A trademark is typically a name, word, phrase, logo, symbol, design, image, or a combination of these elements. Can a shape be a trademark? Is the shape unique? Does the shape uniquely identify the product or manufacturer?
Consider this:
That's the letter "M". It's a shape. It's also a logo, as the particular color and style of the "M" represents a corporation. You don't need to see "McDonalds" to know that's who it represents...just as an FN SCAR doesn't need to be labeled to identify it. Not so with any AR variant, incidentally, of which there are hundreds, and there are hundreds because Colt failed to safeguard the trademark implicit in the appearance of the firearm.
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