5.56mm doesn't have enough mass and energy to split a blue helmet. Need a bolt action chambered in a big necked down cartridge like .300 RUM or .338 Lap
Discussion
But it will sure cause the helmet wearer some issues.
And also why my shotgun is my primary for home defense. Slugs DGAF what they hit. You are going to be hurt badly.
I want one of these for home defense
https://www.tacticalimports.ca/nonrestricted-firearms-c-1/hg105-p-541.html
I've never shot a bullpup shotgun. I'm not sold on most box magazine fed shotgun. I've had to fix one by cleaning up the magazine internals, and even then it was really dependent on the type of ammo used.
What are the thoughts on the 6.8 Remington?