OK, had to go look it up...

The Ultra II was rated at 320 fps with a 6 1/8" brace height. Like Matt said though, you may be surprised when you measure AMO draw length, ESPECIALLY if you've got a Zebra string on it.

Now, let's look at the Ally.

You shot it backed off 8 lbs.... That's a loss (on average) of 16 fps lost right there (using the standard of every 5 lbs. you back off, you lose 10 fps)

You shot it at 28".. Another 20 fps lost (for every inch of draw length you go down, you lose 10 fps)

You shot a 386-grain arrow... Another 5 fps off (for every 3 grains over IBO, you'll lose an additional 1 fps)

Let's add that up now. 16 + 20 + 5 = 41. That means you can take 41 fps off the IBO specs, and you should be right in line -- IF everything was ok.

Bottom of the "smooth" acceptable speed rating range is 318... so 318 - 41 would give us 277... and you got 273.

I'd say you shot an Ally with smooth mods and your chrono is off by at LEAST 4 fps. [8D]