I took out my S&W 66 with 2 1/2 barrell, and wanted to do some serious zeroing with Winchester 145 JHP, for a carry load. My first group was dead on with two flyers to the right. The more I shot, the further my groups went to the right, along with more lead spitting and difficulty in opening the cylinder. I kept adjusting the rear sight to the left, as my groups spread out to the right. Having blown about $100 in full magnum loads, I finally realized the barrell and ejector shroud are actually starting to unscrew slowly but surely, turning counter clockwise as viewed from the rear. I would certainly call this a warranty defect, and it will not shoot to aim, and the cylinder will sometimes lock up. You all think I have a chance of sending it without my original receipt for warranty work?

Are you sure? If your barrel is rotating counter clockwise, as viewed from the rear, the barrel would be tightening not loosening.

They stopped the pinned barrel in 1982 on the 66's.
I believe warranty goes back to 1989. If you call S&W 1-800 number and give them serial number, they will tell you if it is still covered.
Good Luck

If the warranty is no good, you could try Loctite 290 in the barrel threads.
Turn the barrel back to the proper position and appy the thread locker generously at both frame barrel-junctions. It has a low viscosity and will wick into threads before setting up. The barrel can still be removed with the proper tools, but it will not move under fire unless the threads were filled with grease when the barrel originally installed.

WOB

Welcome to the forum

You have a rare Model 66 with the Dan Wesson barrel option

Seriously, call S&W and see if the will correct it. If not have a competent company like Cylinder & Slide do it. I say this as there may be other issues you dont know about, and they are set up to make sure the barrel gets put back in correctly.

IIRC there were issues like this with some Smiths in the early 80's. There was a batch sent to a local PD that were unscrewing themselves during quals. I think they were model 13's, but it was a long time ago.

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Rough waters ahead.

I've sent many guns to S&W for warranty work without being asked for proof of purchase. I don't think it's even a requirement. The warranty is for the original purchaser only, but I've never been asked that, either. Closest they've ever come is when one rep asked me how long I'd owned the gun. I answered "a long time", and that was that.

They built it, they WILL fix it....contact them if you want them to fix it ...but I would remove it completely, degrease the barrel and frame threads,GOOD & clean.....put some Loctite on it and screw it back it, making sure it lines up ( sight on top........at 12 o'clock...)
let it set up, over night before you do ANYTHING else.........

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dan
NRA Benefactor,PMA

Another voice with a ? mark here . If the barrel is rotating CCW as viewed from the rear it is not coming loose - it is drifting past index.

Is this a pinned barrel, or crush fit ? Either way, it's time to stop shooting, and evaluate. That means a trip to a smith .