Here’s what FNH USA‘s Executive Vice President of Military Operations Mark Cherpes told us:
“In the early stages (presolicitation) of the SCAR program, the draft requirement defined a single weapon platform capable of adapting to multiple calibers (i.e. 5.56×45 mm, 7.62×51 mm, and 7.62×39 mm). FN had proposed and offered a single-platform system to USSOCOM that would adapt via conversion kit to SOF current and future ammunition. During the requirements finalization phase, the SOF operators took the decision that the weapon should be split in two platforms, one gun for 5.56 and a second gun for 7.62. The reason this decision was made at the time is that the SOF operators did not like the fact that the 5.56 base platform would have an increase in weight over the M4. The weight difference between the MK 16 Standard and the SCAR H Standard is about half a pound. Upon completion of the developmental test (DT), the operation test (OT), and the full Fielding and Deployment Release (FDR) authorization, AKA Milestone C, a new group of operators reversed that initial decision and said that they wanted to move back to the original spirit of the program: a single weapon platform capable of converting between 7.62 and 5.56.
Basically, to accomplish a multi-caliber system requires that we develop the gun on the basis of the largest caliber and then scale down the conversion kit to go to a smaller caliber. The MK 17 receiver is a little larger to accommodate the 7.62×51 mm ammunition. It is not possible to scale up a smaller receiver to accept 7.62 ammunition, thus the MK 17 was chosen as the base platform. This was almost seven years after the initial decision to split the platform and a new group of operators had rotated into the SCAR program effort. FN finalized the 5.56 conversion kit in late 2010 and it has passed all DT and OT testing, and an initial delivery order has been placed for the conversion kit.”