So what you do to recrown a barrel is first make the cut as flush as possible, then you chuck up a bolt with the rounded end facing out and dip it in valve grinding compound and apply it to the barrel while trying to keep the drill as well aligned as possible. I've also done this with a ball bearing and just chucked up the bearing with about 40% of it protruding and done the same thing that way. So long as you're using a round piece of steel of the right size it doesn't matter exactly what it is. You could probably get away with using a brass bolt as well. Basically, you're just carefully grinding away any uneven areas of the muzzle, because that last millimeter of the barrel is the most important part when it comes to accuracy. If the tip of the muzzle is uneven, then it will add instability to the pellets when they exit the barrel.
There are a lot people here who know how to do it much better than I do though and if you do a search you can probably find a more in depth tutorial on it.
You can tell how hard it will be to cock the gun with a shorter barrel simply by holding it in that location when you cock it. I really don't find it to be a problem with many rifles. I wouldn't want to chop down a Gamo Magnum though. Those are a bitch to cock even with the long barrel.
Don't get carried away with it though. At some point you'll start losing velocity and the piston will start slamming into the end of the cylinder since it won't have enough air pressure there to hold it back.
