They call me Tom

diameter in place of radius, duh
5.5 cubic inches. So, 1.375 cubic inches of barrel volume. Still 7 inches of barrel.
Dart fit is tight, but I don't need to twist them in. I can blow them through rather easily, and I will get orings on the two joints as soon as possible.
I just retested the plunger seal, and a small amount of water leaks past the rubber seal.

Um, I'm about 85% sure it is 1/16.
Cut-down barrel gave a 14" range.

The shell isn't final, it just has that ring to align it in the chamber.
The receiver will be rebuilt with trigger catch in correct location.

I have changed the barrel size, and the ammo type. My barrel is currently cut about 5" too long, but I don't think that would cause enough friction to go from 50-70' to 10-20'.
Please correct me if that's wrong, that just seems like a huge drop off for 5" of friction.

I will cut it down to see if there's any change. I'm currently at around 21.9 cubic inches of cylinder volume, and while I'm running a foot of barrel, it should be just under 7".
As for weight;
I've used a light and a heavy piston, neither made much of a diffence. I don't remember the weight now, but I do know it weighed less then bolt's.
I used o-rings, but then I switched to a rubber cup seal. It does move with a finger push.

Many people have used AR15 springs to great effect, but I do understand a [k26] would be even better. I just cannot believe my spring would be so defective as it is...good thing it didn't end up in a firearm!
I work in the plastic industry, and I use a food-grade oil that is perfect for nerf applications. I don't even know it's name, there's no public product similar that I've seen. We just call it food-safe oil; it's a very very refined vegetable oil.

My rifle is more of a prototype/tester right now, I will need to make a new receiver once I get this problem sorted out, But it does currently fire. I can feel the air it moves, and by my calculations it should be moving the correct volume of air for the barrel size; it's the rate of the air that's the issue. I'll be heading to McMaster soon, I work close and we do will-call orders all the time.