I mean, maybe?
I always thought that the steel doesn't care what direction it pushed the spores it just pushed them away from itself. So while there might be more steel at the front of the gun than the back, the steel at the back is closer, which would force everything towards the center of the barrel, as far away from the circumference as possible. Not to mention that the steel towards the front would be countered by the bullet.
I don't see how that would slow it down, yes the bullet is(probably, do we know if iron is pulled towards spores, I feel like it shouldn't be, otherwise you couldn't sail with it but at the same time it does in Allomancy... I don't know) pulled towards the gas, but that would only come into play after the trigger was pulled, where the strongest pull would be from the gas closest to it.