The little boy happily played with his popgun in the garden, making a popping sound with each shot.

popgun 是由 pop（拟声词，表示"啪"的声音）和 gun（枪）组成的复合词，指一种会发出"啪"声的玩具枪。这种玩具枪通常不会发射子弹，而是通过压缩空气发出声响。