Some of my ship designs include drop-down (or pop-out) turrets. Smaller than the ship's main turret, but heavier than a man-portable weapon, these turrets are intended for anti-personnel and anti-vehicle use while on the ground or at low altitudes. These weapons are usually fired from the bridge and/or the gunner's station, or can be linked to an automated Watchdog program.
These images were created using the Brainstorm thumbnail software, and are direct exports without any Photoshop work. In addition to weapons, I've been playing with the software to create starship massing ideas, and with less success, some robot designs. Anyone interested in doing sci-fi concept art should check out these videos for more info.
Brainstorm - short introductory video
Brainstorm - longer video/tutorial
The software is currently only $10.00 and is very easy and fun to use (if you are into this sort of thing). It works on both PCs and Macs. You can buy the software <here>.