Composition: Cutting off the a portion of the main subject in a photo is usually not pleasing to the eye/brain, i.e. cutting off the grip/mag-well in this case.
Composition: When you have something interesting in the background, say the rifle in this case, cutting off a portion is normally a no-no/not pleasing to the eye.
Focus: If you deicide to use a shallow depth of field with the idea of having the subject in focus and most everything else in the photo out of focus, use a plain (not interesting) background. If you have something interesting, like the rifle, in the frame the eye/brain will strain to try to see it and when it is out of focus it is not pleasing to the eye/brain.
Focus: make sure the entire subject is in focus otherwise the eye/brain struggles to see it in focus, not pleasing to the eye/brain.
For product photos this doesn't apply as much or at all as it does for "artistic" photos:
Composition: Start with the rule of thirds. Then realize we read a photo like we read a book, top left to bottom right. So IF you are going to place the subject out of the center of the frame, moving it towards the bottom right is generally best. Have the less interesting stuff in the top left and build to the most interesting in the bottom right. This doesn't apply for single subject on a plain background where centering the subject is the norm.