Fortunately for you, there's the SAW!
"SAW" stands for "Squad Automatic Weapon." When I was bouncing around Afghanistan, I managed to talk one of my Canadian counterparts into letting me burn through a few hundred rounds on his SAW at the firing range:
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Those were fun times. When I'd emptied the entire box of ammo, I tried every way I could to talk him into trading that beauty for my M-16, but alas it was not to be.
Since I can't (legally) have one of those to hang on the wall, the next best thing is this version from HALO 4:
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It's still pretty imposing and I had to have it. Since it's not something I can just buy from someone, I had to make it.
Here's the finished result:
Here's the finished result:

For details on how I made it, read on...