One problem about using them for display. If by any chance they are seen from outside, or by a visitor/janitor/whatever when you are not home to explain it's a toy... you can get into really hard explanations in a hurry.
Police responding to a call of "This guy has an assault rifle on his wall!" tend to be a bit short tempered.
The costs? They are expensive, even the springers. Not much getting around that. Importing? I think you figured that one out already, it's 'no'.