Back when we thought the 105 was an operationally deployable gun, the C132 round was acquired as the operational round for that gun.
But the round was not intended for general training use in the LG1, even less so for the C3.
Even at charge 1, the C132 is more powerful than the M1 ammo and M67 propellant at charge 7, and has a range ~14km
Problem is the full chg (there are only 2 BTW) is very erosive, chg 1 not so much (~1.4 EFC IIRC).
A study done about a decade ago, determined the LG1 will reach its wear limit (but not fatigue limit) in ~500 rounds or so firing C132 full charge. The C3 was not tested that much with this type of ammo, since it would cost a barrel, and ordinarily there wasn't any training need to be firing ammo that "hot" anyway. This is why there had been a restriction on the C3 firing this HEER (High Explosive Extended Range) rd
The reason for lifting the restriction on the C3 to fire C132 is to mitigate some of the consequences of the ongoing M1 shortage.
Technically, a C3 that has had its cradle inspected and been put back into service could fire HEER ammo today.
Its a good thing someones asked this question about EFC's though, because in this case its reminded some people (at least those that visit this site!) of how very important the units stay on top of the EFC count.