The M40 series is 7.62x51, none were ever fielded in .300winmag, that I'm aware of.
Chad
FWIW, I did find (researched) some good (Rem700 based historical) info, if anyone's interested and doesn't know already.
You sniper types probably already have this tatoo'd, but for the rest of us...(Remington 700 fans):
From what I've learned, the Marine M40 uses 7.62X51 "short action" and couldn't handle being "punched out" (rechambered) to a longer cartridge (.300 winmag, etc.) anyway. You'd have to start with the complete "long action" (M24 type) rig.
So, what I get is the M24
can be converted to the larger(longer) rounds, the M40 cannot.
The Weapon System Matrix Manager for the M24 was Major John Mende and he explains that the long action choice actually had nothing to do with the ability to convert to a .300 Win Mag, but rather the .30-06. The rifle was intended to be chambered in the .30-06 as the -06 was a type classified munition for the Army unlike the .300 WM at the time. They wanted to have a high power load for the .30-06 eventually developed. As development of the system was moving forward they discovered that there was not enough .30-06 ammo in a single lot in the Army's inventory to test and develop the system so they quickly changed to the 7.62x51mm NATO (308 Win) and left the action the same(long) as there was not enough time for the manufacturers of the stock and floorplate to make the change to short action. They also fully believed they would later do a product improvement update and convert all the M24's to .30-06. The fact that they could convert them to .300 WM / .338L was an unexpected benefit to the SF groups (of the M24) and was never actually "designed into" the system. And, it's still unclear to me exactly how much conversion to .300wm has occured outside the SF community, but apparently not enough to cause "a lot of surpluss ammo" to exist in the marketplace...as exists with .308 and .30-06.
(that was MY big question)
M24 UPDATE (03/11/06)
The M24 & M118LR(.308) combination has proven itself very effective during OIF and Afghanistan. There have been kills made beyond 1100 meters, though those are not standard fair. But under 800 meters the system has proven everything that was hoped for. The US Army has accepted a new sniper rifle known as the XM110 which is a 308 semi auto built by Knights Armament. One thing is clear, the field reports on the M24 have been very good and there was a strong outpouring of support for the M24 from field snipers when news of the XM110 was spread.
(This Brief History was "compiled" from various sources, so if anyone "in the know" needs to correct any of this, then PLEASE do!)