If the ATF can ban ammo because it's a rifle round in a pistol format, then here goes:
http://chuckhawks.com/handgun_rifle_cartridges.htm
If it's a cartridge that is used in a rifle, then it's ability to be used in a handgun is apparently the issue. Too much power. A Ruger rifle that can use .44 means the corresponding pistol is using a 'rifle' round. Same for .22 most of all. It's called .22 LONG RIFLE for a reason, therefore, following the OP's line, it therefore is an illegal use of ammunition because of it.
You .22 LR guys shooting from pistols are the real cause of the problem.
Or, we can accept the ATF is just a tool of the current administration and following their agenda, along with it's own. Their definition of "armor piercing" isn't based on the military construction or use, it's based on bullet resistant vests worn by LEO's.
The White House has ALREADY responded to the cries of gun enthusiasts by issuing a statement:
http://www.guns.com/2015/03/03/white-house-supports-ammo-ban/ Which basically restated is that anybody who supports the use of M855 wants to see dead cops in the streets. The gauntlet has been thrown down.
The fact is even the president of an LEO union doesn't support it - because no cop has ever been killed by it. This is an anti-gun agenda item meant to keep the NRA and us busy fighting another brush fire, with the predictable results that there is now ANOTHER panic buying spree going on.
That seems counter productive, but the result of one guy buying 500-1000 rounds of ammo and hoarding it means five other guys can't buy 100 to 200 rounds themselves. Win for the Administration, we now have another wedge between the traditionalists and the modern sporting arms groups, which only causes more friction. And - "less ammo on the streets."
Those who own M855 in an anti AP state will either have to sell it or shoot it, to eliminate the risk of enforcement for illegal possession. Another win!
The fact is that many traditional firearms are based on military actions and cartridges. Those traditionalists who point a finger DO have the others pointing back.
Do I need to list all the military cartridges, milsurps, and C&R's out there? Or that this has been going on a long long time? Anybody own a .30 carbine revolver? Oops, shoots issue AP ammo, dude. Fail.
And a .22 LR pistol? OMG. A rifle round in a pistol. Not. Let's agree that it's ludicrous to accept the argument that is the cause.