Nice sleeves and yeah they do sound like a sling shot but for your knees. Here's hope you'll nail that 730lbs with them!
EDIT: Looks like the Rigor Mortis ones are all sold out...hope there is a restock.
Sorry about that and maybe next season go on a hunt in your friend's dad's memory. He'll certainly be proud when you and your friend bag some good ones!
I bet the plastic will start leaching even the moment you draw into the syringe, so best thing is to load only when actually ready to pin, particularly for those solvents that may be harsh on plastics and rubber. So basically load it and go.
Also it pays to read independent reviews rather than the ones posted on Temu if you are buying on that site. While there may be good stuff there's undoubtedly a lot of junk.