No gunsmith shotgun scope mounts

Looking to get an saddle mount weaver rail for a red dot on my 870 anybody used them ? Are the any good ? Do they stay on zero

Yup. Good way to go.

I have an 870 I put one on so far it's great . Mounted a 3-9x scope for rifled barrel . Holding good after many slug shots

I've had one on my Winchester 1300XTR for the last 4 season and it's still zeroed. No problems at all.

I have one on my 870 with a rifled barrel and a low power scope which I use for deer hunting. It holds zero, but there are a couple of things you should be aware of. First, if you clean your gun often, you will be removing the trigger group and possibly the barrel often. This will require you to recheck your zero after reassembling your gun.
Second, make sure the nut at the end of the magazine which holds the barrel on the gun is very tight. If it is not, the barrel may shift a bit in relation to the action and the saddle mount. This will throw off your zero. If you plan to use only a red-dot type sight, or if you will only be using shot (as opposed to slugs), then you may not notice a shift in your zero due to the fact that neither of these are meant for absolute accuracy or precision.

Oh, one other thing I forgot to mention. The one I use is not made by Weaver. Mine is made of metal (can't remember if it is aluminum or steel). I seem to recall reading that the Weaver mount is made from a polymer (you should verify), so I'm not sure if you will get the same results as using one made from metal.

Wouldent the metal scratch your reciver?

The saddle mount should not be moving with respect to your receiver, so there should be no scratching. That being said, the few times that I have removed the saddle mount, I noticed a bit of bluing wear in the location that the mount contacts the receiver, which was probably due to mounting and removing the mount. Anyways, it's an 870 right - what's the big deal if the bluing gets a bit warn? I'd prefer that and being able to hit what I am shooting at than having a mount made of plastic and worrying if it is still zeroed after sitting in a hot trunk for a couple hours. The large majority of optics mounts are made from some type of metal - there is a good reason for that.