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Another Bullpup (with a post-construction writeup)

homemade writeup spring bullpup pumpaction

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#1 polycarb

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Posted 28 June 2015 - 10:13 AM

What follows is a build process for a blaster I designed the specifics of, with a good deal of inspiration from Xellah's bullpup design (although I've never handled one or seen how the particulars of it actually work). I have CAD files for all the parts and the assembly - if there's interest in those, I'll post them.

They're links to .pdf's:
Templates
NOT ALL parts and holes are needed, sorry for the inconvenience. Rather than struggle though describing which are and aren't needed now, I'll do so when convenient in the rest of the post.

Plans for the non-sheet parts
(the smaller pipe is 1" PVC, the larger is 1.25"):


Mcmaster/parts List:
9691K56 - U-cup seals
91780A735 - 1/2" standoffs (6 needed)
8752K213 - 12x24" sheet of UHMW polyethelene
90480A011 - 10-24 nuts, you ONLY NEED 4
94355A146 - 6-32 set screws
91259A535 - shoulder screws, 3/8" long 1/4" dia. shoulder
91264A240 - same as above but with locking patch - pick one or the other
90402A146 - 3/8” 6-32 screws
4880K314 - reducing bushing
4880K21 - PVC elbow
1/2” nylon rod (only about a foot needed)
a couple inches of 1/2” PVC for connecting fittings
about an inch and a half of 6-32 threaded rod
Enough 1/4” polycarbonate for a 1” PVC rainbow catch
A strong spring with OD <.2" and compressed length ~.25"
2 – 3/4” x 6-32 screws
1 – 1.5” x 6-32 screw
1 – 1.25” x 6-32 screw
4 – size 6 nylon washers (or 2 small pieces of 1/8” material drilled to 9/64”)
a [k25] spring (or a [k26], just be sure this allows it to compress enough to catch)
12ish 1/2” x 6-32 screws
about 2 feet of 1.25" PVC
6" of 1"PVC

Tools:
Scrollsaw
drill press
drill bits
6-32 tap
brain

This is what you're making: (plus or minus a part and two springs)
Nerf1_zpsxfpbvd2x.png

Priming tube/slide
Follow the plans as to where to put the two 1/4" holes. When I made this piece I made it half an inch too long, which made some of the other necessary measurements funky on my blaster.
20150620_134955_zpsdfqseb9o.jpg
On the end away from the holes goes the rainbow(ish) catch. This is a tough thing to make because of how little space there is to make the plunger rod actually fit. I recommend using a drill press as a lathe and making the circles round that way, and then drilling the ½ hole afterward.
If you use screws with heads you'll have to grind one part of the head away. I imagine you could also use 3/4" set screws and not have the head problem and be able to put the holes closer to the sides, allowing for more space for the plunger rod. Only the plate that is on the very end of the 1” tube needs to be screwed in. 3 set screws seemed plenty to me.

To avoid confusion, I used polycarbonate for the front piece and the catch plate, and 1/8” aluminum for the back – yes, it is different.
The slot in the pipe should line up with the space between the two screws such that the catch plate can slide out through it.
20150620_134921_zpskvpog649.jpg

The catch plate needs to have a hole drilled to accept the spring. Make a corresponding hole in the 1” pipe as such (this hole doesn't go all the way through the 1" pipe):
20150620_134951_zpsbuxyv4cd.jpg

Plunger Head
20150620_134614_zpslyi1ciji.jpg

I had to slice some of the square edge of the u-cup seal away to make it fit, but maybe with more lubrication that wouldn't have been a problem. Make a circle for the back, a circle for the front, and a circle to fill the space in the middle of the u-cup. Without this last one, the u-cup will deform while primed, rendering it useless (until un-primed and reset). The front and rear circles should be essentially as big as possible to prevent the front circle from being pushed into the bushing (I had that problem originally) and to prevent the spring from getting to the seal (from the back).

Plunger Rod
Cut your nylon to the lengths described in the plans .pdf. Drill one end of the longer piece and both ends of the smaller piece with a 7/64” drillbit. Join the two pieces of nylon with the length of 6-32 threaded rod.
Screw the plunger head into the other end of the short piece of nylon with the 1.5” screw. Add your main spring and while holding up the catch plate, slide the parts together.

