I have a sigma 600mm k mount lens that I would like to use on a nikon 3100, is there a adapter available?

To the best of my knowledge, most, if not all, Sigma lenses fit most cameras that use interchangeable lenses with the appropriate adapter. As I recall, Sigma uses a bayonet mount and adapters are available for many name brand cameras. For example, I have a Nikon and my Sigma 600mm came with an adapter for a Canon. All I had to do was order a Nikon adapter. If you're not sure, you might want to check with a local camera store to confirm which adapter you need.

That depends on the lens. Sigma makes their lenses with a variety of mounts. If the lens has a Nikon mount then you don't need an adapter at all.

Since the Nikon lens mount has a deeper flange depth than other mounts, you cannot fit a lens with another mount onto a Nikon body without losing the ability to focus to infinity. Because of this, you won't find an adapter for another mount. Adapters are available to fit Nikon lenses onto other cameras but not the other way around.

Sorry, no. If it fit onto a Pentax camera then the lens has a Pentax mount. The Nikon (I assume you mean the D80 digital camera, or perhaps the N80 film camera) has a Nikon mount. Since the Nikon mount has a deeper flange distance than the Pentax mount, there are no adapters available to allow you to fit a Pentax-mount lens onto a Nikon camera.

Sigma makes their lenses with a variety of mounts. If the lens has a Canon or M42 mount then there is no problem. If it has a Minolta mount, for example, then replacing the lens will be easier and just as cheap as changing the mount. If it has Nikon mount, you can buy adapters to fit it to Canon or M42 bodies.

That particular lens should work on the D40 if it is made for the Nikon mount.
If not, you'd need an adapter to fit it to the Nikon mount. (i.e. if it is a Canon-type lens, you'd need a Canon-to-Nikon lens mount adapter).
You'd probably have fair luck selling it on eBay and buying the right kind of lens for your Nikon.

Hi,

The nominal filter size of the Sigma 600mm F8 Mirror lens is 30.5mm(Rear).
The actual filter has a rear screw thread measured across that is approx 22mm.

Does this look like what you have?
If you look for Sigma 600mm mirror filters on the web you may find some.
As you can see by the photo the filters are mainly for film use.

No but you can get a converter for it

You need a T-mount ($30) for a Canon on E-bay or a big camera store. There is NO connections to the camera body, so you have to turn off auto focus and go to "Auto" mode (the shutter speed will change) as the lens is fixed at F8. The mirrors give very little depth of field, 4 or 5 feet when far away but everything in back is a blur of circles. At F8 the shutter speed can go slow and at 600mm you may need a tripod.

There is a ring you must remove then you will have to soften the silocon we use aceton but you must be very carefull DONOT lever out the elemet just turn over the lens and bang it on the table a few times and the front element will fallout with luck

You may be able to find a Konica toMinolta or Konica to M42 adaptor. Some adaptors are available...some aren't; it depends upon the film plane dist. and mount diameter. For instance you can get a Nikon to Canon adaptor, you can't get a Canon to Nikon adaptor. Email KonicaMinolta and ask them.