The two tenants of Special Relativity are:
The laws of physics are the same in all inertial frames of reference.
The speed of light in free space has the same value $c$ in all inertial frames of reference.
So according to 1 above: if I am in a large closed box traveling a constant velocity ie an Inertial Frame of Reference and I set up an experiment to measure the speed of light then my result would be 300,000 kps. If I then set up a a gun that can fire a bullet a $1/2 c$ I would measure its velocity at 150000 kps.
But in this "closed" frame of reference where I am the only observer would the bullet gain mass,and Time dilate? In other words at a certain percentage of the speed of light start would the bullet start to behave Relativistically.
And if so doesn't that violate the first Proposition of SR?