have firsthand experience with M4 and AR15. The trigger assy. conversion is cheap and easy. The ArmaLite Remington 15 is a civilian weapon in branding only. Definitely not a hunting or a defense gun. I own one myself. I would never use it inside my house against an intruder. The rounds would end up inside the neighbors' house. Only people with proper training and vetting should own one.
From a paramilitary's perspective, its a very convenient solution. Great for tactical assault needs.