Devcon - it comes in aluminium powder and steel as well as the stainless version. There are many other brands that will do the same job, Devcon is black which suits black stocks but you can get brown machinable or tapable epoxies which is the main condition you need once the product is set up. The other main one is shrinkage once set - this is a product of curing temperature of the epoxy vs the cured/cooled state after it finishes expanding after set up. The best combination is machinable and minimal shrinkage for a good 'casting' of the action.
The specific gunsmithing products are good, but Devcon works just as well and is likely off the shelf and more easily obtained. The other product you need is a release agent - a non-hydrocarbon and non-silicon product is what you need and the easiest to use I've found is a product called LPS-3. A thin coat of that sprayed on and left to dry and job done. Will save you a metric f-ton of hours fartarsing about coating the action with release agent...
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