Just got in 2 S&W 9mm Police Trade-Ins. I noticed on both that the rear of the slide and rear of the frame do not match up exactly. Frame sticks out about half the thickness of a penny. Not much but you can definitely see and feel the step. I haven't noticed it on any of my other Smith Autos. Is it caused by swapping parts, wear, or do some just come this way from the factory?
Also one has what appears to be a flaw on the rifling just ahead of the chamber. They look like 2 lines that cross the bottom third of the bore and are visble on lands and grooves. Doesn't look like a bulge or scratches and I can't feel a dip or edge to them when I run the tip of a toothpick or metal pick across them. Anyone seen this before? Thanks.

I checked the lock up on my LE 4006 and the rear of the slide is not exactly flush with the rear of the frame, but it is not out as much as you describe on your pistols. On my 4516 the rear of the slide is flush with the rear of the frame.

I have to wonder whether or not there is wear on the top of the slide stop pin or on the surface of the bottom barrel lug/block... whatever the name is, allowing the barrel to move a tad bit farther forward with the slide.

The slide's forward movement is stopped by the barrel locking up in battery, which is achieved by the barrel being fully raised up by bearing against the slide stop pin, and the locking lugs on the top of the barrel being engaged in the slots on the underside of the slide.

The barrel is basically, wedged between the underside of the slide and the top of the slide stop pin.

It stands to reason that if there is sufficient wear on either the slide stop pin or bottom barrel lug, it could allow the slide to travel a tad bit farther forward than normal.

dobk, I have a 4506 I bought new in 1991. The frame and slide did not match up before I even shot the first round through it. Additionally, the slide to frame fit was and still is sloppy in my opinion.

I decided back then when I bought it that I would just shoot the fire out of it without worrying about it.

The loose frame to slide fit and the slide to frame match at the rear have not gotten any worse after several thousand rounds. It will group about 2" at 25 yards if I do my part.

If I were you, I'd just take it out and shoot it and not worry about it.

Now about the bore/rifling flaw; I have flaws in barrels of my Model 24, 29, Ruger Old Model Super Blackhawk, and on many other handguns I used to own. All have shot very well. Personally, I wouldn't worry about it unless accuracy is noticeably bad. It's just not worth the worry.