That clickbait link is pretty useless for determining the most heavily armed states, which ironically is the stated purpose.
"While the ATF's National Firearms Registration and Transfer Record is the only accessible list of its kind, it is not all-inclusive. NFA firearms only include the categories regulated by The National Firearms Act of 1934: machine guns, short-barreled rifles, short-barreled shotguns, suppressors, destructive devices like bombs and grenades, concealable devices with the ability to discharge a shot through the energy of an explosive, and any firearm with a bore over half an inch that has not been determined to have a legitimate sporting use."
As to the other discussion, sanding off serial numbers is so 1970's, today's modern criminal tactic is to stamp over the number using a hammer and metal stamp.