AJ R

Plexiglass aka acrylic is some of the most brittle stuff around. Aj you are prob thinking of polycarbonate.

To tell you the truth, it's completely possible. I really don't know my materials as well as I should.

Impressive, though I question the use of plexiglass for a replacement catch. It is probably weaker than the stock ABS one...

Excuse me? No offense, but no. Absolutely not. Plexiglass is a fantastically strong material. Small pieces won't budge, large pieces will bend, but it's almost impossible to break. Throw a brick at a plexiglass window, and it'll bounce back at you.
ABS, in the grand scheme of things, is fairly weak. for a toy, it's fine. start suping it up, and it won't be fine for long.

Have a sizzlin' day,
AJ

So, where is the breech? In those pictures, I'm not seeing anywhere for the dart to enter the barrel. Maybe I'm blind?

Have a sizzlin' day,
AJ

have you got access to a propane torch? I use torches to help with woodwork. you just roast the wood and scrape off the charcoal. pretty much like burnt toast.

try not to get too carried away with the heat, it causes any cracks in the wood to expand if the heat gets too deep.

it helps a bunch with sanding too, you just burn the surface and hit it with a sanding block or sanding sponge.

you could drill a hole for the thumb and wobble the drill to get it a little bigger, then burn it over and over to get it big enough.

otherwise take an old screwdriver and use it like a chisel. a hammer makes just about anything work like a chisel. torching the wood after will clean it up fine.

early crossbows (real crossbows) used a lever and a dowel pin to trigger the crossbow. the string was in a groove over the hole for the dowelpin (like your hole in the extra block) the handle was tied to the stock and squeezing it up against the stock pushed the dowel up the groove and forced the string up out of the groove.

you could cut a groove in the wood below where the notch on your plunger sits. the wire could be bent into a rectangle and the ends could be bent down like a little handle. the dowel hole would be drilled up through the stock and hit the middle of your groove is sawn. drill a hole in your dowel tip for the tiny handle on the wire.

the dowel would be long enough to stick out the bottom of the stock. you could either push the dowel directly or use a handle like the crossbows used.

as a side note, the notch and dowel setup is pretty darn strong. a friend made a real crossbow completely out of hardwood and the string, no metal at all. a stray shot bounced off a forklift, a steel beam and then went through a 12 pack of soda, lengthwise! it could easily kill someone.

Most of this post made me cringe... I'm a fine woodworker (aspiring luthier, to be specific)
Don't burn the wood, it's a good way to warp the fuck out of it. The only modern reason to expose wood to excessive heat is steam bending. Use a spokeshave. Faster, more effective.

You obviously don't have to patience to use bladed tools and do your sanding by yourself. You don't need a torch to work your wood for you.

Very very good way to fuck over not only your screwdriver, but your project was well. Always best to have a /sharp/ tool, otherwise doing that is dangerous because it forces the person using it to apply more force than is needed. This increases the likelyhood of your hand slipping and you getting hurt.

You could draw your thumbhole, use a .5" drill bit to drill out the corners, and saw between the holes. Afterwards, you use rasps to round things off and then sand it down.
Or, you could make yourself a template and route out the area... with a router, if that isn't clear. I don't know how many people have routers, though... So that may be out of the question.

Anyway, now that I've gotten the safer way to do things across, do whatever you want.

EDIT: and by the way, I just took a moment to process the last part of your post. Nice job having no respect for weapons. Another pet peeve of mine. I'm a hunter's safety kid, and obviously you and maybe your friend need to go through the same course. The reason why I say maybe is because I don't know how he handles his weapon.
Crossbows are not toys, in any way shape or form. You can make a foam tipped arrow, it will still hit like riot shot. The fact that it can kill someone should instill some kind of silent respect for it.
Don't do anything stupid.

Have a sizzlin' day,
AJ

Well, in that case, please share your results. I've been wanting someone to put a rifled barrel on a ridiculous gun. Because I'm sure at the velocity a gun like that could shoot a dart out at, rifling might help. One thing I will say, though, is that it may be a good idea to drill a few air release holes at the end of the barrel. When you're moving 24 cubic inches of air down a barrel, there's always the possibility of the air blowing the dart in to a spin... and I don't mean the good kind.
By the way, I have to ask... is this a spring gun, or is it a pump gun? Because if it's a spring gun, I'm going to have to ask how large the plunger system is.

Anyway, I would rifle a crazy powerful gun myself, if I had one.

Have a sizzlin' day,
AJ