Side of the magazine says STD Long. Is this correct?

The bracket is on the magazine like in the picture.

Seems to be working fine now. I sighted it in today, before i took it apart. No problem when I only loaded 2 rounds, as long as made sure they were not to far back when putting them in. Seems like it works ok with only 2 rounds in it. 4 rounds seems to be a problem.