Slingshots Forum banner
1 - 16 of 16 Posts

·
Super Moderator
Joined
·
1,850 Posts
I played with some goofy ideas over the holiday including building a prototype slingshot rifle to try some wonky ideas out. Not a well finished piece by any means but I had some fun testing a few things with it.

It has been a nice test bed for testing bands and tubes that allows good control over draw length. I also put pulleys on it to see about "compounding" to get a longer draw while keeping the apparatus a bit shorter than the draw.

First up was re-inventing the wheel by testing how much draw length affects power. I set it up with various lengths of 2040 tube from simple shot and drew all to 5x the active band length. And in the spirit of re-discovering the already well known, I found that draw length makes a dramatic difference. Bummer for people like me with short arms who don't butterfly. The table below shows active length vs velocity using 3/8" steel with all of the bands drawn to 5x active length.

I haven't done much with tubes and the 2040 wasn't as powerful as I had hoped- a bit overwhelmed with the 3/8 but the relationship between draw length and velocity is still clear. And in fairness, it did get the 3/8" over 200 fps at longer draw lengths.

Also included are a couple where the tubes went over the pulleys to test if this could be compounded to get a "longer rifle" with a shorter frame. And as expected, there was a cost to turning the wheels. The system worked well but it was pretty much a straight trade between draw length gained vs power lost to turning the wheels for the 2040 tubes. I will note that I built this with wide wheels that were pretty heavy. I did this so I could also test compounding with stronger flat bands. While the 2040 tubes lost a lot going over the wheels stronger tubes may gain some actual advantage- something to test another day when I rig the apparatus up for flat tubes. A 20mm wide band will stay on the wheels so this should be a fun test when I get around to it.

A few thoughts gleaned on the compounding. I put posts on top of the barrel to catch the bands so they didn't fly around and come back to the anchor point under the barrel. I'm not always blessed with great foresight but did manage to ponder the possibility of a return to sending if the ball stayed in the pouch and it wrapped around back toward the shooter. Also, compounding is limited to stretch factor of the band- i.e. a 40" barrel cannot be compounded the full length on the bottom or the stretch factor will be reduced to just 2. So in actuality the length added on the return loop is only 20% of the top length if keeping the stretch at 500% is desired. So a bit limited on the long draw with a short barrel idea.

I can't remember who put it up but there was a video of kids in India (or Pakistan?) shooting really long slingshot rifles. Someday I may test if stronger bands that turn the wheel better can get to long band power with a shorter body.

So here it to those blessed with long arms or the skill to shoot butterfly!
 

Attachments

·
Super Moderator
Joined
·
1,850 Posts
Wow a 30 inch draw is only shooting 150 fps for the 3/8 steel? That seems really slow. What about the videos I've seen of people shooting well over 380 fps?? Must be bb's or something?

Sent from my SM-N960U using Tapatalk
The 3/8" steel was just too much mass for the 2040 tubes. I bet they are better with lighter ammo and would make 1/4" fly pretty well. And BB's would likely come out at or really close to the retraction speed of the tube, especially with a good draw length. Maybe even up around 300 fps. For tube enthusiasts, I get doubling or pseudotapering the 2040 would also get it doing well with 3/8".

This wasn't so much a test of the 2040's as a test of draw length. And on the one hand, maybe I could have done better matching ammo mass to the tubes. On the other hand, this mismatch really helps show the advantage gained from a longer draw.

Maybe the takeaway messages are that 1) length matters, and 2) even an over matched elastic can catch up to the mass of the ammo if you give it more distance to accelerate over.
 

·
Super Moderator
Joined
·
1,850 Posts
Discussion Starter · #9 ·
Lol, I remember the one (and likely only) time I chronied my longbow. I had these visions of 250 fps and was deflated when I barely got 150! I was lofting 750 grain hickory shafted arrows though. At least they looked fast.
The chrony is both sobering and informative all around. I have been trying to make bows and also had visions of lofty speeds. Not sure what poundage your bow is but a 750 grain arrow is really heavy, but will likely penetrate anything it hits.

While being fairly new at making bows I have tried to get estimates for what good self bows should shoot. From what I have gleaned to be consistent the tests should be done with a consistent weight arrow, something like 10 grains per pound of draw weight. And of course anything over that slows the bow down. By that measure, you were dead on if you bow pulls 75 lbs.

After that, all other info I have come across says a good bow with a 10 gpp arrow will shoot ~150 fps. Or, another rule of thumb is that fps should be about equal to draw weight plus 100 when shooting a 10 gpp arrow. So a 50 lb bow with a 500 grain arrow should be around 150 fps. Anything over seems like a bonus.

