I was thinking of the M14.
I carried the M-14 throughout my 17 month tour and half in combat in Vietnam in '66-67. I tricked it out with a light trigger, a clip-on bipod from the M-16 and a homemade rate reducer made of rubber to cut the rate of fire from 800 RPM to about a BAR-like 450 RPM. It was heavy and rusted almost instantly in Vietnam's climate but it always fired when you pulled the trigger and it hit, hard, whatever you aimed at. I became a virtuoso with the thing.
We got the new M-16s in late '66 and they were a disaster: they were difficult to zero with their goofy sights, the safeties stuck thanks to excessively strong detent springs, and of course, they pulled the heads of cartridge cases and left them stuck in the chamber - then fed another round into that mess - all in combat when you life depended on it. The next part made it worse - the M-16 chamber is inaccessible from the outside, so we carried assembled cleaning rods to try to knock the junk back out of the chamber. If that failed, you had to pull the magazine, strip the upper receiver, and then use the cleaning rod to hammer the mess out and then reassemble the weapon and try to resume the fight, all while you're in the middle of the fight of your life. Added to that, the round's performance was lousy. People hit with the thing usually didn't go down, even though you could see the dust coming off of them when the round hit. We could see M-16 tracers ricocheting off wet grass and I can't count the number of enemy I saw run off in complete safety as thousand of rounds of M-16 were fired at them as they ran. Just couldn't hit squat with that thing unless they were on top of you.
I solved my problem by using sneaky techniques to keep my '14. When my grunt company commander asked me why I still had an M-14, I told him that I was "in artillery and we haven't been issued 'em yet" and when I went back to my parent battery to visit and they told me to turn in my '14 and get an M-16, I'd tell 'em "can't - the grunts want me to keep my '14".
Played that game until May '67 when I got hit. My lieutenant came up to me and said "Rick, I'm sorry you're hurt, but can I have your M-14?"