You can always get away with everything, speaking from own experience. Will it always work? No. It depends on the situation, on your story, on how much you succeed in making the cop believe you are telling a credible story.
If you have a MK1 on your back and you're gonna say it's a walking stick, you won't get away with it. But with some smart thinking and acting, a lot is possible. If you have the time to get rid of your magazines, you can claim it's a model, you're a collector, or it's a BB gun. If the cop however has a mag that fits the gun, this could quickly go against you.
TL;DR: Yes, you can get away with it, if your roleplay is good enough. Be creative!