That's a dummy round. We use those for simulating a bad round during training at the range. You'll note the safety orange color so its not mistaken for a live round and the lack of a primer in the shell.
I know it's a Winchester .243 round but what's with the "super short magnum"? It's a very unusual looking round being so short and having such a large diameter. Why would you use this over a normal .243 round?