Big Bad Bow Mod: Part 3--The Final Chapter!

Where we left off...

Sup folks,

Everybody like trilogies, and this write-up serves as the final chapter in the story of my Big Bad Bow. My purple BBB has gotten continually better with each iteration, and it's finally time to wrap things up in a spectacular fashion. This mod is both difficult and time-consuming, and you absolutely need a drill press to complete it. This write-up is mainly based on Zorn's BBB Internals Replacement--I have nothing but respect for my fellow asian -_- This is the ultimate BBB mod that incorporates my own ideas with Zorn's. Without further ado, let's do this!

Required materials:

• Drill press
• Scroll saw or band saw
• 1.5'' PVC sink drain extension pipe 
• 1.5'' tailpiece washer
• 3/4'' to 1/2'' PVC reducer bushing
• 4x 3/8'' length 6-32 set screws
• 2x 1/2'' length 6-32 set screws
• 6-32 tap
• 1/4'' polycarbonate sheet
• 7/16'' diameter Nylon rod
• 1/16'' polycarbonate sheet
• 3'' length 6-32 threaded rod
• Various springs (not a K26!)
• 1/16'' PVC sheet
• Hot glue
• Plunger padding
• 1.5'' hole saw
• Plumber's goop
• PVC cement

Write-up:
Start by measuring out a 6.5'' section of sink drain--use the end with the lip.

Now take apart the BBB and remove all of the internals. Take the back half of the shell and sand down all the ridges. Use the sink drain to check the fit--it should just barely be able to!

Do the same for the front half of the shell. This will allow for a whopping 4'' of draw!

Now take your 1/4'' polycarbonate and clamp it down in the drill press. Use the 1.5'' hole saw to drill out a perfectly sized circle to act as a spring stop. Drill a 1/2'' hole in the middle of the circle.

Now take your sink drain plunger tube and make 4 equally spaced markings at the bottom. The markings designate spots to drill.

Above each mark, make another mark 1/8'' from the bottom. These marks are where the holes will be drilled.

Now it's time to go one by one and drill the holes. Clamp the plunger tube + spring stop in a vise and drill, tap, and insert a 3/8'' set screw into each hole until they are all done.

Drunken drilling

Now let's prepare the front of the plunger tube. Grab the 3/4'' to 1/2'' reducer bushing and the tailpiece washers.

Sand down the inside of a washer and slip it over the coupler like so:

Now bust out the plumber's goop and goop the shit out of the coupler and shove it into the end of the plunger tube. Allow it to dry, then make a mark 9/16'' down from the tailpiece washer--this is where you will be drilling through the plunger tube. Drill through the plunger tube with a 7/64'' drill bit and tap it with a 6-32 tap. Insert the threaded rod through the plunger tube by using two nuts twisted together or a cap nut.

After drilling through the plunger tube, Dremel the spring stop so that the hole becomes square. MAKE SURE THAT THE SIDES OF THE SQUARE ARE PERPENDICULAR TO THE HOLES YOU DRILLED!

Now make a mark on the spring stop and the plunger tube to remember how things line up.

Take out all the set screws and remove the spring stop. Prepare yourself some plunger padding and insert it down the plunger tube. I actually used a foam spacer from a pack of blank CDs!

The plunger tube is now done! Now it's time to tackle the plunger rod. Drill and tap the end of you Nylon rod, making sure to go deep enough for your 6-32 bolt that will be used to secure it.

Now to mark the plunger rod--the left-most dot is 5 3/8'' from the left side and the line is 2'' long. 3'' to the right of the second dot is the marking for the catch notch. 1/2'' to the right of that is where you should cut the plunger rod.

I used a Dremel to cut out the slot. It should be 1/8'' wide, not 1/16'' wide, like in Zorn's write-up. It took forever, and now I know how shitty it is to machine Nylon bars. A cutting wheel gave its life for the greater good.

Now let's do the plunger head. From left to right: 1'' fender washer, 1 1/2'' neoprene washer, 3/8'' fender washer, #6 washer, 6-32 bolt. For reference, the diameter of the plunger tube is ~1 1/4''.

