The M40 was first deployed in the mid-'60s ('65, '66?); the original M40 was just a Remington 700 in 7.62x51mm that was given a national stock number. We're now on the M40A3, which has had more substantive updates and is still in use with USMC.
The M24 was deployed in the late '80s ('88?) by USAR, not USAR + USMC. USAR almost adopted the M40, but they and the Marines had a disagreement regarding the future long-term usefulness of the 7.62x51mm round. The Marines thought "if you need larger than 7.62x51mm, either get a .50 or call in artillery." The USAR thought "what if we want a .300 Win Mag or .338 Lapua?"
The M40 is a short-action Rem 700; the M24 is a long-action Rem 700.
Re: Corrections
The M40 was first deployed in the mid-'60s ('65, '66?); the original M40 was just a Remington 700 in 7.62x51mm that was given a national stock number. We're now on the M40A3, which has had more substantive updates and is still in use with USMC.
The M24 was deployed in the late '80s ('88?) by USAR, not USAR + USMC. USAR almost adopted the M40, but they and the Marines had a disagreement regarding the future long-term usefulness of the 7.62x51mm round. The Marines thought "if you need larger than 7.62x51mm, either get a .50 or call in artillery." The USAR thought "what if we want a .300 Win Mag or .338 Lapua?"
The M40 is a short-action Rem 700; the M24 is a long-action Rem 700.