The third-in-line to the throne was photographed playing with the gun with a friend in the grass.
Another tweet said it wasn't acceptable for Prince George to be playing with toy guns as people had died in incidents where children ostensibly played with toys which turned out to be real guns.
As the royal family (including the new Duchess of Sussex!) gathered to watched the planes fly past the palace, Phillips and George started goofing off a bit.
Speaking of Kate's style, take a look back at her stunning wedding look and see how it compares to Meghan Markle's and Princess Diana's in the video here.
Kate, dressed in a light blue Zara dress and Russell & Bromley wedges, kept a watchful eye on the children, consoling a tearful George at one point.
Little Charlotte adorably practiced her headstands in front of mother Kate, and was even sporting a pair of very cool sunglasses.
Prince George has previously been spotted wearing another style from the Hampton Canvas range at Trotters called the Nantucket.
The family was out after melting hearts at the Queen's Trooping the Colour birthday celebrations.
But his seven-year-old cousin, Peter and Autumn Phillip's daughter, was less than impressed and promptly attempted to shut him up by covering his mouth. I really don't get toy guns - why play with things you should never have to use.