I'm doing some work on a Holland & Holland made .270 Win built on a M1908 Brazilian action. When the barrel was removed, I found a red, neoprene-like O-ring at the end of the threads in the relief cut between the threads & the barrel shoulder. Other than the O-ring, everything else looks normal. Has anyone else observed this or hazard a guess as to its function?

Someone saw a gap and felt compelled to fill it. I guess it might keep fluids from migrating up the threads?

Someone had no idea what he was doing.

2 guesses

1. Was put there to prevent water from entering the joint and causing corrosion.

2. Was put there to prevent rust bluing solution from doing the same.

This won't prevent caustic or water getting into the threaded joint as it can enter at the breech end. What it can do is to prevent an accumulation of said fluid sitting in that space. I have seen guns growing white crystals out of seams from trapped caustic solution and this can also cause corrosion/blueing damage, so that's perhaps the reason.

Mausers are supposed to bottom out hard on the breech end and just touch on the shoulder. So if you get your shank length a thou or two too long it's possible water can wick into that gap from rain. The O ring might mitigate that somewhat.

If the rifle was built by H&H it would have been rust blued. Doubles almost always pick up water between the ribs when rust blued and if not drained, it will sit in there and rust your barrels and ribs. That's why you often see a small hole in the under-rib of a double. Said hole also allows you to squirt some wd40 or similar into the cavity.

Having said that, it's certainly not a common practice to put an O ring on the shank of a rifle barrel.

I have also seen crystals growing out of that joint; that means the bluer didn't neutralize it right.

I can't imagine bluing the action and barrel fully assembled.

I always do......No issues noted in 40 years.

I normally rust blue them together. That way you don't mar the finish by screwing the action on afterward. I don't caustic blue actions or barrels, just some small parts, but I would be a bit nervous doing so. On the other hand, hot blue mars much more easily than rust blue ... and aluminum shims or bushings might not mar the finish, but they smear which is just as bad.

Hot caustic blue; everything together; works fine. No need for me to change now.