20150620_144131_zpsw5pszgie.jpg

Plunger Tube
Cut your 1.25” pipe to the length specified in the plans .pdf. Pick an end and wrap your bushing in packing tape until it just barely squeezes in. Add goop if desired and shove it in. I found that using some lubricant allowed me to get a tighter fit. My blaster has no goop sealing the bushing and can hold air with my hand over the barrel indefinitely.

20150620_141821_zpsrxkoqh3r.jpg

Using a quartering jig (or piece of paper with marks on it) mark a quarter of the way around the 1.25” pipe. Ideally you could put the four holes for these screws under the support piece that will go over this area – that is, the holes should be made about 5/16” away from the edge of the pipe. Drill 3/32” holes and tap to 6-32 (smaller hole to be more sure about air seal). Install set screws.

20150620_140120_zpsltbwvksv.jpg
Mark slots using a ruler and luck or maybe a rolled piece of paper (use the 2 ends of the edge where the page ends as it rolls over itself to mark 2 points). Or use some sort of right angle tool or angle bar to make lines parallel to the axis of the pipe. Or use a mill if you're fortunate enough to be able to. In any event, place these slots where the plans say. Make every effort to make the two slots as close to 180° from each other. If you don't have a mill you can drill 1/4” holes and connect with a scrollsaw (what I did) or dremel or drill a bunch of 1/4” holes and clean with file or something else... I imagine if you've made a homemade before you can figure it out!

Also, drill the hole for the trigger to poke through (location on the plans .pdf). It should be about .4", or a bit bigger than 3/8".

At this point it's probably a good idea to slide on the rear support piece(s). In the CAD file and on my blaster, I have 2 rear supports and one middle. However, after using the blaster for a while, I'm convinced you don't need both rear ones. If I were to build again, I would just use the tall one that holds the wye and the one closer to the middle of the blaster. If you choose to do that, remember NOT TO CUT the slot in the side plate for that support.

20150620_142217_zpsjkhykeyf.jpg

Bear in mind that when these go on (the elbows and wye too) you should be sure to make the long axis of the supports perpendicular to the plane formed by the slots.

Once those are on, mark, drill, and tap holes to attach this piece:
20150620_142002_zpsllq2gko5.jpg

Trigger
This is where my goof making the 1” PVC too long showed...
20150620_140039_zps7d0xtxsv.jpg

In any event, cut the main trigger piece out of 1/4” material, and make the side pieces from 1/8” material. Sand them all gently and glue together, the put set screws in if you feel so compelled (I did – gotta use up those 25 set screws!). Sanding the edges is a good idea. Drill a hole in the back of the trigger as shown – this will let you add a trigger spring.

Attach the trigger to the two side plates with a 1.25” screw: (assuming you've already cut out the side plates)
20150620_140003_zpszofuxrn5.jpg


Putting it all together
20150620_142345_zpsjrf6zcfd.jpg
Align as shown in the picture, then push the two pieces together. As far as fit, you want the 1.25” pipe to be touching the long edge of the side plates when attached. At this point, mark where the holes need to be drilled into the support pieces. Take the bottom piece off and drill and tap those holes. I recommend having a quarter inch of thread engagement on the UHMW threads – that is, use .5” screws into the supports. Again, I would not recommend bothering with the shorter support in the back (or the slot that it fits into).
Start screwing the side plates together.

20150620_143428_zpsuou8coqk.jpg

The longer bar goes there. The standoffs go on the handle and behind, in the otherwise unused through holes. I RECOMMEND NOT BOTHERING with the final hole that's right next to the backmost support. I originally was going to have a plate there to “close off” the back of the blaster, and then decided not to bother. If you want a covering there, feel free to use that hole.

Your blaster should look like this so far:
20150620_144021_zpsesomwyqb.jpg

Now we just have to put the internals in and attach the foregrip!
20150620_134359_zpsdeb0lhfc.jpg
Thanks to hindsight being 20/20, your priming handle doesn't have to look like it was pieced together!
The templates that are posted will give you a decently sized handle (unlike the original). Also, the shoulder screws I listed have shorter shoulders that the ones that I used, so you won't need the spacers I have either (gray polycarbonate). Assemble as shown, nothing new. I don't think you need more than two screws to attach the bottom plate, but if it makes you feel better, go ahead.

