Would you say it takes to "kill" an AR-15 barrel?

Looking to possibly acquire a used SS HBAR off of someone and they have rounds counts in the 2-5k through them, with the bolt they were used with. I can get a new non-chrome 20" (5.56) for 140, the used ones are about $150 with the bolt, Im thinking the new is a better deal. Also..what's the advantage in getting stainless?

Im getting prepped to build my CMP legal 20" A2 upper and just want to start plotting it down on paper before I start buying.

Then it's on to the Garand.

Thanks!