Personally, I think bounty hunters should be able to buy an MXM. I think it could give them a better chance when fighting rebels with 7.62, as they could fire from longer ranges with better accuracy.
But, here's how I think it could be balanced.
1: Make it expensive. Make it maybe 10k-13k so that solo hunters just don't buy one, and it has to be used by an actual organized group.
2: Make it an Honor point talent, such as a level 1 Honor talent in both perk trees so people can't just instantly get one once they become a hunter, they actually have to work for it.
3: Make it only able to be bought at skiptracer, obviously.
I think if it would be expensive it could be a "buff" to bounty hunters, but not so big and really just another addition to the weapons they have to add a bit more variety.