The M14 had the shortest service life of any US service rifle. Because it sucked.
We used them again in Iraq and Afghanistan and my generation was able to discover first hand that they sucked. Pretty much as soon as guys got trained on them the .mil started looking for something else to fill the DMR role. Because the M14 sucks. Your fond memories of the M14 indicate to me that you didn’t use it very much, or you didn’t know any better, or you got indoctrinated in boot that the M14 was so awesome.
I never had a problem with my issued M16A2’s or my last M16A4 (I got out before USMC infantry switched to the M4), and 5.56mm did just fine on dirt bags. Once the USMC infantry switched to M4’s the 5.56 still worked, same for the M27. The 7.62 round offered effectively no advantage over the 5.56 in defeating barriers in my experience. You want to have a chance of chewing up mud brick or cinder block walls get an M2 and get to work. Otherwise an AT4 or SMAW were your best bet. I guess the Carl Gustav is phasing out the SMAW, and the Gus is bad ass from what I’m told by a former 75th Ranger Regiment guy.
Wow! Where to start! Well, how about I start with this, your words still don't explain why the first thing I was told to do when in country, was to guard my M-14 with my life or it would end up missing. I have already stated the reasons. Even as late as May, 68, the sixteens were not reliable. Not junk, but just not fully sorted out yet. The M-14 had a service life of only 8 years, if memory serves me correctly. It wasn't replaced because it " sucked," It was replaced because the Army thought it had something better, so they dived into the swimming pool without looking to see if it had any water in it! The M-14 had seen almost no combat at that time. Ask any Marine in Vietnam at the time if he was happy to have his M-14 taken away and replaced with a jam-o-matic M-16.
I took Basic at Ft. Bliss Texas in October, 1967. There were no M-16s there only M14s. The M14s ignored the sand just worked, as long as you gave them an occasional cleaning. You would need to do the same for any rifle.
In Germany, I was 45B20, Small Arms Repair. I saw a lot of M-14s because there weren't any M16s in Germany at the time. (At least I never saw any) We replaced springs, periodically and the occasional broken extractor or some other small part, but that's gonna happen with any weapons system. I have seen hundreds of M14s that have fired hundreds of thousands of rounds with no more than average maintenance. So don't presume to tell me I don't know what I am talking about. M-14...high maintenance? What a Joke! The M-16 is the most high maintenance military weapon I have ever seen.
The 7.62 offers no real advantage over the 5.56 in defeating barriers....in your experience...? Really? I mean REALLY? Guess you haven't had much experience in that area. You are one of the few people I have ever heard say something that ridiculous. Yes, I would much rather have a 50 cal to " chew up a mud brick or cinder block wall." There isn't always one handy, is there? And all of you acronyms impress nobody, they make you sound like a Chairborne Ranger. Stop already, they have no relevance to the subject at hand.
You need to realize that When I was in, 67-70, the 16 was not all that great. When You were in the Corps, the gun had been fully sorted sorted out and stands today as the finest standard issue infantry rifle in the world. Of at least, one of the best. Read again what I just said. I am no AR hater. I have one, a Bushmaster, and I love it. It has never has a single stoppage, for any reason, in over 2000 rounds fired, including 500 without cleaning. In closing, I work five days a week at the Rock Island Museum. Why do I say this? Because I get to meet a lot of veterans, of all ages and from all wars. I like to ask them is they have ever used an M14. Would they carry one into combat? Predictably the older vets say yes while the Gulf war vets either say no, or they aren't sure because they have never fired one. I run into some vets that have very bad memories of the early M-16s failing in combat.
But I have yet to run into a vet with a bad memory about his M-14.