I'm not a fan of parts washers on aluminum/anodized guns. All steel big stuff like a heavy machineguns they're the cats meow... I don't think kroil will have enough solvent for what OP needs.
OP, if I was you, I'd take the hand guard off, wipe the chunks off with a rag/brush, and then go at it with the garden hose, dawn, and a scrub brush. Then some brake cleaner to get the finite bits off and blast out the nooks and crannies.
Then get oil on the steel parts faster than Clinton fucks over the American people.
Having the HG off also let's you get all around the barrel/nut and gas block.
I recommend spraying the barrel with black grill/engine paint when you have it degreased and clean. Then you don't need oil on it. Let the paint cure for 24 hours before firing. I've yet to have any problems with paint coming off by heat and of course it won't get chipped under the HG so long as you're careful when installing it.