Admittedly, I know very little about the Stoner 63. What little I know started with a memoir of a SEAL in Vietnam who used one regularly. That and an Ithaca 37 with a duckbill shot diverter. (another interesting device).
Anyway, the author is very fond of the weapon and stated it was a primary player in gaining fire superiority even when hugely outnumbered. It is no secret the 63 was the predecessor to the present day SAW concept even though SAWs did not see widespread adoption until a few years after the US got out of Vietnam.
The author also states you had to keep it squeaky clean. No mention of complexity. It just had to be done after every op whether it was shot or not. Getting it wet or submerging it even could be bad (especially if your a SEAL and you do a lot of waterborne ops). This makeshift me think that there could be no laxness of weapons maintenance as opposed to the weapon being to complicated to maintain. This can also be a problem with militaries. They need reliability under the worst conditions. Special ops units may be able to get away with a weapon like this as there ops are short by comparison to days long patrols or S&D operations.
The author also stated it could be magazine fed with M-16 mags. He stated that was of consequence though since establishing fire superiority was paramount to the SEALs combat tactics at that place and time so belt fed was the best way to go. He also went on to say that when on an op he was usually laden with up to 800 rounds on 100 round belts draped over the shoulders across the body.
There was an option to the Stoner 63 though. That was the M60E3 which the SEALs also favored. A cut down M60 with a vertical foregrip and no butt stock essentially served the same purpose as the Stoner but in the larger and heavier 7.62 caliber. For the reasons of fire superiority I’m sure the Stoner would get the nod based on the authors experiences.
A few things:
The .22 caliber barrel is small enough to not be self draining. You have to break the seal of the chambered cartridge in order to get it to drain. The larger .30 caliber bore is self draining, just point the barrel down and the water should run out. It has to do with the weight of water that can get trapped in the barrel and the surface tension of water.
If "the author" stated that the Stoner could use M16 magazines, he really doesn't know much about the Stoner, or the M16, or has a bad memory.
The Stoner used rock-in magazine with a rear catch like an M14, and were full curve, with integral insertion limiters. They are not compatible with M16 magazines at all.
As stated before, NONE of the official test reports on the Stoner by the USMC and Army ever state that there was anything particularly difficult about maintaining the Stoner. And the Army, already committed to the M16, released a very detailed report on all the shortcomings of the Stoner compared to both the M14 and the M16.
One the biggest complaints about the first belt fed versions was the belt came in from the left side, and the ejection port was on the left side. With the 150 round box, or a loose belt the was a good possibility that ejected cases would bounce back into the feed path. This was corrected in the 63A, with the belt feed being switched to the right side.
If fact, here is a quote from the first report on testing at Parris Island:
1. A service and troop test was conducted on the Stoner 63 weapons system to determine its suitability for use within the Marine Corps as the basic weapon and/or weapons system, and to evaluate the operational and organizational concepts, doctrine, tactics, and techniques affected by this weapons system.
2. It was concluded that the Stoner 63 weapons system will be suitable for use within the Marine Corps, upon the correction of several deficiencies, as the replacement system for the present M14, M14(m), M60 and M3A1 weapons.
3. It was further concluded that the operational and organizational, concepts, doctrine, tactics, and techniques of the present Marine Corps rifle squad are not affected by this weapons system. However, the Marine rifle squad armed with the Stoner weapons is significantly more effective than when armed with the M14 and M14(m).
The deficiencies were mainly the aforementioned left side belt feed-left side ejection problem, a poorly located charging handle on the LMG, poor sling locations, corrosion problems in the gas cylinder, the need for a flair on the magazine well, and beefier furniture, nothing about maintenance.
Hardly a statement about a finicky, hard to maintain, weapon system.
One must remember that in 1965, the USMC was still phasing in the M14, and to start procuring the Stoner 63 (XM22 Rifle, XM23 Carbine, and XM207 LMG) in quantity would have been a severe strain on the Marine Corps budget, as it would be independent of Army procurement. At the time, the Army's procurement of M16 was considered a "one-time-buy" of 100,000 rifles (later bumped up to 200,000), of which a of which a mere 240 were allocated to the Department of the Navy as a whole. The initial fielding concept for the M16 was that it would go to airborne and airmobile units and the heavy divisions would retain the M14. It wasn't until 1966 that the Army changed its mind to make the M16 service standard. Unfortunately, that was after the USMC had made the decision to pass on the Stoner.
Oh, and one more thing...
An M60E3 weighs 19 pounds empty, and Stoner 63 LMG weighs a tad under 12 pounds empty, less than half the weight. As I stated, there was no real option to the Stoner LMG.