Re: who can kill an M1 Abrams?
The LOSAT is a diff solution. This technology hasn't been released yet.
The problem with kinetic-energy rounds is that lose about 160kps velocity over 2 kilometers, due to drag. This kinetic energy declines with the square of the velocity, so an armor penetration capability falls off at extended ranges.
The advantage of a scramjet propelled round is that it can sustain the velocity and maintain that penetrating power all the way to its target.
If you consider the fact that a standard M829A2 kinetic-energy round using a finned “dart†penetrater pierced two Iraqi T-72 tanks parked side-by-side, then a scramjet solution would have been even more devestating.
These rounds are travelling at Mach 5 at peak velocity - a scramjet solution could realise a Mach 7 kill - at a greater distance.
The longest recorded kill is 5km by a Challenger 2 on a T72. That means that at almost the end of its flight path there was still sufficient kinetic energy to destroy that tank. Imagine if that shell still had all it energy intact like what a scramjet round can offer.
Its also why the bulk of other militaries aren't looking at PGM gun fired rounds - there are better solutions in place.
Personally I think if you want to have an on board anti-air role, then the Israeli AAM pods are a better idea. The Aust army trialled recessed cannisters on our ASLAV's in the late 1990's, but they weren't seen as essential kit.
As you can tell, I'm not a big supporter of rifled PGM's on an MBT.

Armour is part of a force cohort, hence I believe that anti-air capability should be part of the force structure on the move, not part of a dedicated platform, like an MBT. Stick you Chaparral, Crotales etc on your IFV/AFV's and let the tanks focus on being tanks.
It boils down to an issue of theatre if not battlefield doctrine though.