Next I just added some shading and reflections around the rims of the patches to make them look a little more 3d

Now for the weathering on the gun... make a new layer and call it streaks, then take a color close to black, set the opacity of the airbrush to about 5 and paint on dark spots as shown.

Next, I took the smudge tool, pressure set to about 60%, & streaked the black blobs back untill I got the desired look.
Do this for all three guns (or how many you have)

Next I did the same things for the bullet holes.
Making sure the bullet holes layer was above the streaks layer I just painted on black blobs then streaked them back

I also did this for the patches as well, the reason was because the patches are covering up previous bullet holes.
So making sure the patches layer is above the streak layer I just painted on the black blobs and streaked them back

Grumpy's note

Save the master copy of your work in the native format of your paint program (PSD, PSP etc) to preserve the layers. Then merge all layers before saving the working copy in BMP format to convert into the texture.

Thanks to Tom Tarlton aka G-force237 for taking the time & trouble to write this excellent tute & allowing me to post it. ;)
He can be found on the Simviation forums