Gym

Make sure the dart is loaded after you cock it, otherwise the BBB has a tendency to suck them in. If this has happened it may still allow shots to fire, but with some limitation depending on how the dart inside is rolling around. Based on your pics. I think this could be part of your problem. Open it back up or shine a flashlight down the barrel to see if one has been sucked in.

I don't know that I entirely understand what you're talking about, but I did a barrel replacement for a maverick that used crayola barrels and it had some difficulty revolving because of the added weight. It also didn't shoot the darts at all because the crayola barrel was significantly tighter.

Without a better, more detailed, description of what you did (including barrel material or a reference to a writeup that you followed) it would be hard to know what's wrong.

I'm surprised that the supposed impracticality of nerf grenades has yet to be fully mentioned in this thread. I think they work for indoor wars, but if you're planning on using this outside you may want to redirect your efforts.

You asked for feedback so for what it's worth, here's what I have to say:

I've worked on three grenades, one of which was a failure and the other two worked out alright.
I notice 4 problems: trigger, safety, directional reliability, and propulsion mechanism.

Trigger:
My first (failed) attempt had a couple similar problems to yours: trigger mechanism (or lack thereof) and potential for damage to person or property. I would suggest following Bob's idea for a timer-based trigger. The grenade will deploy prematurely if you don't have a timer.

Safety:
You will also need to cover the whole thing in a significant amount of foam to keep it reasonably safe (this has nothing to do with the knife). Aside from the potential damage to people which may not be as bad as some suggest, damage to property could also be problematic. Since I'm assuming you're planning on using this inside the last thing you want to do is lose an amazing indoor nerf location because you destroyed a window or something with your grenade.

Directional Reliability:
This grenade will not work as a grenade (rather than a mine) unless it deploys or lands just right. You could solve this (and the safety issue) if you create several grenades and put them in a large foam dodge ball. You would want to roll or lightly bounce it rather than throwing it, so it would be a rolling mine more than a grenade in this case.

Propulsion:
Rubber bands are cheap and great, but if you could use an air tank you may have better luck. Perhaps use the above dodge ball concept with a central air tank or bladder and the problem is solved. I just think some form of air propulsion would help in this case.

If you're willing to put time and research into all these changes then go for it. You might learn something valuable in the process, even if it's the result of a failed concept.

-Gym

Loading a bunch of darts into a barrel isn't an inline clip. That's called "shotgunning." An inline clip is where you have a section of larger tubing that holds the darts in between the air output and the actual barrel, such as 1/2" pvc (the clip) with 1/2" cpvc (the barrel) stuck into the end. It's like an rscb without the 90 degree and T couplers.

The darts go in the bigger tube, the gun is tilted down to load a dart into the barrel, shoot and repeat. Wes was using the Magstike to shoot out one dart at a time from the clip. He could have stopped before all the darts had been fired by letting go of the trigger, i.e a controlled burst.

I understand now, thanks! The video doesn't show a tilting down of the Magstrike though, and although there are only a few views of the actual barrel, it just looks like 1/2" sch40, though I guess it could have something smaller in it. If the Tommy Gun is setup with a real inline clip then I withdraw my criticism.

Just in case the video doesn't work, the inline clip does work, and is fully automatic. It fires about as far as a usual magstrike, and shoots about 3 shots per 5 magstrike air outputs (this video was a good one, and fired 3 for 3, at the highest possible ROF).

That makes sense now. I was under the impression that there was some sort of mechanism to load and fire a dart one at a time, which would function in either semi or full-auto. For clarification, is the "fully automatic" function based on the already existing components of the Magstrike? I ask this because I have used what is evidently known as an inline clip (loading several darts in a barrel), but the result was the expulsion of all darts at once as a shotgun, not in an automated, semi-predictable burst (the type of result one expects with the term "fully automatic").

I would guess that the results shown in the video reflect more of the mechanical function of the Magstrike, than the results of an inline clip in general, especially considering what would happen if there were a constant flow of air from the bladder to the barrel (mentioned above) as the Tommy Gun appears to be built for (unless I'm not seeing the rest of the Magstrike components). In that case the inline clip isn't really a clip since "clip," to me at least, suggests an ability to not only fire multiple darts in succession, but in a controlled manner either in semi or full-auto.

This is a cool homemade/modification, but I don't see how it's fully automatic.