I've built about 5 receivers from 4130/4140 sheet from McMaster Carr for myself, and bent a few blanks from this for other folks on Gunco, and have had no problems(both .040" and .050"). Not to mention all of the Tapco flats, and AK Builder flats that I have done this method on. Now mind you, I don't have thousands of rounds through any of my 25+ builds, so I don't know how long it takes for a hole to egg out, or an ejector tip to get peened into oblivion. Have there been failures of these areas from the procedure that I use? If so, I'd sure like to know and change my procedure.
I have actually been looking at some of the case hardening products, (Brownell's and Cherry Red) mainly for firing pins, but was thinking about trying it on the FCG holes as well.