Full thru-flow bolt probe gives the full diameter of the transfer port straight to the back of the pellet. Given the geometry, it also loads the pellet by applying force evenly to the outside of the skirt, which makes every pellet exactly consistent as it loads into the chamber. We don't put the O-Ring on .25 Cal and below because the barrels typically have an o-ring in them. .30 Cal and above the barrel is not generally thick enough for the o-ring to go in the barrel.
These parts are made to order, as not all probes are the same, please send in your breech and bolt, so we can clock and install the kit.