Airframe separation deployment

[dnl2]

I'm not sure what you would call this type of parachute deployment and airframe separation. I want to use the coupler as a payload bay to start becoming familiar with altimeters, but still use the motor for ejection. I have a phenolic Public Missile airframe and coupler. The problem I have is that the coupler is extremely tight in the airframe. Is this common for couplers? In order to separate the rocket in the pictured manner I would think the coupler needs to be snug but not tight. What is typically done here? Sand down the exterior of the coupler, or sand the interior of the lower airframe? I am assuming that couplers are used a payload bays. Is it that payload bays have a smaller outer dimension than couplers? Am I missing something? Thanks.