Hey Oleg,
Others have told me there is no problem with the SU-16 / SU-22 receiver rails, but I've had two different scopes that ran out of elevation before I could zero them - one on my SU-16 and one on my SU-22. Other optics have had suficient elevation adjustment and have worked well.
In both cases, my fix was the same. I placed a shim between the lower scope ring and the scope on the rear scope ring. This raises the rear of the scope. This is the direction you need to go. Shim material should be relatively non-compressible and not prone to rot or decay so paper is out. Thin stainless foil would work well, aluminum foil works OK (especially heavy duty foil), thicker aluminum from something like a TV dinner tray would be good, or plastic shim stock (pretty much any thin plastic sheet). I used electrical tape. It was easy to use because it's self adhesive and it has worked well. It's too compressible to be optimal, but I wasn't too fussy as I'm not planning on shooting either rifle for bench rest applications. If you're good with trigonometry you could calculate the thickness you need, but I'd bet three pieces of electrical tape would be about right.