Yes, a round-nose diabolo pellet, like the JSB Exact series, has a similar BC to a roundball of the same caliber and weight.... If the weight is lighter or heavier than a roundball in that same caliber, you can use the ratio of the weights to get a rough BC.... This is because a sphere has a Form Factor of about 1.55, and since BC = SD / FF, you can get a pretty good estimate of the BC of a roundball or pellet by using 2/3rd of the SD....

The SD is the weight in lbs. divided by the caliber squared.... so SD = Wt. (gr.) / 7000 / cal^2.... If you calculate the SD for your pellet, and then take 2/3rds of that, you will have a close estimate of the BC if it's a round-nose design.... Note that a "domed" pellet, like the RWS SuperDome has a lower BC, because the nose is not hemispherical.... and of course pointed, hollowpoint or wadcutters have a lower BC as well, for the same weight....

Bob