It was a problem with the Sheridan's 152 mm M81 too so they made the keyway shallower on later guns, only as deep as the rifling on the M81E1. The XM162E1, which the M60A2 had when it was standardized, also had a shallow keyway.
It looked good on paper though, fooled the Soviets into thinking it was a bit better than the T-64B and a lot better than the T-72/T-64A.