Yes telescopic.
5.56 telescopic LSAT?
Yes yes it is.
Once you said it was 5.56 and plastic case I cheated and did a google and found the information.
http://modernfirearms.net/en/machineguns/u-s-a-machineguns/lsat-eng/
9mm X 21mm ?
nope, not that 
2.7mm Colbri? Kolibri.
yup, that’s the one mate 
300 blackout rimmed?, lol!
No. 19th century.
Your not playing fair
How about a decent pic?
Preferably of the head stamp. 
is it larger than 30 cal?
Ok then some clues-
Not American
Not Plastic cased
Not Modern
Larger than .300
perspective is deceptive
303 Britt?
Yes, that’s it. Surprised it took an hour.
1 Like
.338-378 Weatherly Magnum.