You didn't want to miss the AR-7 (not AR-4500, I remembered incorrectly), it's a really fun gun. Think a rapid fire GL-303 that fires in a 3 shot spread. Later in the game where most enemies can one shot kill you, this gun can really be handy clearing out a room of them. On the first level in mission 10, you would have found a terminal to remote control a thermatron. After using the thermatron to deactivate a shootable switch, the area in my first screenie will open (you need to roll in, you can't walk in) and there you'll find the AR-7
From your screenies, it still looks like your using the crappy chemical battery you started the game with. Look at the energy level in next 3 screenies. They each show the silencer with 1/4, 1/2 and full energy capacity, respectively, my batteries where the chemical, fission and fusion. Make sure you can have a full red bar next time you see weasel as later in the game, you will be needing it as the RP-32 gets fazed out of usefulness by the last few missions.
Did you see this post here? http://www.gog.com/forum/crusader_series/the_thermatron_must_die/post6
You probably can't pick up new weapons because you are already carrying 5 and you'll need to drop some of your peashooters to pick up decent weapons. Ctrl + D drops weapons. You missed the AR-7 in the last mission but I think it comes up again later in the game (last mission IIRC). May I strongly suggest if nothing else that next time you see weasel, get a fusion battery and a UV-9 (both are in Weasel's weapons section). You will have a tough time if you don't have a minimum of these for the last few missions. After mission 10, the UV-9 might not be available yet but the fusion battery definitely is (5th screenie for proof).