Mike: The Winchester reloading guide, as fars as 170-gr bullets in .40 S&W using 231, backs me up. You may want to look up other references that include pressure data (Alliant, IIRC). The differences are as low as .1 grain in some instances. From what I've read, lead more often gives higher pressures for equal powder charges.
Big Bunny--although I stated it as being the gas cloud fouling up the skyscreen readings, further thought makes me believe it could 'ave been the shock wave in front of the gas cloud. The air out here is clear enough that you can see mirage waves on the ground, from the noonday sun, and skyscreen pointed up at the deep blue sky may not get enough other wavelengths to register the bullet, without adding a diffuser screen. It's a temperature and atmosphere thing.
No blackpowder needed to mess up the readings.