Lens to subject distance has a big influence on the relative sizes of things in the image. The classic example is a straight on portrait taken close up - noses get huge and ears get tiny, and the closer the camera was the bigger the nose seems. Because we all know intutitively the relative sizes of ears and noses, the close up shot with the (relatively) big nose also makes the whole head seem like it's really "deep."

Same thing will happen with a gun barrel pointed at the lens - the closer it is to the lens, the bigger the muzzle will look, AND the (relatively) smaller cylinder will make it feel like the barrel must be a really long one.