[Unclenick]

A chronograph's resolution is limited by its clock speed. One cycle of the clock is a larger percentage of the transit time of a fast projectile than it is for a slow one, so, no, the percent resolution is not constant with a chronograph.

The velocity limits are imposed by the speed gas can get up to. The gas behind a bullet only goes as fast as the bullet does until the bullet clears the muzzle. At that point, it is accelerated to escape the barrel, but that short time and limited gas supply means it tends to be some amount over the bullet speed. That's not the same limit it will have in a longer barrel, for example.

IIRC, the military did a bunch of experiments necking down 20 mm or even 37 mm cases to 30 calibers to see how fast they could get bullets to go, and I think that even when they had half a pound of powder behind it, they could only get to about 6,000 fps. So that's probably closer to a real limit.

Rail guns can, in theory, get to 10,000 fps. I can't recall the velocities for the ones I've seen YouTube videos of.

I did once get a close look at a gun in a lab that fired a 1/4" aluminum ball at 17,000 fps. The way it was done was through a 40 ft long barrel that had two parts that came apart in the middle. The first 20 foot section was chambered for a 40 mm casing that held a pound of powder. The projectile it fired was a disposable plastic piston. At the second section, the bore narrowed from 40 mm to 50 caliber. That portion was loaded with a sabot that held the aluminum ball and a 60,000 psi burst disc was placed behind it.

To fire the gun, both sections were loaded, and the bore of the first section was evacuated and then back-filled with helium. Under equal pressure as air, helium can move about three times faster. Firing the pound of powder pushed the plastic piston (use-once, throw-away) down the first section of the barrel, compressing the helium rapidly. The burst disc would let go, and the 60,000 psi helium drove the Nylon sabot down the second 20 feet, and that's how they got the velocity. The plastic piston extruded into the 50 cal portion of the bore about half way before the pressure behind it bled off. The Nylon sabot had two halves that separated at the muzzle and struck some armor plate that had a hole in the middle for the ball to pass through. That plate was heavily dinged by the sabot halves hitting it with that much energy. The ball went through the hole and only traveled a few feet to its target. There was no rifling involved. All smoothbore.
Powder gases would be much too heavy to get to 17,000 fps. The gun was in a lab that tested experimental satellite armor against small meteorite impact, as represented by the little aluminum ball, taking super high speed X-ray images to study its interaction with the armor.