I have been shooting wildlife with a 200mm Tak f4 M42 and a Lentar 400mm f6.3 M42. I got tired of getting focus lock at f6.3 on the Lentar on my K20D then changing to f8 for sharpness and hoping the focus hadn't changed.
With the 200mm Tak it was better because I could get focus at f8 and just take the shot, problem was focal length. I needed more for the shots I was taking. The Tak is one of the best lenses I have ever shot through, the clarity is amazing. I took these pictures of a Golden Eagle in the wild with the Tak 200mm
https://www.pentaxforums.com/forums/post-your-photos/76876-nature-golden-eagle.html
I finally got the CO (wife) to Ok the Bigma purchase. I researched and checked all the on-line stores (see the thread No More Bigama!!??) and finally went for the Sigma 150-500mm. I had a chance to play with it a bit today and I am totally impressed!! Of course I only have the 400mm Lentar to compare it to, but WHAT A LENS!!!
Wish me luck and I will post some photos soon!