What barrel length or is defined by just shorter than the original for quick handling?

It's just a lightweight rifle with a short barrel.

Yeah, I'd like to know this too. I used to know but I forget. I know that the barrell has to be under a certain length but I forget what it is. Something like 23" or less.

20" or less defines carbine to me

See I would say 18", but thats opinion. Like the SOCOMII. I was just wondering what was industry standard if any.

In the dictionary it says lightweight rifle with a short barrel..

question is how light and how short

Maybe I am wrong, but I bet its like the word magnum, You can tack it to any cartridge without specifications.

Would you identify a NULA as a carbine. I wouldn't

No, a NULA would definatly not be in that catagory.

Thats what I am getting at. It appears to be a marketing name.