I Noticed that I typically Ignore the Engineer and his Weapons for some reason. Let's Change that with the Good 'ol Frontier Justice.
The Frontier Justice is a Primary Replacement for the engineer.
It's Simple, At the cost of half of your Clip you can get a Guaranteed Crit for each Kill your Sentry gets, 2 Crits for kills and 1 for assists. However you only gain the Crits if your sentry gets destroyed.
So right away you can see that this weapon's Main purpose is offense, Often Favoring Sentries being put in offensive positions so it can get kills and so it can get destroyed so you get the crits
You can also Manually Destroy your sentry, but you should only do this if your team has the point.
Even though you got a weapon that encourages Sentry placement, dont forget your other buildings, Put down a Dispenser or a Teleporter so you don't get yelled at by your team.
Keep in mind that you lose ALL of your Stored Crits when you die so don't build up like 35 Crits and go an a rampage, you only get 3 shots per clip anyway, so whats the point of having 30 something crits when you have to reload every 3 shots
Now for weapons to pair this with.
Pistol can help if you dont have any crits and no sentry up, Wrangler can help getting kills or adding extra mobility. Gunslinger for Mini-sentries, Eurika Effect can help you get away or Bail out if your nest is about to die, and by the time you teleport back to spawn you get your revenge crits.
I Personally really like this shotgun, it's fun when playing offensively, just don't try to abuse the Crits, because usually when you got the Crits it's easy for the power to go to your head and you overextend and get yourself killed, Just try to avoid that, even though you have Crits you are still the Squishy Engineer that you are.
And thats all I have to say about that.