Thread: Throat Erosion - New Barrel

Your original reference CBTO measurement for just touching might have been influenced by some roughness in the finish of the cut of the leade, that has now been smoothed out after some shooting and hopefully has stabilised. The improvement in copper fouling might support this idea?

I had something that seems similar recently in a set-back 6.5x47 chamber; 0.2mm (0.008") lengthening after 150 rounds, and initially with significant copper fouling when the barrel had always shot clean for 1700 rounds prior with the original chamber. Opened out the groups too — to a degree, but as we all know, a 6.5x47 never shoots more than half-inch even in the most adverse circumstances, so difficult to compare

Originally Posted by Puffin

Your original reference CBTO measurement for just touching might have been influenced by some roughness in the finish of the cut of the leade, that has now been smoothed out after some shooting and hopefully has stabilised. The improvement in copper fouling might support this idea?

Almost certainly the cause of the large initial jump in CBTO distance. Not uncommon to see then the CBTO measurements settle down dependant upon the cartridge and load being fired.

Carbon build up/ring gives the opposite to what the OP has posted ie gives reduced CBTO distances. That's not to say the carbon and maybe copper isn't the cause of the accuracy drop off. The carbon build up also makes it virtually impossible to get repetitive accurate CBTO measurements if using the Hornady case method.

Best to deal with carbon at each clean rather than letting it get a hold.

Originally Posted by Puffin

Your original reference CBTO measurement for just touching might have been influenced by some roughness in the finish of the cut of the leade, that has now been smoothed out after some shooting and hopefully has stabilised. The improvement in copper fouling might support this idea?

I had something that seems similar recently in a set-back 6.5x47 chamber; 0.2mm (0.008") lengthening after 150 rounds, and initially with significant copper fouling when the barrel had always shot clean for 1700 rounds prior with the original chamber. Opened out the groups too — to a degree, but as we all know, a 6.5x47 never shoots more than half-inch even in the most adverse circumstances, so difficult to compare

Greetings Puffin,
My son had the same experience of an apparent erosion of the leade after a small number of rounds in a 7mm-08. We put it down to an initial smoothing up from the early shots. Still shot the same.
Regards Grandpamac.