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Posted: 3/10/2026 1:30:52 AM EDT
[Last Edit: Beast-of-Burden][Edited]
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Been very happy with my DIY (not a kit) Bullpup I use specifically for when I come across hogs while in the ATV. Compact size to keep handy (not in the rack) & suppressed for ear protection. Details on an old post https://www.ar15.com/forums/topic.html?b=3&f=4&t=788042&tl=-ARCHIVED-THREAD-Revising-my-old-guy-s-AR-15-Bullpup-project The mechanical trigger extension worked but defiantly wasn't a hair trigger for sure... I figured I could come up with a better system so I fabricated a hydraulic setup. I just finished it & the difference is night & day. Most of the time spent was finding small enough 'push-pull' cylinders, hose/tubes & connectors. A whole different area of parts I knew nothing about but the parts became available once I knew where to look.   
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never underestimate the stupidity of other people
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Originally Posted By steviesterno16: It’s cool, but I’m curious the OAL? Seems like you could get into existing pattern rifles or an SBR with what you’re spending fabricating. Yeah, but the adventure and learning along the way are worth way more. Super cool build OP, love the out of the box thinking. |
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That is fascinating, OP, you may have cracked the bullpup trigger code. You're pushing up the rear of the trigger through the pistol grip screw hole? ETA: Do the safety next. 
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I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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As far as it being easier to just buy a SBR or something else available off the shelf your probably right except there were certain features I was set on having on this sole purpose gun. Yes a compact size was first in order to be handy in the ATV but also wanted a 16" suppressed barrel for round velocity on hogs & ease on my ears. I added the digital day/night scope so I could use the same gun from the deer stand at night when I felt like reducing the numbers. I'm not saying this project is good for other uses but it works great for my particular hog shooting needs. The hydraulic trigger modification was just an improvement over the long bulky mechanical trigger linkage I originally made it with. |
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Originally Posted By backbencher: That is fascinating, OP, you may have cracked the bullpup trigger code. You're pushing up the rear of the trigger through the pistol grip screw hole? ETA: Do the safety next. Yes, I'm pushing up on the very back edge of the rear of the trigger but drilled a different hole to match the angle the trigger part as it rotates on it's pin. The farther back on the trigger part the less pressure is needed to activate it. Safety still works as normal. |
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Originally Posted By joeIII223: @Beast-of-Burden what kind of machine tools do you have? Being an 'old fart' I only have the basics. Drill press, Angle grinder, Dremel tool etc. The forward trigger started as a poly 80% lower piece (easy to work with). This latest stage I used real 2.5mm thick Carbon Fiber flats for the sides (strong & easy to work with). This DIY project would be real easy if I had a CNC machine & the smarts to operate it...but. |
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My friend came over to see it & he brought his trigger gauge... 5lb. pull . So much better than the original mechanical linkage !! Sure an AR trigger with a 3 or 4 lb. pull is better but that's not an extended bullpup trigger pull... I'm very happy with the results so far. 
