By defend crops do you mean kill things?

Yes. White-tailed deer are invasive, eat crops, and cause many single-car accidents in rural areas where emergency services can take 30-45 minutes to respond, if you have cell service to call them. It's very desirable to hunt them during mating season to control their population. Wild boar are also invasive, eat crops, and leave giant ruts that damage equipment.

There can be exceptions for specific people to own specific types of guns that would make mass shootings impossible.

There is an unfortunately significant overlap between guns ideal for completely legitimate and responsible purposes and guns ideal for committing horrible atrocities.

Eg. If it is a heavy rifle that takes minutes to reload.

Hunting often involves walking long distances into remote areas. For this reason, hunters often desire the lightest rifle they can find that will get the job done. In fact, one of the reasons the AR-15 was so popular when it was introduced to the civilian market (as a hunting rifle with a 5-round magazine, btw) is because it was two pounds lighter (six pounds instead of eight) than the Ruger Mini 14, which was the most popular hunting rifle at the time.

Also, hunting often involves putting yourself in the same areas bears and other dangerous animals call their home. Not being prey is the first rule of hunting. The type of rifle you're suggesting would offer significant challenges to a hunter who needed to defend themselves from a wild animal.