I wonder how many people still shoot with mirror lens these days ? I like mirror lens for two reason, cost and portability. I am not a pro and will shoot wildlife occasionally, a high quality telephoto lens not only expensive but also heavy to carry. I used many different made of mirror lenses not from camera makers such as Vivitar, Soligor, Samyang/Rokinon, Tamron, Russia....etc., I found 2 made are the best, Tamron and the Russia MTO. Tamron is smaller and the color and contrast slightly better than the MTO but cost double as much depends on where you get it, where the MTO is relatively cheap and plentiful in the used market, most of them are ship from Ukraine so will take some time.
I limited myself to shoot up to 300mm (FF equiv. 450MM) for portability, but some times will shoot up to 500MM with mirror (FF equiv. 750MM), that's more than enough for a non pro and occasion bird-er. I also got a Japan made off brand 300MM mirror and it is quite good too, you won't find a bad 300MM mirror but 500MM is another story, it requires higher precision to produce. There are some new 300MM mirror made for MFT I believe they are good too, but they are smaller at f8 not the regular f5.6 further limited the usability of the 'dark lens'.
There is one note if you happen to find a Russia mirror lens, for instance the 3M5CA, you may find it blurry unclear and produces distorted image, don't return it so soon you can fix it. Early made Russia mirror lenses are not for camera but for weapon use, the mirror is mounted on the cannon for aiming, the lens is over tighten and caused the mirror to deform and produces distorted image, that wouldn't be a problem for weapon aiming but quite for photography. You can search on the internet 'relaxing the mirror lens' to find solution to fix this problem, it is quite simple and you can see the result instantly, my copy does not produce distorted image but unable to focus to infinity.
Mirror lens unlike glass refractive lenses, the infinity focus point is not locked at the far end but over-shoot, you can focus "beyond infinity", this design is to compensate for the metallic mirror expansion due to temperature, also used on early APO lenses due to rare earth glasses in use. The over shoot design actually good for adopting the lens to Pentax camera, coz there is more room for back focus correction, so that you can use any type of M42 adapter, 'sink' or 'flush' type, my lens focused properly after the above simple mod. Please note, Russia mirror uses M42 instead of the more common T mount, so be careful when using and do not force in the lens or you will break the screw thread.
Please see below some casual shots with the 3C5MA, please ignore the shaking and focus (no AF), and keep in mind that mirror lens has lower resolution and contrast than glass refractive lens, handheld shots can not produce the best result especially when shoot in the shade at high ISO, but absent or near absent of CA in long lens is the reason to give mirror lens a chance, also the vignetting on APS-C (K3) sensor is non exist, there are some other high quality shots you can find on the internet.