There is no better way than the magic marker one.
And if all I could do was paint a case/bullet with a magic marker I would have to agree. I want to know the distance from the beginning of the rifling to the bolt face; again, I have tested chambers in rifles that 'just would not shoot'. The one that needed the most attention was a magnificent rifle built by a friend..
He made the reamer etc. etc. he cut the chamber on 4 rifles, one was not accurate, he took the rifle around to other friends 'better than me', they just shrugged their shoulders and or bounced their hands off their foreheads and said "I do not know".
I started out with "I do not know", he then asked me if it was my problem where would I start. I told him, the first thing he did was jump onto the Internet to search the "HOW TO" section.
And then he started; and then he called again. And then? I had to remind him I did not tell him to jump onto the Internet to find the problem. I had to remind him he asked me "Where would you start if it was my problem". That afternoon he came over with a box full of contraptions. Anyhow I had to do the work for him, and before you start by claiming you had an old friend that did it all I will tell you his bullet would leave the case before it made contact with the rifling.
I could have chambered his rifle to a 7mm Shooting Time Western and still have too much free bore. And then he wanted to know what caused the problem. I did not shrug my shoulders, I did not bounce the palm of my had off of my fore head, I just said "I do not know".
F. Guffey