Bull-pup crossbow project

Hey hows it going everyone
After finishing my first crossbow i learn quite abit of better trigger design and after having a look around on here i found a bull-pup trigger mech that i think will work quite well. However today i drew out the stock and cut out the trigger mech in cardboard and found that the pivot point between the trigger and latch are not straight, will this effect pull and strain on the latch. Also i was thinking of building the release out of a boat roller since there cheap brand new and are a high compound plastic will this be able to stand up to a 100lb plus leaf spring prod.
But i will post up the progress of my mk2 crossbow since my inbuilt cocking lever may interest some people.


Thanks Ben

you're right about having an interesting idea with the build in cocking lever.
if you don't mind i think i am going to use that in my next crossbow.
I think combining that with a build in mag will result in verry rapid fire.

Well the idea of the cocking lever i got off the Chinese repeating crossbow but the arm will just sit above the string like a extra rail and i cant see why it wont work. But i do know that it will work better with a bull-pup design since theirs more room at the back of the crossbow to work with hence why it didn't work on my first crossbow since it snapped the stock across the grip.

But im mainly trying to get some info on the strength of the trigger mech that im using and if a nylon boat anchor roller is Strong enough to be used as a sear since i do have a drill press but i have to repair it because it can barely drill mild steel. And the prod that i made out of spring steel is prob around 100 pound plus but i have yet found a way to test it.

Like many things we build, the only way to know is to try it yourself. Of course it is wise to ask first, no need to reinvent the wheel or perhaps better said no need to go down a dead end road. For many of my builds I seem to spend more time solving problems than actually building them. Couple with the fact we don't build the same exact thing twice.

That is what's so great about this place, we share ideas and information so that we can acomplish more than on our own ideas alone.



Todd

@Todd the archer Yes what you say is very true hence why im going to post the progress of my Mk2 crossbow, but i have to stop getting ahead of my self and just tackle it as it comes so to speak.

I read you had a problem with your previous stock snapping on you...*brings back memories of my own attempts* ...there are a few rules to choosing the wood for a stock(or any furniture for that matter), one of which is Wood Grain...

A board with grain running "vertically", or in other words "parallel to the *side* of the stock"...is what you want. Did a little explanation here: >>>link

But basically it looks like this...and the straighter the grain is (vs curving like in the pic) the better in the end.

So, when you are shaping it, the raised areas appear as *rings*...



Otherwise, with the grain running horizontally, pieces will start chipping off...as you must be well aware of from your previous experience. A strong stock is the base for everything else in a crossbow.

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Now for the stock design, you want to keep the original details of P90...



...but gotta keep in mind that in a crossbow version you provided less room to rest your cheek and aim...


...and thus will have to adjust the butt angle and shape to be able to shoulder it and not look like Quasimodo while trying to aim...the original may look like a VCR cassette, but you will definitely want to keep it lower and angled a bit when making it a crossbow.

Basically something like this...



Following the stock building guide line that the length of your forearm from elbow to finger tip is proportional to the stock dimensions fit for you....Also, I see you didn't provide any room for a thumb hole, use the same point to point reference (only this time from elbow to the base of the thumb) to get this dimension figured out.

If you haven't checked out the Stock Making topic, do so...some important info there! >>>link

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As for the trigger, I don't really get what you're asking.

But that looks like the trigger I posted in one of my ongoing builds: link





It was an interesting project I was working on with a few other members n the background and we got figured out a number of ways to get it done, so let me know exactly what you are having a problem with and I'll chip in on the build details.



Ivo

PS: Might want to rename this topic as the Main build topic to keep everything in one place instead of scattering questions all over the forum...lets do this thing legit, ok?

I'll move it to the classic crossbow workshop when you're ready. Good luck!

Thank you so much Ivo for your help, and thanks for the helpful observation of my design and you are right that i have not alot of room for a cheek rest and the thumb hole which i corrected today when cutting the stock out. As for the butt of the stock im going to add a wire frame stock that can be slid out to give more room but i am adding and chopping to my design as i go.

Also the wood im using with this stock is treated pine and yeah its prob a bad wood to use but its light and there is a metal tang going into the under half of the crossbow to reinforce the whole grip from front to back.

