Tips: Always keep the bombs flying. There is little reason, except being dead, to not have the dots ticking 100% of the time. You have an arsenal at your disposal that should never run dry and you'll always have something ready to throw. Don't be afraid to refresh your DoT's early. Use your Gun Blast, Hip Shot, and Focused Fire in Combination with Unshakeable Focus to deal burst damage to finish off a Destuction at low Wounds.

Highlights:
Increased Close Range Damage
Massive AoE Potential
Bonus Knockback from Hip Shot
Strafing Run and Artillery Barrage brought back to back on already DoT'd targets can be very lethal.

The Sniper

This is a mastery built on staying as far away as possible and dealing threatening and hopefully fatal amounts of damage. The focus will be Snipe, Gunblast and Hipshot. You'll have a 15% increase in Critical Strikes, great for hard hitting un-avoidable attacks like Snipe. With Signal Flare on the target you'll deal 15% more damage. With Unbreakable Focus up, 115%. Pick a target with a little Health lost and take it out. Find them squishy's and make 'em bleed, baby!

Notes: While I have not been able to try this, it it mostly suited for Keeps. Picking off targets on walls or picking off Healers and Ranged DPS below.

Highlights:
Snipe for picking off foes
15% increase to all damage when target affected by Signal Flare
Magnet for AoE.
15% increase in Critical Strikes.

1.5 - Tactics -

Your best bet for tactics are almost always the same. There are few you'll grab in the Mastery Trees that are worth it.

Basic Trained Tactics

Steady Aim - You will always want this. (160 Ballistic Skill @40)
Pierce Defences - Good against targets that defend often, Tanks.
Tangling Wire - A good way to snare a target who will most likely break the Root. Much more safe over using Spanner Swipe.

Mastery Tactics

Tracer Rounds - 15% Critical Strike. Can't really turn that down.

Throwing Arm - A must have for T4 if you plan on using your Grenades. 50% more range is a blessing. Disables and Knockdowns will be the end of you if you have to get so close.

Stopping Power - Knock Back added to Hip Shot. Always nice to have another knock back. Not recommended for Rifle builds. The knock back can cause range issues and Hip Shot is a valuable instant shot to apply pressure.

Extra Powder - A nice little 75% increase in range for Fragment Grenade and Acid Grenade. If you are focused on these two, you'll probably want this... However, with such short CD's and low costs, you can spread the Fragmentation Grenade DoT pretty easily.