Chamber cutting issue

[delucy]

I'm sending a couple of barrels back to the smith that did the work, I was wondering what causes this in the throat area?



Dale
SW Michigan

 

[Ccrider]

What is the problem that you are referring to?

 

[LVLAaron]

The "U" shape in the lands. It's not a problem.

 

[Alex Wheeler]

Does every groove look like the last pic or not? 5r?

 

[delucy]

Alex, no not every groove looks like that. I have 4 barrels that were all done at the same time, 2 are 4R, 2 are 5R. all 4 look this way. Its worse on one side of each of them.

These chambers are supposed to have a .170fb. A bullet will stick in each one of these (and make a mark on bullet) at around .060 to .070.

Every other barrel that I have had cut are relatively 'flat', no u shape.

 

[LVLAaron]

Ah... that makes more sense.

 

[Alex Wheeler]

Then Im going to say a combination of a throat thats not perfectly centered and a worn reamer. Freebore diameter is too small. If it was real crisp on the one side Id say the reamers fine, but its even feathering on the "heavy" side. Even with the throat being out slightly, (the small free bore makes it look worse than it is) chasing it with a good reamer will probably make you happy. The u shape is push off from the 5r, looks worse because of the small reamer.

 

[delucy]

Thanks for the info fellas, I appreciate it

Dale

 

[gunsandgunsmithing]

Are the lands cut evenly? It does look like a 5R bbl and what Alex described but if the smith uses a pilot on the reamer that registers off of the lands and they are cut evenly, the only thing left is variance in land to groove height differences. While it may be seen with a borescope, it's actually a very small amount and doesn't mean the bbl or the chamber is bad at all. Nothing wrong with seeing something like this as long as you understand the real world value of it. Again, very tiny, probably something less than a tenth here. I'd shoot it and see how it does. The camera is what will affect your confidence in it more than anything. Bottom line, if the lands or grooves are not perfectly the same height, relative to one another..then someone else will have to tell me how it can be avoided when chambering. It's out of the smith's control and IMHO, is nothing to worry about and does not imply the bbl is bad nor the chamber. I've had some very good bbls that show it. Don't let the borescope carry more weight than the target, physically or mentally. An air gauge would and did tell the story if it was checked before shipping the bbl., better than anything I know of to check that area of uniformity in a bbl.