Step 0) (Not seen) Clean up source image very sloppily

Step 1) Reduce image. These options are available in other programs like GIMP I'm pretty sure, just follow their methodology. For the most part when spriting it's best to use nearest neighbour, although there are times where you can get a better silhouette with Bicubic Sharper (although it brings up another set of problems like transparency issues that I personally use actions to correct in 1 button).

Step 2) Comparing it to the size that you originally intended. I noticed that you wanted the gun a little more compact, so I reduced it further FROM the previous reduction that already gave me the proper colour hues that looked good. Because nearest neighbour is on, it preserves the HUES, which is excellent. From this stage, I actually also attempt to add more detail before the new reduction, so in this instance I tried to add the gap in between the barrels and picatinny - but it never quite looked right, so I scrapped it.*

Step 3) Fixing up the silhouette. I also noticed on the reduction that a dark pixel was lost in a place that looked really good, so I added some darkness to that area on the new reduction.

Step 4) I compared their lighting to the lighting that I personally like to use, and darkened any areas that would be shadowed by said light.

Step 5) Adding little 'glints' from anything that would glint from said light. In this case, the back metal piece of the gun. Don't go overboard with this.

Step 6) Figuring out what else should be accentuated. *Obviously the hole in the gun would be great to add that gap, but again, it didn't looked right so I left it out of the design entirely. To make up for that, you should try to add something in this area for eye-candy's sake.

Step 7) So I added a holo sight. It looks kind of okay but maybe a little bulky?

Step 7a) I added a scope. Even more bulky.. yikes! but not too bad - this could still be indexed and look fine in-game.

Step 7b) The picatinny itself. Yeah, not bad! gives a balance to the image. Mind you, this is entirely opinionated. You could've done anything here really.. as long as you made it exaggerated enough to be clear what it is.

Step 8 ) Deciding what else the silhouette was missing and just adding in tiny amounts of pixels. Literally 4 pixels were added here and look how much difference it makes

And then to show you how much was actually done after all that, I added the shading layers without the scaled image. I think it's only something like 33 pixels. Mind you, I kind of lucked out that the hue indexed so well... sometimes you have to play around with saturations in the end to make sure that something doesn't look really out of place, but it's usually only 2~5 pixels anyways unless your entire hue for the device is on the edge of one palette color and another. If that's the case, sometimes it's best to hue-shift the image in the hue saturation of your choice a bit, index it, and then work on it from there. Seems like a lot of work, but I've been doing this for 10 years and have had projects with over 250 layers, so 8 steps is like nothing @_@ so don't be discouraged. It does help a lot to have epic programs though >.>