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Originally Posted By LeadBreakfast: That is extremely cool. It is a shame that the main benefit of the bullpup is negated by the buffer tube. There are short pistol buffer tubes that you may look at I have seen a bolt carrier cut off some to make a very short throw on the amount of buffer tube needed |
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Originally Posted By LeadBreakfast: That is extremely cool. It is a shame that the main benefit of the bullpup is negated by the buffer tube. Normally yes but it does have an advantage when I'm sitting in the deer stand... As it turns out when seated with the forward hand grip resting on the open window the buffer tube rests on my shoulder. My hands are free & it's easy to scan with the day/night scope without supporting the weight of the gun with my hands. Not a planned advantage of course but it is nice over extended stand time. |
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Originally Posted By kychas: I would be interested in seeing if a temperature change would change the length of the trigger pull? If you put it into a freezer overnight then test it and then put it into a hot box so if you leave it in a car in the summer would it create a problem? Down the line if it remains a working option that would be an option but seeing I just built it it's too soon. I don't know yet if this works well over time or if I have to fabricate a mechanical cable setup (or go back to my original setup). It's in the test stage now. The one thing I'd keep if I had to go back to a mechanical setup would be the pressing up on the back edge of the trigger part . |
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OK, Update... there is a problem. Probably an issue with the cylinders I chose for this project (learning curve for this hydraulic parts world). I had the trigger set up & working great but haven't messed with it for a few days. I picked it up to do some dry fires today & it felt like there was an air bubble in the system. Pulled the trigger & it traveled to the stop & didn't move the rear cylinder enough to fire ! I don't see any signs of a fluid leak but some air must have gotten in. I'll bleed the system & see if I can discover the problem. If it turns out I bought the 'wrong' hydraulic parts to work reliably then I'll convert to a mechanical setup. Oh well, work in progress. |
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OK, update, While undoing the connectors I discovered one of the elbow fittings had a really bad crack on the threaded section that connects to the cylinder. I obviously over tightened the little 5mm section. I managed to remove it (in pieces) and will be more careful reinstalling the new part. While I've got it apart I'll still replace the Dot 3 fluid with a thicker velocity oil. Seeing the oil only needs to travel a max of 10mm a thicker fluid should work well. We'll see ... 
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Been busy all day trying different fluids & oils in the hydraulic setup. The original issue was caused by the cracked fitting I over tightened with that fixed & testing different viscosities of fluids . The motor oils were too thick to work well with the little 2mm hole in the connector fittings. The thicker oil severely increased the trigger pull so I tried manual clutch fluid, auto trans fluid & power steering fluid... the dot 3 brake fluid worked the best. All that testing & ending up with what I started with. Dry firing so far shows no issues. I'll take a break for a few days & see if anything changes . 🤞 |
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OK, Big Update !! Not all 'good ideas' work out. The idea of this hydraulic extended trigger setup was good in theory but not dependable in practice. When the cylinder shaft moves back & forth enough times or sits over time enough air get in the 'closed' hydraulic system to cushion the pressure to where shaft travel isn't enough to trip the trigger dependably. I'm sure there are ways with different parts to resolve this problem but I'm not the one to take it on any further. I quickly looked into small DC electric linear solenoids as another option. Small enough yet strong enough and low DC power driven is looking like another field I don't currently know enough to find the right working parts. So I'm now thinking 'more old school' about a sectioned 'push-pull control cable' mechanical setup. With using only small sections that enclose the open cable I should be able to minimize friction/drag and keep the trigger pull weight down. Oh well... on to the next chapter of this quest. (good thing I'm retired) 
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This is some seriously kinky s#!t.... I like it. I like the idea of hydraulic line trigger idea. I know nothing about it, but was wondering if cannibalizing hydraulic brake components and lines off of high end mountain bikes been considered? Long live the kinky stuff!!!! |
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Originally Posted By turbolarry: This is some seriously kinky s#!t.... I like it. I like the idea of hydraulic line trigger idea. I know nothing about it, but was wondering if cannibalizing hydraulic brake components and lines off of high end mountain bikes been considered? Long live the kinky stuff!!!! I know nothing of high end mountain bikes but there is concern about the weight & size of the components used in a gun application. As for me I've moved on to an 'old school' cable project. I'm going to use a couple of brass 6mm pulley wheels as low friction options over the basic enclosed cables (like bicycle derailer/brake cables) and build from there. waiting for the parts now. |
