Animating within video games - Robot Sprite work

Here I have some experiment s that I have been doing with the robots. I have been trying to create a sprite sheet that can be put in the game engine. The sprite sheet isn't finished, and there are changes that need to be done in order to improve the movements. I have done an Idle sprite for the small robot, a Walk cycle and a weapons sprite. They are just very simple movements so I can get used to drawing out sprite sheet. Here's what I have so far.
Here is the idle sprite sheet:

Here is the walk cycle:

And here is the gun sprite sheet:

These are the sprite sheets in motion as you can tell there needs to be a lot of improvement.

Gun

Idle

Walking

Walking on the spot