The Pancor Jackhammer is a fully automatic Shotgun, with mediocre damage, bad accuracy, and a decent sized magazine.
The real downfall to this gun is the lack of firepower per shot, it's easily beaten by Judgement in terms of damage, and accuracy. That said, the Judgement's advantage here is for it's one-shot-kill capability.
This gun is best used with a very fast, or very strong soldier.
Actually, this weapon has one of the highest DPS (damage per second) of any shotgun. With a high rate of fire (faster than many assault rifles!), you don't really need hugely powerful damage. So, while the slower but more powerful shotguns will need to take a second to get any extra shots in, the Jackhammer, when used correctly, can mow down literally anything in its way with a little bit of practice.
So in summary, this is a really powerful weapon, but it takes some getting used to, and doesn't get one-shot kills.
Recommended use with: A powerful sidearm, a Katana would be the best choice. If you're trying to balance out your weapons, a sidearm with good accuracy, damage, and range is good, like the Kriss Kard or the Beretta Px4. Attachment/Skill wise, it is best to use Hair Trigger to further improve the already high rate of fire, or using Extended Mags to get more rounds into your enemies before having to reload. Using Fast Hands or Full Auto along with wither of these further improves how many enemies you can handle without reloading and how fast you can dispatch them
Recommended use against: Any target which doesn't have an extremely powerful close range weapon. Since this weapon can literally mow down groups of enemies (provided you don't run out of ammo), pick a group of lower-end enemies (Generals, Snipers, Engineers, etc.) and start destroying the enemy team.
Note: This isn't really a shotgun to use if you wish to have good survivability since it has low range and can't one hit kill.