Which Challenger, and which T-80?.
Using solid shot, the 120mm L30 94 gun on unit No 27 will kill most later model T-80 (about 70 or 69 frontal armour) to 1750 metres, and possibly out to 2250 on rare occasions. And the earlier ones with 60 and 50 something frontal aspect, to 3000-3500.
The earlier 120mm tank gun (No 102, 84 model) will deal with the contemporary armoured ones (B/BV) to slightly over 1 KM. (45 at 1 km, with occasionally up to 50). best 47 at 1500, so it could do a T-80BV at that range against the turret, on occasion, and the HEAT (HESH) will deal with those models at all ranges, if it gets a good roll (no fuse failures etc) and the ERA is not triggered. The 1998 "U" model T-80 is somewhat difficult from the front...
Best HEAT performance will be about 80 - so the Russian advanced armours get frontally impenetrable by that in about the late 80s or early 90s.
Of course - we have the L30 CHARM 120mm available from 1994, but if anyone knows different, it could be moved back a year or 2 to deal with the T-80U. Or, the L11A5 could do with a bit more "poke" - if you have any info, please feel free to share.
Naturally - if you fight to their flanks, then side shots (at low angles of attack) will whack them at any useful range. The best steel side is about 26 - which the 1984 gun will reliably penetrate to 3500 metres. So try to get them into a "U" shaped kill sack
!.
Cheers
Andy