This plunger head design is excellent for many reasons:

1. The large bottom fender washer flares out the neoprene washer to get a perfect seal on the forward stroke (firing). The plunger stops cold when you plug the front coupler and fire. It also prevents cut-to-length springs from hooking over the plunger head.
2. The smaller fender washer on top allows the neoprene washer to fold upwards when priming the gun, allowing air from behind the plunger head to enter the plunger tube. This prevents vacuum loading, which can cause misfires and hopper misfeeds. I have confirmed that there is zero velocity drop when I load my slide breech with a dart before priming.
3. The overall length is very short, allowing for more plunger travel/spring room.
4. It is dirt cheap. The entire plunger head cost me less than $2!

I ended up replacing the little neoprene washer with a regular one.

I also Dremeled the end of the plunger rod to catch smoothly.

Now it's time to prepare some springs for duty. I started with a K26, but I failed to consult the Python program I wrote to get the K26's maximum draw! It ended up being too small, so priming the blaster essentially was the same thing as ripping the trigger catch out of its well. I had to go with four random springs that I have accumulated over my career.

It's a little ridiculous, but they nest beautifully!

Slap on the polycarbonate spring rest and secure it to the plunger tube with the 3/8'' set screws. In case you didn't know, the BBB has two catches. One of them is deactivated by a couple of "fins" that connect to the plunger rod. I made mine originally out of 1/16'' polycarbonate sheet, but soon found out that it was a bad idea. I then opted for 1/8''. Here are the schematics:

Here are my first four attempts--little did I know how precise the fin really had to be! 1/16'' turned out to be a total shitstorm (blaster locked up after first shot), so I had to go up to 1/8'' thickness.

Fifth time is the charm, I guess.

I scraped the polycarb through the holes with a paper clip to mark the holes for drilling. Drill, tap, and insert your 1/2'' set screws.

I snipped the threaded rod down to size with diagonal cutters, then used J.B. Weld around the threaded rod holes to prevent air from leaking out. In Zorn's original mod, he made the coupler detachable and solvent welded the spring rest (opposite of mine). He ended up leaking air from around the threaded rod on both sides, so this is one major improvement in my mod. The shell had to be Dremeled to accommodate the epoxy humps.

Drill holes at the front of the shell for the threaded rod to stick out of.

Solvent weld a piece of 1/16'' PVC sheet to the trigger catch for extra support.

I also found that I needed to solvent weld another piece of 1/16'' PVC sheet to the shell since my plunger rod was having trouble catching.

The orange "shell catches" (side nipples) that hold back the rear portion of the shell mush be shaved down in order to prevent the blaster from locking up. Here are the before and after pics:

Finish him! Lube up all moving parts, especially the trigger and trigger catch and put everything back into place. Put and #6 nut and washer on either side of the threaded rod and you're ready to rock! Notice how compact the BBB is now with its draw increased.

Primed and ready with 4'' of draw!

With a 12'' brass slide breech:

Yes, a BBB can finally take a hopper!

Final thoughts:
Wow, that was a ton of work for a single blaster, but I'm extremely satisfied with the results. No more catch problems, no more plunger tube rotation problems, and no more seal problems. There really isn't much more that can be done to improve this blaster without extensive shell modifications.

The prime is very similar to a K26, but slightly smoother. It feels rock-solid, and I'm confident that things won't be breaking for a very long time. The accuracy with my 12'' brass slide breech is spot on, and I've been having fun sniping cans with it. As far as ranges go, I don't care. There are too many variables (dart type, atmospheric conditions, angle, etc.) at play, so it's not very meaningful to say "I get 100-foot ranges." Instead I post my muzzle velocity, which is less dependent on external factors.

This mod was my first project involving heavy use of a scroll saw and drill press, and I made many mistakes along the way. I'm fortunate to have a father that has always been a tinkerer, and he helped through some of the trickier parts of the mod. It was a great learning experience, paving the way for future homemades. Now that I have the tools to tackle almost any problem, I can't wait to see what the future produces! I hope you all enjoyed this very long write-up and now see the true potential that lies within the Big Bad Bow.

Cheers,
~T da B