I made 542 last night. Go for the line drives, not the flies.
Whoa, discovered a new technique. 586.6!
OK, on preview, I’ve been beaten to this by a lot of people, but I already typed it, so you’re getting it:
One of Einstein’s great realizations is that there is NO absolute reference point for motion. One of my dad’s favorite little annoying phrases is “relative to what?”, which he picked up at MIT.
The fact is this: You are forgetting that you are already on a huge ball twirling at a tremendous rate of speed. This speed can be proven, because you/we are rotating around a fixed point. It can’t be argued that we’re still and everything else is moving.
OK, so we’re moving very fast already. BUT, relative to the gun in your hand, your speed is 0mph. This goes to show that no matter what you do, your relative speed to the gun will be 0, as long as you aren’t dropping or throwing it.
The point of having the gun is to eject a bullet at Xft/s (900, ok) relative to the gun. The bullet is not smart, and it doesn’t know that you’re on a whirling ball, or in a plane, or on the dark side of the moon. The pressure of the expanding gases will eject it at 900 ft/s relative to the gun.
Ok, you have the concept that the gun and bullet are at rest relative to each other, but then you start factoring in the speed of the plane again. Forget it. It’s literally irrelevent.
The bullet and gun will part ways at 900 ft/s, period, end of story, forever and ever, amen.
NOW: relative to the ground, the bullet will be traveling not at all. Imagine trying to throw a fastball off the back of a fast truck. Compared to the ground, it might fall directly down, which you could prove by throwing just as you passed your buddy who is standing on the ground. He could tell you that it didn’t travel at all compared to him.
BUT: you did throw really hard, right? You really gunned it, just like normal. It took all that effort to counteract the speed of the truck. It was hard, and the ball left your hand as hard it ever did. It just didn’t travel relative to the ground.
You’ve been mixing up the idea of speeds. There is no “SPEED” written in stone. There’s one compared to you, and one compared to the ground. You’re just used to having be the same thing, because you’re standing on the ground.
Here’s another example: You’ve seen larger birds hover over the ground. How did they do that? It’s because their speed relative to the air was kind of high, but it was the wind doing all the work. It was blowing past their wings, and they get the lift, regardless of the the fact that “they’re not moving”. They’re just not moving compared to the ground.
Yes, I’m a nerd.
Generally, the “on preview” excuse is acceptable. But in your case, you’re asking us to believe that it took you more than 18 hours to compose your post, and only after that time did you become aware of the earlier posts that covered the same material.
Much better to admit that you didn’t read the thread in the first place. 
593.5 Cop that!
You are confusing relative speeds and speed relative to an outside observer. The bullet’s velocity relative to the plane firing it, is 900 ft/s. To an outside observer, the plane is flying at 2700ft/s and the bullet is going at 2700 + 900 = 3600 ft/s. So the firing plane can’t run into its own bullets (if we leave out drag on the bullet due to air resistance for the moment)