I was in the peacetime military, so I don't know anything about combat, or what's important in combat. To me it seemed dumb that some guys were really impressed by their own ability to disassemble and then reassemble their weapons quickly, as if it was something that made them more Soldiery. Has that ever come in handy in combat?

yes, If you have a broken part, misfed round, stuck brass.........ect.  even changing hot barrels under pressure.

Only in the sense of knowing how the parts go together and affect malfunctions and troubleshooting.  I am not sure mindlessly slapping parts together is very helpful, but understanding the root causes of a fail to extract is.

First to posts for the win.

I always just figured it was a symptom of being a peacetime army, that since we couldn't gauge ourselves by how well we were able to shoot the rifle we would instead gauge ourselves by how fast we could put it together, or how clean we could make it, or maybe how far we could throw it, or any other quantifiable measurement.

FPNI. Stuck brass in chamber when taking fire and need to swap barrels on your crew serve lickety split.

sweet, my first FPNI!

To me it was more about completely knowing your weapon. In a fraction of a second you know exactly what is supposed to go where. I was not an 03xx Marine, so being a machine gunner was out of the question. However, there is a course for non 03xx Marines. I went through the one at Combat Skills Training School in Camp Pendleton, CA. It's an intense two week course that covers all of the crew served weapons. As part of the qualification we were expected to disassemble, assemble, and functions check the M-249 and M-240 in under 1.5 minutes and the M-2 and Mk19 in under 3 minutes I believe. After doing it hundreds of times during that 2 weeks, most guys blew that time limit out of the water. Oh, and for the M-2 the bolt was completely disassembled as part of that qual.

We never did any kind of timed drills, but we were expected to know how to take down anything that was assigned to us to the operator maintenance level. The funny thing is, they never explained to us what any of the parts did, they just wanted us to be able to take it apart, clean it and put it together. I didn't know how the thing worked until years later when I put one together myself.

Being able to disassemble & reassemble the SAW in under a minute got me the day off once.

Really the main point behind that is to ensure that Privates are completely familiar with their weapons inside and out, as well as teaching them to complete complex tasks in a time crunch. It's also good for fostering a competitive spirit amongst the guys.

I was in SF for over 20 years, I was a 18B and a 18B instructor. When I went through the course in 1987 this was our final hands-on test for light weapons:
FULLY (20+ parts) disassembled 1911A1
FULLY (lots MO parts) disasembled M3A1 SMG
FULLY (yet even MO parts) disassembled M1 garand (yes, the trigger group too)
FULLY (can I get an amen) disassembled M1919A6 machine gun
So, you go into the classroom, having no idea what to expect (I think we trained on 60 or so different weapons) and on your table is the above- all piled up in a hideous pile of fuck. This was known as the pile test. I want to say we had 20 minutes to put all this shit together correctly, and that included headspace & timing on the 1919A6. To get a go you had to get 3 out of 4. The reason I passed the first time is I didn't even try to get the 1919A6 together- I just made dam sure the other 3 were right. This test was stupid then, and its still stupid, and years later when I was an instructor there it had been eliminated. Instead individual weapons (that the student might actually see/use one day overseas)were disassembled and assembled, and more training was given on troubleshooting common problems with specific weapons and having a real understanding for how they worked. Things that need to be worked on for speed are things like crew drills for machineguns (changing barrells, tripod mounting, etc.) clearing malfunctions, reloading, and the like. Oh- and delivering effective fire, since that's what they are built for.  I'm reasonably sure that the chances of coming upon a pile of fuck in a gunfight and the feasability of quickly assembling said pile of fuck into several functioning weapons and ultimately putting them into action (presumably with the required ammunition, mags, tripods, etc. that the fuck-pile maker conveniently left) is more than remote. i hope this war story (even though there wasn't a war at the time) helped you. Or made you laugh. Or whatever.

Listen carefully. Under your chair is one bullet, a box of weapons parts from four different weapons, none of which are similar in form, fit or function. The instructions for those weapons are not written in English. In three minutes the door to your right will open and a man-eating tiger will enter the room and tear you a new a$$hole. Your task is to reassemble a working weapon, load, and prepare to protect yourself. Any questions? Begin .......

I remember being in a fight and the bolt on my m4 kept jamming whenever I hit the bolt release. Apparently there was dirt or something in there, so I figured out that I had to pull back the charging handle to load a new mag. Never could get it to do it again. Took two mags before I started pulling the handle back. That first time of watching the bolt hang up was like a big moment of "what the fuck!?!?"

Were you with a whole bunch of other guys whose weapons worked, or were you with a small group? Did it make you an AK guy?