1) Get a bag smaller than you think you need. This will limit how much you put in it. You really don't need much in there at all aside from a few tools, maybe some water, snacks, and a few other odds and ends.
2) I have an Eberlestock X3 and have been thinking about downsizing for a good while. Its too big for what I need. That said, that scabbard is great for not being a scabbard. I use it to store fired brass. Its always open and easy to just toss in. I throw trash in there, water bottles, etc. If you have to throw your rifle in there, it is great for a long haul, but no need for most matches. I just toss my gun on my shoulder and carry it to the next stage. I really only use to the top part of the eberlestock X3. I have a small bag with tools and attachments, a can of dry lube spray, and some propel packets. That is literately it. I have another small molle pouch on the front of the bottom part which I throw all my ammo in.