my Dremel, this step is likely optional as the spring's role is more important. The longer version is below. More details: After cycling many more rounds through it and replacing the spring with new OEM one, the number of FTFs reduced but was still at about 3-5% for some specific ammo brands. Since reliability of a weapon is the most important thing, I wasn't satisfied with that. I wasn't able to replicate the issue with A-Zoom dummies or other "no-name" dummies and live fire gets expensive really quick. As per the Internet anecdotes, Fiocci dummy rounds have more FTF issues than the others, so I order them. Once the dummies arrived, I could clearly replicate the issue with FTFs happening 30% of time. It was great as I finally got a way of reproducing the issue reliably and with no extra expense for live rounds. As for any engineer's satisfaction, the ability to reproduce reliably is like 50% of fixing the issue. (Side note, Fiocci dummies are NOT suitable for dry-fire as their cartridge head is just flat, no primer, and it bends if you dry fire). After studying the anatomy of the feeding system, I saw for myself that if the mag tube is properly installed all the way in, there were limited number of variables in play: 1) Friction inside the mag. 2) Friction from the receivers wall. 3) Friction from the rim engaging the carrier latch. It basically the carrier latch surface itself and the spring that pushes it outwards. 4) The power of the spring pushing the rounds out. So, before going into troubleshooting 1-3, I decided to order a couple of Carrier comp springs and try them out. I asked CC rep on the phone if they tried it with 5 round mag tube but was largely referred back to this focus, so I figured I'll need to experiment by myself... Once the springs arrived, I noticed that the springs are stiffer and LONGER that the OEM ones. The OEM ones when NEW are about 27'' long, after few hundred cycles, they shrink to about 25''. The carrier comp ones were about 31''. So, I installed CC spring and found that while it's longer, you can still fit 2 3/4 slugs into it, so it wasn't that big of a deal. After about two hundred of cycles of Fioccy dummies and rotating them to account for the rim wear, it was clear that the behavior has changed. Basically, I wasn't getting any mid mag TFTs anymore, the ones I was seeing with OEM springs, but was consistently getting 70% of FTFs on the very last round from the tube. It didn't make sense to me, so I re-installed the OEM one and it was back to "random" FTFs but no FTFs on last rounds. GOOD, I could clearly reproduce different failure modes with different springs. What didn't make sense to me was WHY the STRONGER AND LONGER spring was giving me such a high rate of FTFs on that last round. Anyway, I decided to see if reducing the friction will make things better. I replaced the carrier latch spring with a new OEM ones. It actually made things worse as that new spring needed some break it time likely, so I re-installed the old one. Then, I decided to give Dremel with some rough polishing compound a go and polish the carrier latch. After making the entire round rim facing surface slick and shiny, I immediately noticed an improvement. No, CC spring was giving me about 30% FTFs on that last round (down from 70%) and I could clearly see that on average, the rounds would get pushed further away from the follower and into the carrier. OK, it was great but still not enough. I did understand that slow and manual cycling wasn't the same as firing live due to no inertia that could help to push the round further into the carrier However, the fact that CC spring had these reducible FTFs on the last round didn't sit well... I came up with the idea that the bent shape of the spring in the mag tube may have a bigger impact on consistency and applied back pressure, which I was committed to test. Knowing that I could easily measure the impact by looking how far from the follower the last rounds get pushed, I decided to start chopping it in. The springs in 5 mag round M4s are exactly the same as in 7 round mag tubes. I made rather simplistic, and potentially flawed, assumption that I should be safe to go up to about 18'' inch length by using the "post break-in" OEM spring length of 25''/7 (size of the mag it was designed was) * 5 (size of the mag tube I have). Because it was easier to reproduce the issue on CC spring, I decided to use it and apply and start cutting 2-3 inch of length at a time. The end result was unexpected, the more I trimmed the spring, the farther away from the follower that last round was going. Once I got to 20'', the round was making it it to the retaining notch 50% at the far back of the carrier 50% of the time and was always at a solid distance from the follower. WOW. After manually cycling the dummies about 100 times, I saw 0 FTFs. Then, I installed NEW Carrier comp spring, untrimmed, and it was back to about 30% FTFs I had after polishing and before trimming. So, the experiment was clear to me, I switch to 20'' Carrier comp and went to try it out with the same mix of previously problematic live ammunition and saw 0 FTFs after 100+ rounds, half of which were cycled by only loading one round into the mag. I suspect, the OEM spring will exhibit similar behavior but had no time to experiment with it too. All in all, I learned a lot about the M4, the way in functions and can probably disassemble it with my eyes closed and replace the carrier latch / spring in minutes lol. Thank you for all the hints given in this thread and hope the information above helps somebody else.

Every time I shoot this gun(and it’s always no less than 250 shots a session) something comes loose. I added the shell carrier, almost fell off so I added loctite. The light, almost came off, added loctite. The strap for the light, almost came off, added loctite. Today the shell carrier was loose. Strange because I added loctite. Must have broken it free. I did use the red M-242 because of not knowing if I wanted to remove it down the road. Well, everything is freshly tightened. Not bitching, just amazed at the power of this gun . Love it.