Doesn't necessarily mean it was shortened at the barrel end. It might have the stock removed and replaced with a pistol grip.
There's also stuff like the Sig Sauer 556 Pistol .
I've always wondered who buys those (and the other "pistol" versions of battle rifles). A friend had one at the range once...it was a fun but useless ammo muncher. Adding a foregrip made it a bit more controllable, but it turns out that it is illegal as well.