Based on the CMP measurements, how much wear is acceptable, and still give you a good shooter?
The lower the numbers the better.
The CMP rifle sales page indicates that for Field Grade rifles:
"muzzle may gauge more than “3” on muzzle gauge. The Throat Erosion will gauge less than 5 – well within US Army standards."
If you are ordering via mail order, read and understand the grade descriptions. The CMP is very upfront and honest about condition of each rifle.