However I have buds who do regularly shoot 6mm and 6.5 mm rifles in Highpower and Long range competition.
The typical "accuracy' life of a 6.5 mm is around 1500 rounds, I think the 6mm cartridge adds a couple hundred rounds on that.
If you are speaking of the 6.5-284 then I agree with you as it is known for very short barrel life.
On the other hand the 6.5 creedmoore, 6.5 Rem. will go 4,000 rounds. The smaller 6.5 Mannlicher and 6.5 Japanese will generally go much more than 4,000 rounds.
You do not mention which 6mm cartridge as there are many target variations of the 6mm bore size especiall in bench rest shooting.
Several friends of mine that have tried ..243 Winchesters have all told me that they got very low milage out of their barrels,some with as little as 2,000 rounds.