Just got a bore scope, what's up with my bore?

Al Bear said:
My guess (and it’s only that ...a guess) is these are rings from each hammer blow as the rifling die is pulled through the barrel during the hammer-forging process. I watched a video on this once.
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the marks on the Marlin barrel cannot be drill marks because they continue both through the lands and the grooves and are uniform and continuous all the way around the bore. I think Al Bear's explanation is the most probable choice

cheers mooncoon
 
Kind of a necro-post, but I used to work at IBI so I can shed some light on what you're looking at. Those are marks from the reaming process that takes place after the bore is drilled. IBI used to hand lap at two stages of the barrel production process - The first lapping takes place after the reamer, specifically to remove those marks.

The second lapping takes place after button rifling and heat treatment and really gives a beautiful mirror shine to the bore. Barrels would be bore scoped at each step to make sure that the bore was nice and shiny and free from those reamer marks. I know because I used to be one of the dudes doing the hand lapping.

Thankfully for those of us who had to do the lapping, IBI bought a Sunnen Horizontal CNC Honing machine, the only one in the country. It replaced the first lapping step, and allows IBI to control the bore dimensions down to .0001" (or ~2.5 microns). This was much faster and much more precise than the hand lapping method (and the machine itself is cool AF).

What I'm trying to get at is that's not one of IBI's blanks, unless MRA specifically ordered non-lapped blanks from IBI, and I'm pretty sure they didn't. I've seen a lot of factory take-off barrels from a number of different big-name manufacturers, and it's pretty common to see those reamer marks in the bore. That Marlin barrel is one of the more noticeable I've seen though. Factory barrels with the reamer marks can still shoot nice groups, but a quality barrel with the extra time, precision and attention to detail like an IBI can make a huge improvement.
 
Milofficer said:
Kind of a necro-post, but I used to work at IBI so I can shed some light on what you're looking at. Those are marks from the reaming process that takes place after the bore is drilled. IBI used to hand lap at two stages of the barrel production process - The first lapping takes place after the reamer, specifically to remove those marks.

The second lapping takes place after button rifling and heat treatment and really gives a beautiful mirror shine to the bore. Barrels would be bore scoped at each step to make sure that the bore was nice and shiny and free from those reamer marks. I know because I used to be one of the dudes doing the hand lapping.

Thankfully for those of us who had to do the lapping, IBI bought a Sunnen Horizontal CNC Honing machine, the only one in the country. It replaced the first lapping step, and allows IBI to control the bore dimensions down to .0001" (or ~2.5 microns). This was much faster and much more precise than the hand lapping method (and the machine itself is cool AF).

What I'm trying to get at is that's not one of IBI's blanks, unless MRA specifically ordered non-lapped blanks from IBI, and I'm pretty sure they didn't. I've seen a lot of factory take-off barrels from a number of different big-name manufacturers, and it's pretty common to see those reamer marks in the bore. That Marlin barrel is one of the more noticeable I've seen though. Factory barrels with the reamer marks can still shoot nice groups, but a quality barrel with the extra time, precision and attention to detail like an IBI can make a huge improvement.
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MRA used Green Mountain barrel blanks before they started using IBI blanks. I'm 99.999% sure that is a Green Mountain barrel.

Right now MRA is once again sourcing their barrel blanks from an undisclosed US manufacturer instead of IBI. I hope quality doesn't once again suffer as a result. The MRA barrels I have that were made from an IBI blank are really great shooting barrels.
 
Milofficer said:
Kind of a necro-post, but I used to work at IBI so I can shed some light on what you're looking at. Those are marks from the reaming process that takes place after the bore is drilled. IBI used to hand lap at two stages of the barrel production process - The first lapping takes place after the reamer, specifically to remove those marks.

The second lapping takes place after button rifling and heat treatment and really gives a beautiful mirror shine to the bore. Barrels would be bore scoped at each step to make sure that the bore was nice and shiny and free from those reamer marks. I know because I used to be one of the dudes doing the hand lapping.

Thankfully for those of us who had to do the lapping, IBI bought a Sunnen Horizontal CNC Honing machine, the only one in the country. It replaced the first lapping step, and allows IBI to control the bore dimensions down to .0001" (or ~2.5 microns). This was much faster and much more precise than the hand lapping method (and the machine itself is cool AF).

What I'm trying to get at is that's not one of IBI's blanks, unless MRA specifically ordered non-lapped blanks from IBI, and I'm pretty sure they didn't. I've seen a lot of factory take-off barrels from a number of different big-name manufacturers, and it's pretty common to see those reamer marks in the bore. That Marlin barrel is one of the more noticeable I've seen though. Factory barrels with the reamer marks can still shoot nice groups, but a quality barrel with the extra time, precision and attention to detail like an IBI can make a huge improvement.
Click to expand...

Thank you for chiming in.

I think it was a Green Mountain blank.
 
Green Mountain is a truly impressive operation. I've seen what goes into making (relatively) small numbers of barrels at IBI, and the sheer volume that GM can pump out is staggering. Granted, they are definitely geared for quantity over quality, and their stuff is priced accordingly.
 
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