Personally, as a former sniper, I am not a huge proponent of the BMG based sniper weapons. That is to say, they indeed have their uses, but I don't find them the end all, be all of sniper weapons, and bigger isn't always better.
The BMG .50 caliber round has truly impressive range, and getting hit with a projectile the diameter of a banana and weighting a good half pound is no laughing matter - but effective shots are made far more often, every day, with weapons that weigh a third, are more portable and equally accurate.
But- that said,such weapons have their uses.
What is the intended modern use for this weapon? I can’t imagine being anything but a long distance sniper rifle. For that I would assume that you would want a longer barrel to allow the charge to burn up.
The barrellength/charge hasn't anything to do with it...its the overall size here thats telling. More and more sniping and enagagements are occuring in urban environments, and a smaller and more compact weapon just makes sense - hustle a 5 foot long 25 pound Barret up six flights of stairs in a standard stairwell, and you'll see what I mean. And don't forget the ammo, lay pad and gear you always hump along with you...its an ungainly process.
Soft skinned ( lightly armored if at all) targets are now more of a priority than ever - car bombs are a reality, as are escape vehicles and "crashers" - vehicles beefed up for ramming road blocks. A .50 cal. round in the engine block slows the most persistant driver. The same is true of my favorite use for such weapons - resources destruction. I am an old timer, but I have no doubt some doctrines havent changed - heavy rifle fire at commo relays, commo antennaes and such I am sure are still the practice, and think about it - a bullet in the trunion of a SAM radar disables the installation, as do well placed rounds to the missles themselves.
You also see these heavy rifles used in denial missions - directed fire against mortar tubes, heavy weapons emplacements ( such as an emplaced DShK or similar heavy weapon, a round to the reciever puts one out of action for good) and their usege on human targets is a welcome incidental, but not always the point.
This weapon would also be useful in mountainoius terrain, again where the small size is a premium, and cover is scant.
BTW I am not a gun control nut but what possible purpose could this rifle/cannon serve? Home defense? If that is the case I suggest moving. It seems to me that if you need more than an M-14 you need to move.
Home defense??!! Obviously not.
As to purpose, refer my previous comments. There is of course no reasonable civilian usages, barring perhaps police use ( I can't see what purpose it would have here in the US directly, but many countries have issues with previously mentioned car bombings, and routinely establish roadblocks, such a weapon has apparent utility there)
For Traveller purposes, the weapon is very like the statted LAG ingame, and most parties that absolutely must have one generally intend them as a simple method of defeating high tech personal armors.
In one of my campaigns in an alternate universe, they were a common civilian weapon, used to attend to neo-roos ( a raptor like native predator) and other inimicable creatures that were generally in the plieocene megafauna stage of evolution. I imagine on planets where the native predators are built like rhinos and have a taste for humans , I would want one around.