I used Pro-Bed 2000 when I did the bedding on my LRB. Very easy to work with and very strong. I'll use it again on my next bedding job.
In the past I've used gray Marine Tex (don't ever use white) with good results, Devcon steel works well (I'd like to try their titanium), and Brownells Acra-Glas gel mixed with their stainless steel powder.
I've fooled around in the past with JB Weld, but it didn't seem very strong and didn't take to machining very well. YMMV
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