As for the trigger what was the highest pull weight used on it if any, could it stand up to 100+ pound if the sear was made of a nylon anchor roller.
And yeah that would be a great if you could move it since i have some pics of what ive done today, and sorry for the scattered questions misunderstandings im so worked up at the end of the day lol.

But as for progress here what i got done today

The grip fits my hand like a glove and makes for a great and mobile frame for firing since both hands can firmly grip the stock, the prod in the image is the one i used on my first build and will reuse it for this crossbow.

And as you can see i gave my self i fairly big trigger hole for my long fingers lol but i still have abit of sanding to do tomorrow and if i have the
time i can cut out the slot in the front for where the prod is bolted it.
Thanks Ben

Wow,Nice

Howdy people
Got quite alot done today so much that tomorrow ive got nothing to do intill i get some more steel to start making the trigger mech, side plates and the cocking lever which i have redesigned but heres some photos of what ive done.

Here ive got my cardboard trigger mech so i can see how much room ive got to play with, and i found a piece of box alloy that was perfect for the bolt track and will act as a backbone for the crossbow



So i cut out the arrow grove in the box alloy with a angle grinder then using a 6.5mm steel rod in-graved the grove to seat my bolt nicely, the whole rail is mounted on two L bit of light alloy then i riveted them together. The gap at the front is for the side plate that comes right up for the cocking lever to lock into. Also feel free to laugh at my bowstring it was the product of 2 broken strings yet it did work all i used for the string was blind cord and is quite strong 69kg breaking strength and 5 strands held up to the 28 shots i got in with my half broken crossbow.



And this is the new design for my cocking lever so it will look apart of the crossbow and act as a sight mount for the crossbow fingers crossed it works well.

If there is anything you can see that ive done wrong please let me know since i understand the dangers of building crossbows so far lol only thing im mainly worried about is the extra draw on my new crossbow ive increased the draw by 8cm is this pushing into the danger zone of the prod breaking?.
Thanks Ben

Looks all right but I don't know about the prod angle, it might be a bit to much.
About the draw, I think you just have to try. If it is a steel prod, it will just bend, if it is a fiberglas, it might snap on you.
I have to say the wood is comming out nice.

@Nils i can reduce the angle a tiny bit but ive seen a repeating crossbow which the prod was at the same angle, as for the prod i know that steel has a breaking point mild steel will bend but spring and tool steels will flex to a point then snap rather dramatically, as ive seen a few times with cane knifes but thanks for the impute and observation.
Thanks Ben

Howdy everyone
Been busy working but in my spare time made a new sear and the start of my latch

Heres how i made my sear since my drill press cant drill stainless and i had some scraps laying around, i cut out 9 bits of flat mild steel then drilled my centre hole. The 3 inner bits of flat i cut the nook and trigger latch which go in the middle so i then have 3 bits on each side of those, then with the grinder started to grind away the excess.

And a few hours and beers later i have a strong solid sear and the start of my latch which still needs two parts pinned to the out side so it fits the locking point snug. Now all i need to do is finish pinning the outer plates to the latch, make the trigger then wack together the trigger mechanism box so the whole system can be installed and removed as one part.

Good work,

Howdy everyone how we all going?
Just wanna update what ive done so far with my trigger mech and get the thumbs up from the more pro builders on here.


Its still abit rough around the edge's but is solid all parts have been pinned together and only wielding i will do is 3 plates along the top and sides, ive still got the trigger to make but will be fairly quick and simple and big thanks to ivo for his trigger design.
Now is there any issues that may arise with the trigger mech that ive overlooked?
Thanks Ben

Looks good. Nice design. However, a steel roller nut is HEAVY and absorb energy when you pull the trigger, making the action somewhat less powerful.

You asked above about "nylon" anchor roller as a roller nut, and no one commented. I don't know that "nylon" will stand up to the stress of 100# draw, but Delrin (a specific formula of "nylon") certainly will.

You could certainly build a lock test bed out of a 2x4 to try out the concept.

You can find short lengths of 1-1/2" diameter Delrin online through Ebay.

