Rifling is not concentric to barrel...

I have a gun on its way. However, a common problem seems to be that barrels are not concentrically aligned with the outside of the barrel. I assume the Chinese put the gun in a vise and drill through it rather than using a lathe with a solid rod and bit.

Is there a way to fix this? The only reason it matters is if I decide to put a suppressor on it, or a brake. Now heres the other thing that some of you might laugh at...it is an air gun. Shangai QB79. But its powerful and I like to tinker. You can get .5 inch groups at 30-40 yards, and better with some tuning. Im converting it to PCP and going to do some valve work so that itll be pushing 900 fps with 18 gr pellets. That makes some noise with that type of pressures and speed. Around 1200-1500 psi max as far as I know for now.

Like I said, it really just boils down to tinkering. I thought maybe it would take a chuck with individually adjustable jaws to center the piece and check it with a dial indicator. The lathe id be using is a pretty old, industrial lathe that has some slop, but is pretty precise for what it is. Im not worried about super super preciseness, but closer than what they are factory. I need to recrown the barrel when I get it anyways.

dsm said:

I have yet to see a barrel that the bore axis is concentric to the outside. Any good smith knows this and will dial in the bore to make any attachment concentric with it. If the outside of the brake needs to be tapered with the barrel for a seamless look...the outside of the barrel gets dialed in.

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Oh, I didn't realize most barrels where like that. This gun is visibly misaligned so its pretty bad. Im no gun smith, but I understand basic principles and what not. Usually the things that hold me back are not the knowledge, but the tools and equipment to do so. There are a few things I would hesitate to do, but for something like this, ill put it on the lathe and get her turned. I have the instruments to measure it, its just a matter of if the lathe wants to be in working order or not. Thanks!!

BBBBill said:

Not only is the bore not centered relative to the outside, it may not be parallel either. Proceed accordingly.

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I never thought of that...it could be concentric on one end, not the other, or non concentric on both ends. Either way, it can be fixed I assume by adjusting lathe. If it was a very long suppressor on the barrel, I don't think itd hardly be possible to lathe it precise enough if the bore wasn't off the same on both ends. Seems to me that one end would be centered in the lathe and the other would be way off.

Well, not to disagree, but yes I have seen them. They dont look like drill bits. They look more like a piece of rod that has been sharpened with a cutout. Its hard to describe them... They also have a small hole coming out of the bit in which fluid is flushed.

brickeyee said:

WE used them often enough for deep hole drilling work to have a permanent setup.

You should have seen younger machinists trained on CNC equipment eye's bulge.

We finally had to put a dike around the equipment to limit oil spills.

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Hehe...Id love to work in a CNC shop for a short time making gun parts. I don't know how long I could stand to work inside, but I like tinkering around with designs, sending it through CAM software, and watching the chips flow, or fly depending on the metal and lubrication applied. It would be interesting to see the rifling process in person. Mainly just seen videos and such so I understand how they work, but its just so cool to see the intricate work being done to make a bullet hit its mark at over 2000 yards in some of those guns. One little mistake and that barrel isnt gonna make its owner too happy.

brickeyee said:

For cut rifling many places are still using WW2 Pratt & Whitney sine bar rifling machines.

We had a huge number left after the war and they are still around.

Old overbuilt cast iron machinery has a very long life.

I have a 30+ inch bandsaw that is still running.
The hardest chore was adding ball bearings to replace the old Babbitt bearings.
Custom reamers are not an inexpensive things in larger sizes (think multiple inches diameter with pilots).
At least the old cast iron cuts like a dream.

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Yes sir, it does. If there is anything I learned from running some farm equipment with cast iron parts, its that it cuts easy with tools, cuts terrible with oxy-acetylene, and welds best with steel wire or bronze brazing for thin parts. I learned that from our machine shop who said welding cast iron with iron rods is not as strong...