Hey guys: Geezer here with a few observations concerning chinese repeating crossbows. I have made a few of them, so can speak from some experience.
First: string wear is inevitably pretty bad, given the geometry of the machine. Using a really hard string-serving and lots of lubricant will help, as will making the bolt-track really slick, with rounded edges and paying close attention to detailing the lock/notch and the push-pin that releases the whole thing.
Of course, adding things like goose-quills to the string could increase string-life, but will also add to the string-weight and cost some modicum of performance... not that you should expect particularly high performance in the first place. Try to keep any slack in the parts, particularly where the magazine and table slide on the lower stock, down to a minimum. That will really help accuracy. I found I could keep all my bolts near the middle of the target, shooting from the hip at 20 yards, shooting about one shot a second. You can walk them on target as you go. Much more than that may be beyond the machine's ability.
As for fletching bolts, there are two fairly workable solutions. I have made bolts with heavy heads and grooves cut from about mid-way to the tail. 3 or 4 grooves should suffice to give you higher friction at the tail and heavier weight at the head of the bolt. This helps accuracy a LOT, and the grooves won't hang up in the magazine like feathered bolts will do. The original Chinese solution is to fletch the bolts with a very low (1/8 of an inch high?), thin line of feather or even twine, spiralling down the length of the bolt. Start your spiral about midway and go all the way aft. One single line of fletching will do, if it makes a couple of turns around the bolt as it spirals aft. The spiral fletching won't lighten the tail of your bolt, but as long as you use narrow heads, like target points or bodkins, you should have enough fletching to make the bolts spin and keep 'em straight.
There's one more problem with Chinese repeaters that hasn't been mentioned yet. Well okay, there are two problems. Payne-Gallwey's plan shows a handle made of wood. If you shoot at all fast, you'll probably tear your wooden handle to pieces in a day or two. Get some mild-steel strap from the hardware store and make your handle from that. Second, unfletched bolts disappear beneath the grass MUCH easier than fletched bolts do. Be prepared to make a whole bunch of bolts and don't be surprised when they disappear. Hint: If you take off your shoes and walk slowly back and forth between your firing-line and the target-butt, you should be able to feel the bolts buried beneath the grass.
Anyway, have fun with the repeaters. Geezer.