BudsGunShop.com - is this shotgun, considered a handgun?

Hi Steven and welcome. No. The Mossberg 590 Shockwave is classified as a “Pistol Grip Firearm.” That is how it will be entered into your A&D Record and recorded on the 4473. It is neither a long-gun nor a handgun. Only long-guns can be sold to those under 21 years old so this firearm can not be sold to anyone under 21 years old. Cheers and stay safe!

it is just a Firearm and requires purchasers to be 21

Hi Steven, the ATF considers it a "Firearm". It is not a pistol and not a shotgun. It is classified as "AOW", or Any Other Weapon". It is not a pistol grip firearm, as it has no pistol grip. A buyer must be 21 yrs old to purchase.
Stay safe & have a great day!