While watching lots of videos on bow making I saw many examples of people making self bows shooting over 170 or 180, and occasionally over 200 fps. Pretty dejecting when mine were coming in around 150 to 160. But digging a bit deeper, I found some of those were shooting lighter arrows with those breaking 200 fps shooting 7 gpp.

After doing all this background work I feel better about mine. While I am not in the running for any world records mine seem to be a bit above average. My last two were a 42 lb that shoots at 150-155 fps, and a 46 lb one that shoots at 163-165 fps. I drop arrow weight down to ~8 gpp they start hitting around 170 and 180 respectively. A bit dejecting at first but more uplifting after putting the numbers in context.

Then I looked into compounds for a bit and found they test speed with 5 gpp arrows. And while they are still faster than longbows and recurves the numbers are not nearly as astounding when they are pushing 10 gpp arrows.

So maybe think about your bow a bit more. While not getting to 250 fps I am betting it is an outstanding performer if it is getting 750 grain arrows up to 150 fps.

And in the end, I am trying to remember that speed is just a number. One that can be used to make informative comparisons but in the end still only just a number. And for that number, size matters. Especially draw length, projectile weight, and force used to push it.
 

·
Super Moderator
Joined
·
1,850 Posts
Discussion Starter · #11 ·
Kokopelli.. Nice job! All those facts and figures make my head swim. But one fact hits home, longer is better, assuming you can put it where you are aiming. Otherwise it's a moot point. :D
A perfect short summary to my many many paragraphs. And also why I am looking forward to taking up butterfly someday when I have enough time to learn how to do it well and make it hit where I am aiming. Until then, I'll be happy that current elastics are plenty snappy to be useful with shorter draws.
 

·
Registered
Joined
·
18 Posts
I'd say you're doing quite well if you're building bows shooting 10 gpp at those speeds. This was a custom reflex/ deflex longbow with a lot of tweaks. High foc arrows with barrel tapers and special strings for energy transference.

I do belong to the weight over speed camp. Plus those heavy arrows were so quiet with that bow. About as loud as a mouse fart when I'd shoot.

Back to the topic at hand, I've been kind of confused why more sling shooters don't mention how long they draw when talking band set up. Short draw or butterfly can mean a wide range of lengths between shooters.
 

·
Super Moderator
Joined
·
1,850 Posts
Discussion Starter · #13 ·
Sounds like an awesome bow! And I love the "quiet as a mouse fart" comparison! Almost fell out of my chair laughing at that!

I had been away from slings and bows for a couple of decades and am doing a lot of re-discovering and catching up. No regrets as I had a lot of rich experiences coaching sports for my kids and other things during that time. But also having a lot of fun getting back into things I loved as a kid.

Along the way to rediscovering things I have to agree with the weight over speed idea, especially when the weight gets traveling at a pretty good speed.

I have also noticed the weight vs noise trade off- mine are noticeably quieter with heavier arrows. I still have an old 45 lb bear recurve I got as a teenager and tested mine alongside of it. It was one of the basic models, not a super nice grizzly. But I almost jumped out of my skin when I shot it as it was so loud compared to the ones I made. Maybe I need to tune the string to the bow better and add some silencers. Very happy to have mine shooting almost as fast but much much quieter. Never appreciated that until now.

And I am really happy with my current bows getting around 160 fps with 10 gpp arrows. I am looking forward to figuring out how to bend them to add some reflex and / or tip flip, and also trying to back them with bamboo. If I can do those and get a few shooting 10 gpp arrows at 175 or better I'll be ecstatic. And if not, I am really enjoying shooting the ones I have made so far.

Feeling really fortunate to have some enjoyable pass times that enrich my life and very sorry for those who haven't found the same.

Cheers and happy flinging!
 

·
Registered
Joined
·
80 Posts
Kawkan had an old post from years ago where he cronied long bands, then bands exactly half the length. The result if remember was almost exactly half the fps. Your result is a bit different as the 4 inch bands are more then half the fps of the 8 inch bands.not sure what it means
 

·
Super Moderator
Joined
·
1,850 Posts
Discussion Starter · #15 ·
Kawkan had an old post from years ago where he cronied long bands, then bands exactly half the length. The result if remember was almost exactly half the fps. Your result is a bit different as the 4 inch bands are more then half the fps of the 8 inch bands.not sure what it means
I am going to guess the difference is the mass of the ammo tested. I think I would have seen less of a difference testing light ammo like BB's and and even bigger difference if I tested heavier ammo.

I think the length advantage is biggest for heavier ammo since it gives the bands more time to get up toward full retraction speed, and the heavier the ammo the longer this takes. With light ammo the bands probably get to full retraction speed at shorter draw lengths. But just a guess since I haven't tested it.
 
1 - 16 of 16 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top