difference between br30 and br40

The only difference between these two is the bulb's size. The number shown after the letters gives you the size of the bulbs.

BR30 = 3+3/4 inches bulb size
BR40 = 5 inches

All the other differences are from the manufacturer, and involves wattage, light, light color or other specific characteristics.

Check this website for info about BR20 vs BR30.