What is the fine-grained topic of the following text: Not only will PolarStar be offering Airsoft rifles with their HPA Solenoid Engine pre-installed, they will be allowing the engines to be sold separately as drop-in kits for those that already have an AEG body to drop one into. Only return the topic.
PolarStar's offering of HPA Solenoid Engine for Airsoft rifles and drop-in kits.