i kinda think ya need ta slow down a bit..GM barrels have been "proof" tested for years,, they exceed all recommendations for safety standards.
the best thing you can do to "learn" your new barrel is to use moderate loads for the first 50-100 shot's. after that patch and/or lube properties can be changed for the most accurate/best load for your rifle and your shooting technique. Your new barrel will be accurate and consistant only after about 200 rounds using your own shooting style. I'd use the .490's and keep it loose,,ya wanna just cut them threads out and experiment anyway's don't ya?
ain't no need ta "proof" a GM barrel, an ain't no need ta stuff tight ball down a new barrel, you all', can do that if'n ya want.
But it sure seems ta be the hardway ta get ta bein a good shooter
just get sum powder, an sum ball an shoot