Welp, looks like I was partially wrong, there was no rule for 1981-1985 guns:
They obviously made a few exceptions to the rule with the Magach and Finnish T-55s. Israel did have the M-426/DM-63 but it's post-1991 (Leo 1A5 NO2 uses this round).
For instance at 2000m:
- the 1982 AMX 30B2 had the OFL 105F1 ammo penetrating 420mm.
I had this discussion with Oktoberfest a while back - the ammo the B2 has is correct. The best 105mm round available at that time was the Israeli M-111/DM-23 and the Soviets were forced to uparmour their tanks in response.
the US have I believe the M774 with 390mm of pen, then from 1983 on the M833 with 460mm of pen IIRC (might be worth giving the M1IP Abrams 16 AP and 2275m range)
Those figures are on the high side but 16 AP is reasonable.
The rule is pretty simple, 30mm penetration is 1 AP. If you have a round piercing 300mm, it's 10 AP.
The AMX-30B2 can fire the OFL105G2, which has 440mm of penetration, giving it 14,67mm of pen, or rounded 15AP (the APs are rounded at the superior value IG, for every value superior to 0,5).
The OFL105F1 would have exactly 14AP. It's an improvement from the 12AP the 30B2 currently gets, but I think it wouldn't differentiate it enough from the 13AP HEAT the AMX-30B should get with its 400mm penetration. The HEAT round would be the get go given the advantage HEAT round gives over KE rounds at distance (the stupid does not penetrate therefore I don't shoot rule). But that's another topic.
The M833, with 460mm penetration would have 15,33 AP, so rounded to 15 AP. The M774 would be a round 13AP.