There was an ABM treaty in force which is now defunct but itit limited any ABM development to track or engage targets travelling at less than 4.8 meters per second or Mach 14. That is the limit upon which both USA and Russia wrote their algorithm to be used in their ABM. ALTHOUGH treaty Is expired but mABM manufacturers still mention Mach 14 as their upper most limit for detection of any target. S-400 has also the same stated upper limit of target detection.
You can check it yourself from wherever you want.
Other than that you may be aware that there are only two systems in the world which have been specifically developed for ballistic missile defense and can defend against all types of ballistic missiles. The Russian A-35 and American GMD.
ALL other so called ABM are SAME with added capabilities of "trying " to hit an incoming ballistic missile.
The nature of the systems limits them to reliably defending against tactical ballistic missiles only. Which are generally missiles which don't rise above 50-60 kilometers and hence don't have speeds above 6-7 Machs. It is stated in S-400 specifications by manufacturer. They only mentioned tactical ballistic missiles and even that limits system's tracking range to just 60 kilometre as even those type of missiles are too fast.
S-400 or Patriot or any other SAM + ABM cannot defend against strategic ballistic missiles or missiles who rise to altitudes of above 100 kilometers and freefall to silly Mach numbers. These missiles are too fast to be tracked or hit by S-400, Patriot or any SAM+ABM.
SO that's why I said Shaheen series us safe. Nadr,Abdali and may Ghaznavi are vulnerable if India gets S-400.
