I work as a commerical photographer. I studied photography for 4 years and Im still learning ,have been running my own business for 8 years now. My advice is to keep shooting and experiment. Some of the best photographers in the world are self taught.
1: photography is all about light ( photographing something like this in the middle of the day with the light being very harsh or flat ,will make your subject look like that as well)
2: Keep your backgrounds simple: At the end of the day you want people looking at your image to focus on that -not a big tree in the background ,a car etc etc
3:Clarity: When shooting a rifle like this , you need to position it so it makes it interesting for the viewer ,your 3rd picture as you described is the better one.
Laying the rifle down in the grass and shooting as a plan view dosn't make an interesting photograph. A friend of mine who does alot of still life work ,could spend two days setting up a still life shot!!! ( thats to the extreme of course)
4: As I was taught by some very good pros ,their advice was " keep it simple "
" have your light source behind you ie: the sun or a light.
5: f stop: Im not sure what kind of kit you have ,but depth of field is very important when shooting something like this. We could chat about this for hours ,but quickly -the more depth of field you have IE: f16 /f22 the more in focus your still life will be ,the less depth of field you have ie: f2.4 /3.5/5.6 your rifle won't be as sharp . Hope that makes sense.
Good luck and play around with it ,thank goodness for digital cameras where you can view your handywork.