Acraglass, devcon, marine-tex, JB Weld, basically any two part self hardening epoxy that has a good strength rating will work. As far as the actual application goes there are videos available like this one from Midway
https://www.youtube.com/watch?v=oOo-_Ss7aIs
Make absolutely sure you have enough release agent (I've used clear shoe polish before that worked really well) on the action and even though it may look like a huge hole that needs a lot of bedding material don't over-fill as it can be difficult to clean up at times and may stain the stock.
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