@Kenh yeah thanks mate i didnt think of that since the whole trigger mech is rather heavy plus the roller nut, as for that delrin i might know a place in town that may have it if not as you said ebay.Also how much of a diffrence would it make keeping the steel roller by the way,
but i am trying to keep this build cheap yet so far it hasn't cost me a cent since im reusing bits and parts from my first crossbow and what ever i find laying around.
Thanks Ben

Hey every one how we all been?
Just wanna show where im up to on my bull-pup crossbow in fact its pretty much done just needs a few touch ups and my new spanning device to be build since the top rail cocking lever looked well crap.
The whole side plates and top arrow grove all come off as one part including the trigger mech that is held in with one pin then turned into a slot in the arrow grove so if i ever need to check a part in the trigger its easy to get to with little hassle.
As for weight it is abit on heavy side but because of the bull-pup design this weight is bearlly felt yet the weight comes mostly from the trigger mech and side plates but giving the whole crossbow a nice solid feel to it.

As you can see it doesn't look to bad for a crossbow ive built in just under 2 weeks i do how ever have to decrease the draw from 27.5cm/10.8 inches to 20cms since im over drawing my prod into the danger zone, which ill do just by increasing the slot at the front.
The bolt at the back is for the windlass system which with out a roller i was able to nearly span the prod and was falling short of 7cm but the prod was nearly a perfect U at this stage and was most likely close to its breaking point.
Thanks Ben

lookin' good

Good woork

Hey hows it going everyone
Well ive finished my crossbow and finally got to test fire it and after reducing the draw down to 18cm i found i was able to hand cock it, the trigger mech works very well with a easy short pull to fire.
I was able to get 15-25cm grouping from 20 meters but thats as close as i can get it with the short bolt im using which is around 10cms long but all in all it was a fun quick project that ive learnt even more from some time down the track im going to rebuild my crossbow but instead make a epoxy molded stock.
Thanks Ben

Howdy every one
Ok so ive got some pics of my bull-pup crossbow which ive named the adstringo which is latin for compact and ive redesigned the stock since when i reduced the draw the stocks fore grips where way infront of the prod and two the old stock was a pain when it came to cocking the crossbow since the handgrip and back would dig into my leg.

This is to give you a fair idea how compact this crossbow is and would make a great survival crossbow.

The front fore grip can be folded forward and back to allow more flexible handling, also the stock is build in two sections allowing the whole crossbow to be dismantled to fit all into a small backpack.

The trigger is a very easy pull so much now that i can fire it with my pinkie lol one thing I haven't put on is the spring tongue because the bolts i made nock into the sear, but in the next few days im going to buy some bolts then test how well it fires also record it and post it up. Since so far effective killing range (if i was to go hunting with it) is 15 meters and from 30 meters i get 1.5 meters drop.
So thank you every one for your help and input i hope my crossbow inspires people to build a bull-pup compact crossbow and in 3-4 days ill post up a video of it action.
Thanks Ben

The front fore grip can be folded forward and back to allow more
flexible handling, also the stock is build in two sections allowing the
whole crossbow to be dismantled to fit all into a small backpack.

Now you're speaking my language.

As for the stock, seems you completely re-did it, corrected and improved quite a few things. I was hoping you could pull off the cocking lever, however, I did also mention earlier that built in spanning devices are a bit in the advanced part of crossbow building and isn't a task to be taken too lightly....but certainly worth the extra mile. Hope you don't stop here, and make this bow truly live up to the name it was given.

Thank you for sharing this build with us, jot me a note over PM sometime if you plan on taking it a step further.

PS: *fires*

http://crossbow.wikia.com/wiki/Basics_of_crossbow_triggers

Good job, Ben. Looking forward to a vid of you shooting this one!

Thanks Ivo and Kenh that means alot, Also Ivo just to cull my own curiosity i put my fitting the crossbow in a small back pack to the test and was quite surprised at how little room it took up i still had enough room to fit more gear in like food etc if i was going hunting or if i had the bag as a bug out pack.


Heres a pic of it dismantled takes about 5-10mins give or take and only needs a flat head screw driver. The prod sits in a slot that is wedged in with 4 parts of thin tin.


And here with the whole crossbow in my skater back pack and with plenty of room for other things if need be.
Yet im happy with how well in packs down but im already drawing up new plans for a more compact crossbow with a plug and go assembly and maybe a few extras.

Ivo ill take you up on that offer but i wont be starting my next one intill next year just wanna get as much design in first and take my time and make it perfect.
Thanks Ben

Howdy
Here the video clip of me firing my crossbow was gonna get a few more shots in but my hand was pretty sore.