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There is a route to cheat, OP: https://digitrigger.com/ |
I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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Never seen that, looks cool as a digital binary trigger to pop off a bunch of ammo but not my thing. Looks well engineered & well made (also expensive) but I don't see it as a possible remote trigger setup. As soon as the parts I ordered arrive I'll start on my current plan. Thanks for the input. |
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I was also wondering if going electric was an option. I played paintball around '99-2000 and folks were fiddling with solenoid activated MAC valves. I imagine it would be possible to operate a fire control group via solenoid. Electro -mechanical fire control is a gray area, however in this application it's not being used as a giggle switch. |
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What kind of tactile feedback do you get with that? |
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| The quick (& not in depth) search I did on electric 'push-pull' solenoids looked like a normal 'on-off' one wouldn't be the best option for a momentary 'push & release' action on a trigger setup. I looked at a pulse solenoid but only found fluid control ones. Most were not low voltage DC power. I'm sure something is out there that would work but I'm going to go with the cable route this time. The parts are scheduled to be delivered next week, eager to start !! |
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Originally Posted By Beast-of-Burden: The quick (& not in depth) search I did on electric 'push-pull' solenoids looked like a normal 'on-off' one wouldn't be the best option for a momentary 'push & release' action on a trigger setup. I looked at a pulse solenoid but only found fluid control ones. Most were not low voltage DC power. I'm sure something is out there that would work but I'm going to go with the cable route this time. The parts are scheduled to be delivered next week, eager to start !! We are all pulling for you, man. A decent bullpup trigger would be a godsend. |
I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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Well it took a while for the parts to come in & I ordered a couple parts that wouldn't work with my plans (cost of not having them in my hands) but I fabricated ones that worked better. The part that presses the trigger piece I threaded a brass tube so it's length can be fine tuned. I decided to run the bare cable over brass pully wheels instead of inside the cable's outer sleeve. Not as easy but I believe it will reduce the trigger pull drag. One advantage cable has over the hydraulic setup is weight. The 1.2mm cable & fittings are super light. The cable was for the derailer on a 10 speed bicycle , thinner than brake cable (still way stronger than needed here). Anyway it seems to work well but the true test will be after I assemble the side panel to further stabilize it. Without it there's some flex that when eliminated might cause an issue in tension adjustment. Anway, here's the progress so far.  |
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A trigger linkage in the opposite direction: https://www.ar15.com/forums/t_1_5/2839222_Unbullpups-your-AUG.html |
I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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Originally Posted By backbencher: A trigger linkage in the opposite direction: https://www.ar15.com/forums/t_1_5/2839222_Unbullpups-your-AUG.html The creative mind at work !! Most are happy with things straight out of the box... then there's us.
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Now I've found this 'low tech' dependable extended trigger setup I'm modifing it yet again. Now that it is together it's obvious to me a simpler route for the cable would no longer require the thick panel spacing that was required by the hydraulic hoses & the cable pullies. Because it's such a straight line from the front trigger along the flaired edge of the magazine opening to the rear trigger linkage I'm going to use the normal 4mm outer cable housing (like normal). Without the extra spacing a new carbon fiber side piece will lay flush & blend better along the cable cover & with the rear shoulder stock. Now just waiting for the carbon fiber piece to be delivered. Never leave 'well enough' alone. |
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So the safety and mag release is still at the original receiver position? Can't imagine the mag release would be a problem in a hunting scenario, but safety could be. Interested to learn how the trigger feels compared to a normal setup. Overtravel, reset, pull weight and such... |
Finishing up... Glad I changed from bare cable & pully wheels to a normal shielded cable. Much cleaner & easier setup. Still not an 'off the shelf bolt on' modification but a smooth fuctioning setup. The trigger feels light & reset works great, repeatable with no issues. When I see my friend next time I'll use his trigger guage for an true pound pull but it feel light to me. It has been a long project with the hydraulic path a failure but I'm really happy with the current setup. (wished I started off with this route in mind ) Yes the mag release & safety are untouched. 
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Originally Posted By Beast-of-Burden: Easter Greetings Y'all... Met with my friend today & we used his digital trigger guage about 10 times on the bullpup trigger. All results were under 5 pound pull. The range was between 4.6 & 4.8 I'm a happy camper.Mumtaz! |
I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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