I have a fixture that is 120v going to it. I have the right tap hooked up. Checked the cap. And is good. The bulb starts to fire and then burns out. The voltage in the screw shell is 296v. What should it be? The bulb is a 250w M58/E

Need to confirm the ballast is correct, should have the same ANSI code of M58.

Don't let the 296 volts scare you, as the ballast creates this voltage, here is a typical ballast with a open circuit voltage of 310 volts, which is close to your reading.

http://www.1000bulbs.com/images/PDF/M0250-81C-213-specsheet.pdf

Here is a troubleshooting guide I have used for years:

http://www.hydroponics.net/productdocs/HID_Pocket_Guide.pdf

It does not address lamp burning out right away.

I suspect that the ballasts, cap and lamp socket is not connected properly, or the ballast is not correct, or the cap is shorted.

My money is on the lamp being 250W and the ballast being 400 or 1000W.

I was repairing some pole lights once and the boss told me the last guy had one he couldn't get working. It had the same symptoms you describe. Turned out he was putting 250W lamps into a fixture with a 1000W ballast.