Cost- free if you have all the tools and the gun
Items needed- Crossman P30 (or gun with similar clip) airsoft gun
-Small Phillips and flat head screwdrivers
-Small abrasive files (an emery board might work too)
-(optional) Dremel Tool with 6mm BB sized tip (this will really speed up the process not to mention make it much better looking)
Step 1: Remove and Disassemble the Magazine
First you want to remove the magazine from your gun by pushing the button located near your thumb on the gun. After you have removed that, get a screwdriver and remove the screw on the bottom of the magazine. This will cause the bottom piece to come off and the spring to pop out, make sure you don't loose any of the parts. Next remove the two screws on the side of the magazine and pry the magazine open gently with a flat head screwdriver. You will see a large heavy lead weight that gives the magazine its heft, if you don't worry about the magazine weighing a couple ounces less then proceed to remove it. If you like your magazine to be very heavy then stop now and put it all back together, this is not for you. This open area is where the extra 65 BB's will be held. You are now done with the first step. Go on to modifying the magazine.
Step 2: Modify the Magazine
So now that its open its time to modify the magazine. Look at the pictures to get a better visual of what to do. First you want to extend the notch on the left of the slot so that the tab and spring can move further down than normal to expose an opening that the BB's will flow through. I used side cutters and a file to make the notch about an 1/8th longer. Now put the halves back together with the spring and tab back in place. Once you have done that pull the tab down as far as it goes now and make with a sharpie where you see the top of the little plastic piece that pushes the BB's up. Open the two halves again and using a file or Dremel (lots easier) make a hole above the marked line, a little bigger than the BB so it can fit through it easily. That's it! Put everything back together and look at the video in the introduction to see just how it works. I'm sure theres more you can do to modify and make better my loading feature such as a tap on the bottom of the magazine, but I'll leave that one for you to figure out. Hope you win all your neighborhood battles now. Be sure to always use eye protection when playing airsoft.