DefiantDad;
I've shot my HK91, G3, and CETME's all to the point of failure numerous times. Usually I only clean them if I know I'm going to do a
bunch of shooting with it. Or they fail WHILE doing a bunch of shooting.
The delayed roller blowback operation results in a massive amount of carbon crud getting thrown back in to the bolt, around the bolt, in to the receiver, and so on. That design is the most heavily fouling one I own, period.
The failures I've experienced are universally failure to feed. These rifles eject so violently you don't want to be on the receiving end - you MUST give shooters to your right fair warning before shooting to avoid raining burning brass on to them - even if they're 50+ feet away...
Failures start happening when enough "crud" builds up to slow the forward motion of the bolt carrier group down. This is a combination of residue in the receiver/on the bolt carrier, and to a much lesser degree, the chamber proper.
Powder Blast (made by break free) is probably THE best solution for these, in the field, as you can take the bolt out, hose it down, hose the chamber down, and the inside of the receiver.
In a pinch, lacking cleaning supplies, you can also get some motor oil off the dipstick of your truck and liberally coat the bolt with it and the chamber. This will return this type of firearm to functional for awhile, but it will also rapidly accelerate the rate of fouling from that point because oil and carbon crud makes a nasty combination after awhile. (Whatever oil doesn't burn off starts to form a thick sludge).
They're great rifles, don't get me wrong, but DUDE.. they are horribly dirty to shoot.