It's a good AND bad idea. Ok, I have a blow pipe and I use it regularly. Here is the conclusion I have come to; Anything other than LARGE ammunition is totally useless. Target and/or spear tips are unsless unless you happen to have the recipe for a decent poison or can milk some neuro-toxins or hemo-toxins from the local flora/fauna.
Realistically you want to use something like the cold steel stun dart. If you can guarantee a head shot,,,,do it. otherwise aim for center-mass (the bit with the heart/lungs) this should shatter the ribs and kill the animal in seconds. If you do HAVE to do this then don't bother with anything bigger than a large rat.
You could try a fishing jig and use a line to "teather" what ever you hit but you have to carry a fair old weight in gear so it's not a good trade off.
Also; Also if anyone tells you that they can kill with a target/spear dart you should instantly call bull*hit on them. Doctors use adrenaline needles on dying patients by impaling their hearts. These are 5x-10x bigger and cause no lasting damage.
Just look at it another way, a blowgun dart is about 1mm across,,,,,, what are the chances of hitting something vital?
Do what I did; learn to make an atlatl. They are accurate, powerfull, you can (with practice) craft a high quality one in a day and those things can take out ANY animal you could consume on your own. Hell, in a survival situation, if there are a few of you, make one each and take out a cow.
cold steel stun dart