I actually would suggest the use of single pistols over machine guns. Only because if you like fortegunner and arent thinking of going gunmaster, maybe you like striking a bit more than I would suggest to you to lvl up the pistol shots and the single saber. It is a classic combo on PT. Many people neglect the single saber and pistol combo, such as I do but people that have them lvled really seem to like the combo and after lvling single saber pa's up I can see why.

I would go like this:

Shotgun (all elements)
Grenades (Ice, and one other element and boma durenga)
Twin Pistols (well if you are going to strictly stay FG then none, make use of crossbows and sabers/daggers or machine guns and saber/dagger or as I suggested pistol and saber/dagger)
Rifle (all)
Laser Cannon (Slow to lvl but once you do youll love them so all elemental)
Spear (dus majarra, maybe dus robodo)

Now you asked also about some of the ultimate's and to tell you the truth the machine gun ultimate blows, you need to be directly behind enemies but it will stun lock bigger slower guys such as Jarba's. Its difficult to use against all enemies because you need to be directly behind them but it will also do this to carriguine's but they turn so fast you have to keep moving, a simple rifle or grenade works best on them I have found.

As far as grenades are concerned, if you know how to work grenades real well you can keep enemies from moving out of a corner, I play PT and use them selectivly so I have ice because there are many fire creatures and I have light. I use boma durenga the most however. Basically ice and maybe one other element and boma durenga should take care of most boss's.

I do not play FG but I feel my input is useful because I bring something else to the mix. I think its important to play and think like other styles so you can find out what others are trying to do so you can help them out when playing your class. Sure it might be awesome that you cause knockback with those rifle bullets but if you keeps blowing enemies away nobody is gonna wanna play with you for being annoying. This game isnt hard the hard thing is being considerate to other players you play with. With such a thin community now, this game is still fun but its hard to find considerate people to run with.