About a half-century has passed since I had to thoroughly clean a AR-rifle (US M16A1) for turn in, subject to a very thorough inspection.
We used immersion heaters in galvanized steel trash barrels, raising the water temperature to a brisk boil. Strip down the weapon, all the small parts (including bolt, bolt carrier, etc) in a wire basket, and everything dunked into the barrel of boiling water and dish-washing soap. Then into the barrel of boiling clean water.
Coming out of the barrels the metal is too hot to handle, but the water immediately evaporates off leaving the parts completely dry. All the grease, oil, and crud are removed. Normal cleaning with solvent, then a light application of LSA (Lubricant-Small Arms), and your weapon is ready for Staff Sergeant Gilcrest's closest inspection.
I still use boiling water and dish soap when cleaning heavily fouled or seriously neglected used guns that come my way. I have a couple of large kitchen pots that I bought at Goodwill for a buck or two for this use (I don't recommend using my wife's good kitchen stuff). 5 or 10 minutes in boiling water with a bit of Dawn dish soap, then 5 or 10 minutes in clean boiling water, then normal cleaning and light lube.
Sounds extreme, but you probably never met Staff Sergeant Gilcrest.
"Dawn takes grease out of your way".
