Gunsmithing Chambering anomaly- yeah, another borescope question...

[Wannashootit]

but this one is different from any others I've seen.
Just chambered this barrel. There are perfect half-moons cut ever so lightly into the grooves at the beginning of the leade cut- full diameter of the barrel.
All perfectly symmetrical, all aligned radially meaning the bore was correctly indicated. I don't see this affecting the way the stick should shoot- but I've not seen it before-even on this forum- so it's more of a curiosity thing.

I've cut over a dozen chambers with this reamer (pre-bore, flush system), no indications that it's related to the reamer.

All I can think of is that the groove diameter of this barrel was slightly undersized- and caused very light removal of material from inside the groove at the transition to the leade, but I also can't quite wrap my head around how a form cutter could cut a semi-circle in rotating stock?

What do ye opine?

 

[Ledzep]

Rifling cutter geometry was a tighter radius than the groove nominal diameter.

 

[Supersubes]

I’ve seen that a number of times, although ledzep wouldn’t you say the radius is larger not smaller? If it was smaller radius it would grab the edges, and not reach out in the center. How my brain sees it anyway.

 

[Ledzep]

Yeah that makes more sense.

 

[FCrifles]

5R barrel?

 

[Wannashootit]

^^^
Nope, standard button-rifled McGowen.

Still can't wrap by brain around it, the rifling cutter is what creates the groove diameter by displacing metal (?) so how can it be "different"?
I guess I'm misunderstanding and the reference is to the diameter of the drilled hole before the rifling is done?

 

[Supersubes]

^^^
Nope, standard button-rifled McGowen.

Still can't wrap by brain around it, the rifling cutter is what creates the groove diameter by displacing metal (?) so how can it be "different"?
I guess I'm misunderstanding and the reference is to the diameter of the drilled hole before the rifling is done?

Not the diameter of the drilled hole, the radius of the groove. Your reamer has a radius, the groove has a slightly larger radius. You see how the tapered portion of your reamer “reaches” out further(towards the muzzle) on the lands? It does that because the land is taller, yes? The “reach” is greater in the center of the groove because effectively it’s taller(closer to your reamer center axis). The edges of the groove are slightly further away. Might help to take the right angle mirror off your borescope and look straight down the bore. It should pop for your then.

Edited for typos

 

[Supersubes]

Shitty representation and exaggerated, but maybe this will help. GRV is the groove profile if you were looking breech to muzzle, and CUT is your reamer.

 

[Ledzep]

Totally not to scale representation of what we're saying.

To scale .256/.264 bore with 1.5 degree throat cut in it, Groove radius larger than nominal.

 

[Wannashootit]

Got it now! Thanks for taking the time to sketch it out.

So this raises the question of a smaller than nominal groove diameter.
I could slug the bore to see what the groove diameter really is -a thou or so wouldn't be a big deal, but much more than that and we're talking increased pressures.

Do you think it's a point of concern?

 

[Ledzep]

The surface finish looks good, I wouldn't worry about diameter unless it's extreme, which it doesn't look to be. You could slug it and measure it but I doubt it's a problem.

I've seen factory savage barrels that were .002" under nominal with poopoo surface finish. Still shot fine without pressure issues using factory ammo.

 

[Wannashootit]

Looked like it barely cut the surface, but wanted to make sure given I haven't seen this before.
Thanks again!

 

[Supersubes]

Totally not to scale representation of what we're saying.
View attachment 7441504

To scale .256/.264 bore with 1.5 degree throat cut in it, Groove radius larger than nominal.
View attachment 7441505

Thanks man, that’s puts my napkin sketch to shame!



@Wannashootit, I agree with ledzep on the finish and shootability. You won’t notice a thing.

 

[FisherT&C]

I've seen similar when I cut a 6 Dasher chamber in a Preferred Blank. A JGS reamer and a 3R barrel.
I put the link to my video of it on my instagram.