My M240G is a little different to yours, but I think its probably because I stuck to the FF&S design sequence pretty rigidly. Your ROF mod is fine if you're just designing guns and want them close to reality, but I think you'll find it has a major impact on gameplay for Traveller.
I use a barrel length of 63cm. And a 100-rnd belt as per FF&S rules.
Tech Level: pre-stellar (7)
Weapon Type: heavy self-loading longarm
Ammunition: 7.62x51 mm TL-7 necked
Muzzle Energy: 3689 J (Tranq: 2213 J)
Weapon Length: 126.5 cm
Weapon Mass: 14.632 kg loaded, 12.332 kg empty, with no magazine
Weapon Price: 2768 Cr
Magazine Mass: 2.3 kg loaded, 0 kg empty
Magazine Price: 0 Cr (100-round belt)
Ammo Price: 0.46 Cr (Ball), 0.92 Cr (DS, HE, Tranq), 1.38 Cr (HEAP)
Ammo Mass: 23 g each, 1.15 kg for box of 50
Features: plastic stock, optic sight, bipod, long muzzle brake, shock absorber