Sigma 300 Pantel

I took delivery today if a 300mm f/5.6 Sigma Pantel. It's in Nikon AI mount. I have had Sigma 135/2.8 Pantel lenses in various mounts for many years. It's a decent regular 135 and I have made some interesting images at the f/64 setting. Yes, diffraction sets in at f/64 so I don't have any 16X20s shot this way. Sharpness was helped by using a tripod. I have used medium format enlarging lenses at small apertures on a bellows with 35mm film. The 300 I received today needs some work. The aperture mechanism is slow and will not close down all the way. Has anyone used one of these?