The following factors are not considered in BlasterSim at the moment:
The projectile outer diameter is assumed to be the same as the barrel inner diameter. Loose projectiles with less cross-sectional area and also leaks are not handled.
Projectile length is not a factor in BlasterSim’s model. Projectile length would impact projectile friction, however, projectile friction is modeled purely empirically. Presumably the friction pressure would increase in roughly linear proportion to the projectile length due to the contact area staying the same if the projectile outer diameter is equal to the barrel inner diameter.