I have all the necessary parts to build my legal Sig 551 16" rifle, starting with my JDI 551 pistol. I know I can have a local gunsmith pin & weld my barrel extension, which screws into the flash hider, to make the necessary legal OAL but he's raised an alternative that I want to run by you guys...
As he described, he can drill a hole through the flash hider where it threads into the barrel extension (but not through the latter obviously). He could then insert a ball bearing and press it into the seam, rendering a different type of permanent attachment. The smith would then file/sand down any portion of the remaining metal from the ball bearing, making it seamless with the outer diameter of the barrel.

Having never seen this technique and not knowing if there are any concerns that might arise in using such with a factory 551 barrel, I wanted to ask the collective wisdom here if this approach is ideal or raises concern(s). What say you?

Functional and compliant with NFA are different things. You have two options to secure a barrel extension and be compliant; pin/weld (ball bearing could be considered a pin I guess) or silver solder.

quote:

The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to
the furthermost end of the barrel or permanently attached muzzle device. Permanent methods of
attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver
soldering, or blind pinning with the pin head welded over. Barrels are measured by inserting a dowel rod
into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at the
furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and
measured.

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