How do you know when it's time for a fresh barrel?

[Haven]

My .243's groups started going south after too many hot rounds, what made you finally switch yours out?

 

[gunrack]

I get it when my groupings get worse, the ammo's cold and the rifle feels worn out from too much firing.

 

[CoyoteHunter]

There are several things to do that will help.
1) change to a different bullet will often times help.
2) try different seating depth's with bullet you are using.
3) try different bullet weight will also help.

I was loading for a guy for his 243, his 105 gr bullet starting shooting terrible, but after much testing with different bullets, what we found that really worked good was a 85 gr. bullet.
So his barrel was getting bad and it was time for a new one.