A .200 diameter on the transfer port would wipe out the shoulders that fit inside the o-rings. The shoulder diameter is only .187. Additionally, opening up the transfer port won't really gain anything. The firing valve and the breech hole are smaller and will then become the restriction. In older guns, the ports were 2.8mm (.110) and newer guns went to 3.6mm (.140), but the valve and breech were drilled the same size. On another note, if you drill the breech and barrel over-sized, there is a chance that the pellet head could drop into the hole when loading and shear the pellet.
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