I wouldn't think you need "perfect" crowning, only near perfect so as to vent the air around the BB as it's leaving the barrel which will stabilize it as it it exiting the barrel. If there are only two burrs, and if they aren't gigantic, it will cause such minuscule turbulence it wouldn't really matter.
Also from what I understand, It has the greatest effect closest to the BB, where the air actually reaches it before the BB leaves entirely. So if the burrs are a bit farther away from the BB, it might just fly past without the turbulence getting a chance to touch the BB. It also depends on the shape of the burrs. The burrs might be letting the air pass next to them without causing any effect. Like when you hold up a deck of cards in the wind, hold them sideways and they are easy to hold on to, but turn them and they will be ripped out of your hand.
Hope I helped.
Also from what I understand, It has the greatest effect closest to the BB, where the air actually reaches it before the BB leaves entirely. So if the burrs are a bit farther away from the BB, it might just fly past without the turbulence getting a chance to touch the BB. It also depends on the shape of the burrs. The burrs might be letting the air pass next to them without causing any effect. Like when you hold up a deck of cards in the wind, hold them sideways and they are easy to hold on to, but turn them and they will be ripped out of your hand.
Hope I helped.
