Yup ranger nailed it on the scrapping. If you are missing that little ring, try and find one of those old plastic film canisters. You can cut the bottom off, and cut down the side. With a little fitting you can get the canister to fit and you have a nice little sleeve 
For the spring, yes go very slow as one wrong cut and you are out a spring. And yes you should file and sand the end down. This will make sure that it doesn't grab on to anything on the inside of the cylinder, and none of the scrapings get into your bucking or barrel.
As for the rattling it could just be the tension of the spring releasing. Springs of that high of an FPS rating are already under tension when you close the cylinder up. And when the sear lets go of the piston all that energy in the spring is released and snapped forward very quick. When the spring hits the end of the space, it will still have momentum, and will thrash around inside of the cylinder. And that is probably what you are hearing after you fire the shot.
It is really nothing to worry about, but just watch the stress on the cylinder head. You may end up finding that the cylinder head will start to pull out of the cylinder itself. So I would look at the cylinder after every game just to make sure that there are no signs of wear or anything like that.
For the spring, yes go very slow as one wrong cut and you are out a spring. And yes you should file and sand the end down. This will make sure that it doesn't grab on to anything on the inside of the cylinder, and none of the scrapings get into your bucking or barrel.
As for the rattling it could just be the tension of the spring releasing. Springs of that high of an FPS rating are already under tension when you close the cylinder up. And when the sear lets go of the piston all that energy in the spring is released and snapped forward very quick. When the spring hits the end of the space, it will still have momentum, and will thrash around inside of the cylinder. And that is probably what you are hearing after you fire the shot.
It is really nothing to worry about, but just watch the stress on the cylinder head. You may end up finding that the cylinder head will start to pull out of the cylinder itself. So I would look at the cylinder after every game just to make sure that there are no signs of wear or anything like that.
