Page "MGM-51 Shillelagh" Paragraph 12

While the maximum range of was acceptable, the Army felt it could and should be improved.

Ford received a contract to develop a longer range version in 1963, and returned a slightly larger design the next year.

Test firing of the new MGM-51B started the next May, and production began in October 1966.

Besides the changes to the missile, the gun was modified.

In testing it was found that the key slot in the gun led to cracking after firing only a few shells.

After further study a version with a shallower slot and new barrel was selected, creating the M81E1 / MGM-51C.