Over-boring a .22lr barrel

Tolerent

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I have a .22LR Ruger 10/22 rifle that I want to convert to shoot standard ammunition subsonic. A standard barrel would accomplish this somewhere between 4" and 5" long. Cutting the barrel to less than 16" creates a SBR which is a big hassle. I am considering using a long piloted bit to overbore the forward 12" of my standard 16.5" barrel.

Challenges I am considering:
  • Long Piloted bits are not as available as I had hoped.
  • I worry that turbulent gasses passing the bullet in the over-bored barrel will contribute to projectile instability.
  • Thinking of over-boring the entire barrel and inserting a 4.5" liner with a 9" twist on the chamber end to mitigate the line above.
  • I did note the rules for target rifle competition specifically allow for barrels that are not rifled full length and wonder what they had in mind.
  • Worried that any crown recessed 12" will be difficult to properly dress.
  • Wondering as dirty as .22LR ammunition is if cleaning the fore-bore will be problematic.
  • Still anticipating threading muzzle 1/2x28. I see some 9mm threaded 1/2x28 so think an 8mm over-bore will be easy to accommodate.
Looking for perspective from those with more experience in such machining challenges as well as ballistic and maintenance considerations.
 
The unthreaded portion of barrel in match guns is called a bloop tube and primarily is there to lengthen sight radius.

It would be tough to crown the muzzle with it recessed inside the barrel.

Easier route would be to cut barrel to length and then use a tube for the extension and pin and weld in place. That way it is legal length but has none of the drawbacks you mentioned.

Over boring and still threading the barrel is probably a bad idea. At least as far as using it with a suppressor. Too much travel to avoid Murphy and baffle strikes.
 
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If I was to cut to 4.5" then thread for the extension it might be challenging to retain 16" of a 16.5" initial length. I guess the threads would not need be very long if welding it all back together. If I weld it I would definitely need to re-finish the weld bead better than my current abilities.

I might be inclined to run the piloted overbore bit to take material out incrementally and test for crown problems. If I am able to get a reasonable crown most of the time I imagine I would be willing to test results at 5" and progress in small increments if accuracy was problematic. More so If I concluded I needed a 9" twist liner.

is 12" of 8mm diameter bloop tube likely to be deleterious to 5.56mm projectile stability at 16" (or 9") twist? I always assumed bloop tubes were much larger diameter.

Regarding baffle strikes: I see your point. Probably would need to use a 9mm suppressor design if attempting to suppress something 12" from the end of the effective barrel.
 
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Couldn't you pick up a used 10-22 barrel to screw onto your stub (4.5") barrel?
 
Bloop tubes for centerfires are usually same outside diameter or larger than the barrel because the iron sight mounts are like scope rings on the barrel.

If it were me I would use a different material for the tube. Aluminum would be a choice but welding it may be more difficult with it in contact with steel. I don't do welding yet.

Threading the stub and then the barrel extension would be likely to shorten the total below 16". Plus there is so many possibilities with a length of .22 barrel.
 
Couldn't you pick up a used 10-22 barrel to screw onto your stub (4.5") barrel?
My original idea was to use a salvage 10/22 barrel the inside would still need to be bored out to avoid conflict between any tiny offset or rifling alignment if I grabbed a barrel for part of the project. I could find an 18"+ barrel but I don't value the existing barrel as much as the effort to change it out on the received. I figure if I ruin the barrel it is one I was not going to otherwise use anyway.
 
Because they think it is all Eley TenX judging by the price.
The cheapest subsonic ammo I see is twice the price of Federal bulk ammo. I shoot a lot and want to eventually form 1 a suppressor. I imagine that is going to be applied for about 2 weeks after I get the lathe and by the time it is approved I will have the skill to build it. After I receive the form 1 approval and build the suppressor and have adapted a few firearms to mount it on I would primarily target shoot with firearms adapted to subsonic performance with bulk ammo.
 
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