I'm feeling really nice today (must be the weather) so I'm going to address the OPs question directly.
No it's not OK to play on the open field nearby. No you don't need to contact the police since you're NOT going to play on that field. If you're looking to play, go to a local AS/PB field that hosts Airsoft games.
I'm sort of curious, exactly how old are you? Just wondering since I don't know how "playing with realistic bbguns in an open field" in this day and age constitutes as a good idea.
Scarecrow - "old geezers like myself and Starlight"
- "senility prevents me from recognizing anyone except people who owe me money.."
Greylocks - "The heck with 'getting'. I am old."
SF - "Neal! You are the one!"