Which CO2 or multi pump?

Preset your shooting spots around the barn and get cover in place way before you start shooting. Let them get use to it.
I shot way more starlings here than I ever got on the farm, simply because on the farm I just started shooting in plan sight. They picked up pretty quick what a gun was. Here it's taken them a while mostly because I've always shot from the bathroom.(I have both windows covered off and a drop sheet down one side of window because they set sentries on a tree that could look in from above on that side which I had to stick me head out the window to see. When nesting season comes if you lay off them let them nest and lay, you'll get better results as they have to come back to chick's.
On farm I started in nesting season by fluke, as soon as nesting season was over they split to other trees on the farm. Set up a couple of hides or cover on the opposite side of where your shooting for later on when they get onto you. They will get smart and start coming in from behind cover thats blocking your view. They can get into and out of thier crevice nest at a surprising rate. Way faster than you could aim and shoot.

Good advice Kruz, I've been ready your posts about shooting the starlings and mynas and how challenging it could be.  I used to have similar challenges shooting jackdaws back home. They nested down the chimney and took nestling birds etc. I could walk around the garden all day and they'd sit on top of the chimneys, no sign of them flying off but the first sign of me holding an air rifle and they were off. It got to the point that as soon as they saw me going into the house they'd fly off. In the end I had best results shooting them from a bedroom window, curtains drawn as they fed on the bird feeders.

Green shade cloth if you have any. It has to be dark inside so you don't silhouette. I had a lot success with that with just propped up against the fence like a pile. I'd get into it with a single layer where I could see through it, double everywhere else and done in such a way I could push the barrel through where the single and double layers met.one place in it I had a hole in the mesh and put a flap over it.
It they are close you can leave the scope behind the mesh where barrel is poking out. Further away in the branches gets bit to difficult to distinguish them. Scope will be your biggest give away in the end. So less they see it the better. Its why I have camo cloth over mine. But at the end of the day that big lens looking strait at them is pretty noticeable.
However for some reason they have hard time seeing through the shade cloth. Mind you I'd usually have a camo shirt, hat, and fingerless gloves on. But they never picked up on the scope when it was behind the mesh. They occasionally did when I was pushing the scope past the netting.

I'd skip the 24inch barrel, just turns it into a rifle. Also more weight out past balance point. Can't help you with which one to choose, I haven't used a CR600. If you want to mod your gun at any stage 22xx will have a heap more mods available. Billbob has owned both, he can prob give pros and cons if he reads the post any time soon.

I'm just measured mine and it looks like it's about 14inch. I thought I'd gone for 16 or 18. Anyway at 14inch (it's a 22xx barrel) I finding it way accurate to the distance in shooting.
Give me 5 and ill line it and my two rifles against the wall take a pic so you can compare sizes.