Alot of people seem to want to upgrade the stock bolt in the B2K4. I see no reason for this, unless you are getting some blowback (Bad factory bolt) then you can just mod it to lower your cycling mass and reduce friction!
Before attempting this mod, read all the directions, and don't blame me if you screw up, your modding it at your own "risk".
What do I need?
You will need a dremmel, your bolt, a vice, a sharpie and a rag. Maybe some fine sandpaper if you are no good with a dremmel.
What do I do?
Basically, you just cut it.
Step 1:
Hold your bolt up to the light, you should see where it get's dark from there being solid delron there. This is where you should cut it, but don't go any further because there is a screw and spring holding a ball bearing in place, and this ball bearing is what holds your link pin in the bolt and ram+hammer assy. Mark a line with the sharpie a little bit into the lighter part of the bolt.
Step 2:
Now, just wrap the rag around the front part of the bolt, and put it in the vice. Don't tighten the vice too much, or you could put too much stress on the bolt and crack/split/warp it. Now take your dremmel, and put a 1mm thick cutting disk on it. On mine the speed settings went from 1-10, and all I needed it on was 3 to cut the bolt.
Step 3:
Just cut directly through the bolt on your line, and pull out the little melted bits, then blow in the back of the bolt. There should be about a 5 mm buffer zone before the screw.
Step 4:
Now just slowly remove the delron at the back of the bolt, until you are about 1mm away from the screw.
Step 5:
Clean out the bolt with a Q-Tip, and sand the back if it's too rough. But it probably wont be.
What did I do this for?
Cutting the bolt is the first part of a series of mods/upgrades you can do/buy to squeeze all the performance out of your marker that you can.
It get's rid of the fatter back section of the bolt, which get's rid of a major friction point. The bolt should slide back and forth in the upper tube of the marker more smoothly now.
It reduces the cycling mass. If you get rid of mass on the moving parts, it takes less energy to accelerate them, so less pressure is needed to operate the marker. You won't notice much difference with just a modded bolt, but like I said, there are other mods and upgrades to do.
Reducing the cycling mass, increases acceleration, which increases the attainable cycles per second of the internals. The bolt mod alone allows you to achieve a rate of fire about 1-2 bps faster more consistently.
It also makes the marker look better, would you rather have a nice looking mill and anno job with a big ugly bolt hanging out the back of your marker like this:
Or a smooth look of milled and annodized aluminum with no ugly bolt, like this:
The choice is yours.
The other mods/upgrades are as follows:
Lightened/lighter link pin.
Lighter hammer.
New (High Flow) valve.
Lighter valve spring.
Ported/Polished ram.
Magnetic Trigger.
These are the things you need to do/upgrade to get the ultimate performance out of your B2K4. With everything done, you will have an extremely low operating pressure. Your HPR output will be about 100 psi. And your LPR lowering that even more! (40 psi even!).
Pics of the lightened stock pin:
Stock Both vs. Lightened Both (Profile wise):
How to: Mod your stock bolt. *Pics*
the way to go...i did it and its fine
it shall set you free 
!