I've known a couple of fellows who served as armorers in US service. Their philosophy seems to be that the people to whom they issue weapons aren't smart enough to do anything but cleaning, and they view with a gimlet eye any user improvement or fiddling.
And, with good reason. Because we're the guys who have to fix things when "user improvements" break them, or otherwise render them unserviceable.
If "the lads" are allowed to tinker, things that would otherwise be working get bent, broken and often LOST, rendering a formerly serviceable piece of equipment unusable, and in extreme cases only useful for some spare parts.
There are valid reasons why "users" are only allowed to disassemble items to certain points, AND NO FURTHER. I understand, getting things clean enough to satisfy one's superiors can be a real PITA without being able to take certain things apart. Been there, done that, etc.
BUT, lose one little pin (say the one that holds the extractor..) and your rifle is now useful only for holding the bayonet on the end. Don't put that lower group (that you weren't authorized to take apart anyway) back together CORRECTLY, and you could have anything from one that goes full auto when it feels like it, to one that is non-functional or both, in that order.
Got any idea how many M1 Carbines wound up needing repair because of the BS barracks rumor that, to improve the rifle, "all you need to do to get full auto is file the shear pin"??? (just one example, among many)
The scene in Enemy at the Gates where the raw conscripts (recruits isn't the right word) were given rifles to every other guy and the odd guy got only a clip of ammo weren't the norm, but they did happen. Those guys almost certainly didn't get trained on proper bolt take down and maint, until after the battle, if they survived..
The Soviet system wasn't big on teaching anything that wasn't needed to get the immediate job done, and when in emergency mode, more than a bit less so...
Some troops got full training, other's ...not so much...