>>64189714
The Mk48 has a 19.75 inch barrel, which is perfectly long enough to be taking advantage of 7.62. It's not like there are significantly longer 5.56mm options. The long barrel SAW is .75 inches longer, which in practical sense makes it the same length.
For your context, the MK48 barrel is only 4 inches shorter than the standard M240 barrel, and it is the same as the short barrel for the M240.
So, from approximately 20 inch barrels for both you can compare 5.56mm and 7.62 machineguns, and you've got obviously better cover penetration with the 7.62. You've also got longer effective terminal ballistics, though with something as lightweight as a MK48 I think the emphasis on something more portable than super steady at ranges beyond what 5.56mm can kill. Not totally discounting the ballistic range increase though.
I deployed and embedded with guys who had MK48s to replace their M240s and they absolutely loved them. The barrier defeat of 7.62 was the same as what the M240 gave but in a package about 10 pounds lighter, which for machinegun guys was very appreciated.