Re-reading the NDIA 2012 notes on the LWMMG for the N-th time made two things very clear. One, the brief is slanted. It exaggerates the range gap by comparing the M240 on its bipod against the M2HB on a tripod (page 5), but tabulates weights for the M240 with tripod against the LWMMG with tripod (page 15).

See http://www.dtic.mil/ndia/2012armamen...662steimke.pdf

Two and more vitally, the reference to ".338in variant” (page 15) indicates the internal mechanism of the LWMMG can be modified and mated with an appropriate barrel to operate with some calibre(s) other than .338in.

One likely calibre is 7.62mm and especially a magnum cartridge, with 7.62x51mm NATO as a less likely contender. The .338in reference covers both the current 8.59mm NM cartridge and longer cased 8.59mm Lapua. An amateurish effort at mensuration suggests that the LWMMG feedslot might be able to admit something even larger such as a 9.3 or 9.5mm cartridge.

The power of such ammunition would seem too much for a lightweight gun. However it also seems that no-one inside/outside GD is prepared to fully describe what it is intended for and with the LWMMG.