Re: How do you know if crown is bad
You didn't say if you are working with a rifle that isn't shooting to your standards. Sounds like you are looking at all avenues of improvment. A good crown is important. In my experience a slightly worn crown can cause groups to open up but for some hunting rifles it can be quite acceptable. A fresh crown can often improve groups if the barrel has been used for hundreds of rounds. Just shooting it can cause the edge to become rounded. If the barrel has been cleaned improperly this wear can be accelerated. Pulling a bore brush back into the barrel and across the crown is highly suspect and some guys will not do it. Vigorous movements with the cleaning rod dropping down on exit then pulled back while sitting on the rifling is another.
If the crown is really bad you can see it with good magnification. Before my borescope I used an eyepiece from an old rifle scope, inverted. If the crown is just not crisp but rounded it will be difficult to see without a borescope.
I have had a few hunting rifles that were very consistent shooters shoot larger groups the next time out. One crown was obvious and when recrowned it went back to shooting like before. Another rifle also began to shoot poorly, it still shot groups but not as tight as before. I couldn't see any issue with the crown (pre borescope) so suspected the scope, bedding etc. After a lengthy elimination process I crowned it anyway as part of the control. It solved the issue.
The past few weeks had a rifle, I intended to take hunting this coming Friday, not shoot to my satisfaction. Just yesterday I decided to deal with it. The borescope showed the crown to be ever so slightly worn with the sharp edges of the crown to be somewhat rounded. It is a rifle with 400 rounds through it since it was new. It wasn't anything that was a glaring issue. I crowned it on my lathe and took it out to the range. Every pair of shots were cutting into each other as I sighted it back in. #5 and #6 shots were one ragged hole. The confidence in the rifle is back. I have a coues wt hunt this friday and it will be going as originally planned a year ago!
If this is a new rifle for you, whether new or used, and you are going through it, I'd say crown it too. Then you know it is fresh and there will be no doubt. If the rifle still has issues you can move on to other areas in your attempt to improve its performance.