As far as I know only 120mm Abrams went into action in the Gulf War with some 105 models in reserve.
This makes sense given the time frame.
The 105 used in the Centurion and M60 tanks was countered by the Soviets in the T-62 tank being fitted with a 115mm gun.
Although the 105 could defeat the T-62/64/early 72 armour the 120mm gun was developed so it could take advantage
of different types of rounds and still ensure a high percentage of kills against later opposition.
British tanks used HESH rounds (squash head rounds) so rifling was ok.
APFSDS (fin stabilised discarding sabot) rounds were better if they didn't rotate so smoothbore was best for them - the fins
stabilised the round in the air anyway.
HEAT rounds needed a bigger diameter barrel because they have an internally shaped charge - the bigger the diameter the better.
Heat rounds suffer badly from rotation as on impact this skews the effect of the shaped charge and lowers it's ability to
penetrate.
The smoothbore also meant larger charges to increase velocity as the chrome coating helped the round slide out of the barrel. This
in turn gives better ranges and less wear although smoothbore barrels still wear over time.
Soviet tanks then went to the 125mm with the T-72 but from there it all becomes a matter of ammunition rather than barrel diameter.
For example, the 125 barrel was made thinner than the 120 since the T-72 and on are more of a medium tank with a big gun rather
than a full blown MBT like an Abrams. Replacment time for an Abrams barrel is around 1500 rounds. By then accuracy is unacceptable.
For the 125 because of it's lighter construction - 250 rounds before it is no longer accurate. A big difference.
Everything possible had to be done to keep the weight down on soviet designs and it has shown in conflicts
where soviet era tanks have come up against modern Western types.
There. Now I've run out of stuff so I'll stop.
