Edd there is no real way to measure the amount of curve accurately unless you cut the barrel in half and measure the opposing wall thicknesses. This would be just a little costly. I guess one could indicate both ends true, than a 30 inch grizzley rod w/bushing, held by the tail stock, rod run 15" into the barrel. Then indicate on the rod, right next to the breach, and take that measurement x2 would equal runout.
The effect? I guess you need to clarify the effect on what?
Based on what I have seen with barrels going thru the shop is that the curve is usually very minimal, if I had to guess I would say in the center of the barrel, with both ends indicated, there may be about .010-.020" of TIR. I have seen some bad enough that it may be .035", If you ever got one with .050" I would say it was a rare and extreme case.