Well, to be fair, the AKS-74U (The AK-97 Carbine, essentially) is just a large SMG in size, despite the light assault rifle cartridge (5.45mm Soviet.). The H&K HK53 is marketed as an SMG in the company catalog, despite being in 5.56mm NATO as well.
For most people, SMG requires a pistol cartridge, but not everyone agrees on that one. Just like a lot of firearms terminology, unfortunately.
The HK53 Carbine (note the full name) was a 5.56 carbine...specifically an SBR. It was a variant of the HK33, with some influence from the MP5, designed to fill the role of an SMG while having the armor penetration of a rifle. As such, it may well have been marketed along with SMGs, but it was, indeed, a carbine.
Firearms terminology is very much NOT vague and spotty - it is much more specific than most lingo, because people tend to be specific when their lives depend on it. (As a side note, that is probably the reason that the military and law enforcement folks tend to rant about the abuse of these terms. It makes them...itchy...when people around them don't know their gear.)
The problem is that people spend all of their lives being saturated by Hollywood with misleading information about firearms, and they start to believe it. Most people spend more of their lives watching inaccurate stuff about weapons than they do on their entire education. Point out particular bit of bullcrap, and it's like blasphemy.
Blowback is a myth, people. Glocks don't have hammers, no matter how cool it is to cock it back when you enter the room. When you rack the last shell out of your shotgun for dramatic effect, you're left with an empty shotgun. You can't shoot the weapon out of a bad guy's hands, and you never, ever shoot to wound.
-Jn-
City of Brass Expatriate
EDIT:
Clarification - By "blowback" I mean the whole hurl-someone-across-the-room-with-a-.45 effect, al la Last Man Standing, not the mechanism by which self-loading firearms operate.