Some years back McMillan told me that they use Marine tex. They sill may use it, I don't know, but if Marine Tex is used by McMillan it must be good. I've used it once with good results, but I've used Devcon Steel Putty for all other bedding jobs over the last 30 years.
Years ago I used to use Devcon F-1 Aluminum Putty for the first "coat", and then followed up with Devcon Aluminum liquid for a "brush-on" coat. That worked well but I heard that the Devcon steel putty had such a low shrinkage rate that two coats aren't needed. I agree with the person above who wrote that it is best to weigh the resin and the hardener separate to get the correct ration of the two.
I use Devcon for bedding the action. For gluing the pillars into the stock, I use Tap Plastics 4 to 1 epoxy mixed with tiny glass fibers. In my opinion, pillars require an epoxy with tensile strength, and I think that the 4 to 1 epoxy mixed with glass fibers has more tensile strength than Devcon (Devcon has more crush strength). I can not substantiate this, however, but I have never had pillars come lose, even for big-caliber cartridges such as the 300 and 338 RUMs.