The burden of proving a claim that a person committed a crime lies with the government's lawyer. Having the gun as evidence in a felon-in-possession prosecution would seem to be quite important. Apparently they are going ahead without it, presumably based upon witness testimony. Since the witnesses may be codefendants, it seems possible that they are being rewarded and threatened for their testimony, which gives rise to an inference that they are false witnesses. Each person accused of a crime has the right to a speedy trial, especially people being held in custody. It is not always to the advantage of the defnese to have a speedy trial. However, if the defense wants a speedy trial, it must assert that right at every opportunity, verbally and in writing - including motions for a speedy trial which can be combined with a motion for immediate release from custody based on a denial of speedy trial.
There are other ways of proving he had a gun other than the police actually finding it. Obviously, its a much stronger case for the prosecution if the police find a gun (presumably in his possession), but the other passengers saying he discharged the gun is also considered evidence.
As for his release, this is not automatic but can be requested by his attorney. If he formally requested a speedy trial and it was then reset, he can move the court to release him. In addition, he can always bring a motion to reduce bond in the alternative.
I hope you find this information helpful and feel free to contact me if you have any other questions about this case. I wish you all the best of luck.