Pump Action Crossbow

Blaster Design by Ryan McNumbers and KaneTheMediocre.
Writeup / Instructions by Ryan McNumbers

Forum Threads:

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This writeup is out of date and the files are no longer available. There are much better options to building this blaster seen here and here

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Build your own uber leet blaster!

Essestial Tools:
-Scrollsaw
-Drill press and/or power drill
-7/64", 5/32", 3/16" drill bits and 1/2", 9/16", 5/8" spade bits
-Mitre box or mitre saw (need square cuts)
-#6-32 tapping bit (usually includes 7/64" drill bit)
-Scissors, or file.
-Screwdriver
-Vice, wrench, or pliers 
-Computer (obs), printer, and full sheet label paper (templates)
-Duct tape
-Super glue
-Safety glasses

Helpful, but not necessary:
-Drillpress
-Belt sander/disc grinder
-CNC machine
-Leet skillz

Part List:
Download

All items available through McMaster-Carr (www.mcmaster.com)

Fabrication:

After you have proper tooling and materials, it's time to get started. Download your templates and print them out on full sheet label paper.

Cut our your templates w/ scissors and put them on the appropriate materials. Please follow picture.

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*Please note - There are three extra pieces that aren't in the picture above. There are two small circles ( 7/8" OD x 1/2" ID), one needs to be cut from 1/4", the other 1/8" polycarbonate. The third cirlce (larger circle 1 1/4" OD x 1/2" ID) needs to be cut from 1/4". I also used delrin for the 1/8" pieces, obviously this is not necessary.

Now, it's time to drill all those holes.

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After that, cut out the pieces on the scrollsaw.

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Align the top of both the catch frame, and the rear frame piece with the top of the side plates. Mark in the side of the plates, where you need to drill holes. Drill those four holes (7/64") in each piece, and tap them. Also, you'll need to tap the four other 7/64" holes on each plate.

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Take your plunger spacer (flower-shaped thing), and put four 1/2" long machine screws around the holes on the rear frame piece, as shown. Optionally you can also add an extra screw port for the handle to attach to the rear frame piece. Unfortunately you'll have to line up that hole, after the basic frame and handle are assembled.

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Now you can start assembling, You'll still have more components to machine, but they'll be made as you go along. First, start off assembling the handle. This picture should be pretty self-explanatory. 

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Next assemble the catch to the catch frame. You need four 1 1/4" screws, and eight nylon washers. Screw, followed by nylon washer, followed catch, followed by another nylon spacer, all screwing into the catch frame. 

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Attach the rear frame piece, and the catch assembly to the side plates using (8) 1/2" machine screws.

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Attach the handle to the rest of the frame, with four 3/8" screws (in the standoffs), and another two 3/8" screws (if you did the optional extra hole for the handle).

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Next, it's time to make your plunger tube.

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To make your slot, drill two holes (1/2") going through both sides of the PT. Be as straight as possible. The slot is plenty big so you just have to be kinda close. Use the scrollsaw to cut out the slots from hole to hole. In case you are oblivious, the PT is 1 1/4" clear PVC.

Attach your plunger tube to the front of the frame. Put in another one of the plunger tube spacers, in between the screws. Make sure the back of the PT is a square cut so it will be aligned with the rest of the blaster. Mark off the six holes (three on each side) and drill them with a 7/64" drill bit.

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Tap that shit, and assemble to the rest of the blaster with six 1/2" screws.

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Now do the same thing again, but to the stock piece. I used 10" for my stock (you can use whatever length you want).

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Now it's time for the bow arms. First, you'll start with the PVC Tee.

Insert the two 1 1/4" x 1" reducing bushings into the Tee. You'll have to either, cut off the end where it reduces, or dremel out that ridge for 1" PVC to go all the way through. Then put a length of 1" PVC through both bushings, flush at the ends. After all that, you'll have to drill a 5/8" through the top of the Tee, and coming out the bottom of the 1" PVC. Add a small segment of CPVC (or anything 5/8" OD), and fill the open gap in the bottom of the Tee with hot glue. Here is the process fully completed.

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One other extra thing that is not pictured until the last picture, is the PVC stub that your barrel will attach too (or hopper in my case). You'll need to ream out a small chunk of PVC with a 5/8" spade bit to nest over the CPVC.

Now it's time for the actual bow arms. This is all that is powering your blaster. Take two 1.5' segments of 3/4" CPVC and cut V-notches in the end of both pieces.

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Next, cut two lengths of CPVC about 4-5" long. Wrap them in duct tape until they nest inside the 3/4". The smaller CPVC prevents the 3/4" from bending straight at the end of the Tee, and make the bow curve more gradual. 

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Now, wrap the 3/4" CPVC in duct tape until it nests inside the 1" PVC in the Tee.

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Onto the plunger rod. Cut to these specifications. The hole in the rod should be 3/16" diameter to fit the vinyl tubing. I used two catch notches, you can cut however many you want. They are set up for 6" and 4" of draw.

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Plunger head assembly. You'll need a 3/4" screw, followed by the 1.25" x 5/32" circle, the two 7/8" x 1/2" circles (over plunger rod + inside skirt), and the 1.25" x 1/2" circle.

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Add a small strip of duct tape on the end of the plunger rod to keep the polycarbonate circles and the skirt from moving back.

Cut your foregrip out of the 2" OD polyester tubing, 6 inches in length.

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One more piece you need to machine, the catch spring holder. Cut a .5" long section of 1/2" nylon rod. Drill a hole in the middle straight through both sides. Drill another hole, perpendicular, but only half the way through. Tap both holes.

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Assemble with a set screw to hold the spring, and two 3/8" screws on each side.

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Almost done. Cut a 6" section of the (3/16" OD x 3/32" ID) vinyl tubing and put it in the hole through the plunger rod. Feed your string through the tubing.

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Tie a knot in the string, insert in V-notch. and wrap the string around the CPVC a couple times, end it with it coming back through the V-notch. 

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At this point you can tension it however much you want. The more tension, the further it shoots. It's really up to the user to decide what level of power they need. 

And you're done!

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The vinyl tubing prevents the string from damaging itself. The plunger is stopped by the vinyl tubing hitting the end of the slot, which means you can dry-fire them all day, and you're blaster will be fine. The bow arms are also removable, and so is the front and back PVC Tee. 

Ranges will vary depending on the amount of tension but, they can easily shoot ranges of Plusbows.

Add on your barrel of choice, and go have fun. 

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