Take your internals (plunger rod, spring, and catch slide thingy) and slide them into the plunger tube from the front of the blaster.
20150620_134429_zpsq9u5zae2.jpg

Bend the foregrip like this to get the shoulder screws into the internal slide. Do this with only the back-most set of shoulder screws installed in the foregrip.
20150620_144237_zpsqj7topql.jpg

Use your finger on the inside to hold the nut still while rotating the shoulder screw from the outside with an appropriately sized Allen key. Once you've done the shoulder screws that are further back, THEN install the ones closer to the front. Unless you have wildly long fingers it'll be really hard to get a grip on the further back nuts with both tiers of screws installed.
20150620_134106_zpsh1ivucbr.jpg

Add a barrel and a clip and enjoy!
20150620_133629_zpseyyuudt3.jpg

For size reference:
20150621_104509_zpswpzz5pkj.jpg

If you have questions, comments or concerns feel free to post!
Also if there are typos or unclear parts I'd love to fix those – yay crowdsourcing error finding!


Edited by Aeromech, 23 November 2015 - 01:07 AM.

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#2 Astech

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Posted 03 July 2015 - 10:09 PM

I'm fluorescent yellow with envy here. You've made a practicval bullpup and clearly put a lot of though into placement of parts etc. Really, this is rather impressive. My main concern is that the square pump grip will chafe after a long skirmish,but that can be easily fixed with epoxy putty or silicone or whatever. I might have missed it, but is this thing slam-fire? Xellah somehow managed to do it, and that alone is a massive step up in homemade design. I would be very interested in a slam version of this particular bullpup, even if it isn't already slam capable. Once again, amazing build - and good writeup.

Edited by Astech, 07 July 2015 - 06:52 PM.

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#3 Xellah

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Posted 04 July 2015 - 01:13 AM

Typo.

Xellah somehow managed to do it, and that alone is a massive step up in homemade design. I would be very interested in a slam version of this particular bullpup, even if it isn't already slam capable.


I've been considering posting in this thread with a multitude of comments, differences in build techniques, features, and tips on building a more produceable version of this type of bullpup, but I would ramble on and on. As the picture thread indicates, I've gone through almost 4 different versions of this design in my quest to fully optimize it.

This is a good design, polycarb, but it is more of a crossover between my first bullpup and Drev's design. My current version has a sliding trigger, not hinging. It also utilizes a 1" thinwall priming insert, a different rainbow setup, and a tapered plunger rod that sits outside of the catch in the unprimed/resting position. The sliding trigger is smoother and allows for very consistent slam-firing.

A big reason why I abandoned your type of design was to optimize part fabrication and cost efficiency. Every plate on my current version can be built out of sheets measuring 12"x12" or less, eliminating the cost of 12"x24" sheets (and shipping costs less too.) Plus, all of my templates require no overlapping when applying them to the materials. If you've had the misfortune of 2 plates not lining up due to maligned adhesive label paper, you'll know that overlapping templates is a pain in the ass.

Edited by Xellah, 04 July 2015 - 01:20 AM.

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#4 polycarb

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Posted 07 July 2015 - 09:27 AM

My current version has a sliding trigger, not hinging. It also utilizes a 1" thinwall priming insert, a different rainbow setup, and a tapered plunger rod that sits outside of the catch in the unprimed/resting position. The sliding trigger is smoother and allows for very consistent slam-firing.

A big reason why I abandoned your type of design was to optimize part fabrication and cost efficiency. Every plate on my current version can be built out of sheets measuring 12"x12" or less, eliminating the cost of 12"x24" sheets (and shipping costs less too.) Plus, all of my templates require no overlapping when applying them to the materials. If you've had the misfortune of 2 plates not lining up due to maligned adhesive label paper, you'll know that overlapping templates is a pain in the ass.


I hadn't seen Drev's design; I guess there are only so many things to do with a backward facing plunger and a foregrip.

Earlier on in planning, it had occurred to me that using a sliding trigger lets the whole thing be shorter, after which I realized I was happy enough with this, even though it isn't ideal. As a side note, this is essentially as short as you can get with a pivoting trigger.

Using the thinwall 1" pipe would have been a great idea, if for no other reason than making the catch easier to make, what with more space for it to fit into.
And it did seem to me at one point or another that using 1212 sheets (and good on you for doing so) goes along much better with a sliding trigger - it keeps functional parts more towards the back and thus needs shorter side plates.

As to slamfire - kinda... When the 1" slide is in the primed position, the trigger pops up into the 1.25" pipe too much. That could've been prevented by extending the 1" pipe towards the front a little more.
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