A while ago I bought a red dot sight online.
They send me a few free tins of pellets as well. (Gamo Pro Magnum, Gamo Pro Match, Apolo)
Whatever pellets you buy, don't bother with APOLO.
They are actually more expensive than the Gamo's, but is without a doubt the worst pellets you can buy and may very well be the world's worst pellets ever.
On the box it says the calibre is approximately 4.5mm
It says "approximately" because the manufacturer wasn't sure of the size either.
Some pellets don't fit into the barrel at all and some fall right through. And those were the good ones... some look like they have been used aready!!!
I took about 30 pellets out of the box and found these among them... (There were more, but I threw a few away before I decided to take a picture)