Why is the max effective range for area/point targets for a M240B different in the M240 TM and the Army Machine Gun FM?

Check the dates on the two references. I'm guessing the FM 3-22.68 is probably from the late 90s, early 2000s. TC 3-22.240 was just updated in 2017. Always err on the side of the newest reference.

I have a couple guesses why the maximum effective range might be different: 1. They may be using a new method to determine max effective range. 2. We are using different ammunition with a higher ballistic coefficient now. 3. Some new update on one or more of the components of the 240B. I would guess #2 is the most likely culprit, but ultimately it doesn't really matter unless it comes up on a board. I know people that can effectively engage targets way outside the listed max effective range, and I know people that can only effectively engage targets at about half the listed range. Soldiers should get enough practice so they know their capabilities with their assigned weapon system.

CPT (Join to see) has it right. Bare in mind that the FM for the M240 is no longer published and has been superseded by TC 3-22.240, so as he has pointed out, this is your best source of information given that it was published in 2017 vs the TM that was published in 2002.

According to my 240B son at Fort Campbell they use TC 3-22.240 and not FMs and TMs anymore. He said as far as he remembers the only difference is between the bipod vs. tripod. Not sure if that helps you out.