Transfer Port (TP) is the ‘channel’ through which the air passes inside an angun when the hammer hits the valve and it opens to release the pressure (air). That instant opening produces a shock wave that finally reaches the pellet, pushing and sending it out with great velocity. 

It can be located in different positions and shapes as well, depend of the air-gun, but always fulfilling the same task. Its position influence the possible efficiency that a particular gun can have; and frequently people that enjoy tuning their air devices for fun and looking into increasing the speed (fps) and energy (fpe) of their rifles, drill in the TP to make it wider, thus allowing more air to pass through.

To reach the desired efficiency which is a combination of speed +energy + shot count, other aspects need to be added into the equation. They could be the opening hole in the valve itself, the spring quality and tension used to hit the hammer, and the valve’s spring as well. Of course many other things take part as a whole to send a pellet out in an accurate manner, as the barrel’s class and grade, the way it engages the main body of the rifle, the supports, and the pellet itself, etc. 

Even more objective and subjective parameters must be added to that mentioned equation; however, the reason of this page is to show up a chart that can be helpful to ensure you do not damage your barrel, as it has happened many times to many air-gun enthusiasts when they got into the tuning business and used the wrong drill speed and bit.

Here a complete picture / diagram of the TP, Valve System, Spring, Trigger, and other parts of the Benjamin Bulldog .357 caliber.

The little blue part on the top of the picture is a different type of TP; because it can be removed from the barrel without having to dill-in, and replace it with another pre-made for the desire opening diameter. In this case is a Hill 105 i.d TP used in a .22 caliber Marauder Pistol.

These are two: 12 mm and 8 mm, solid carbide cutter slot drill bit.

Next a chart that can help you in understanding the variety of speeds. 
Different speeds that you can adjust your Dill Press depending of the drill bit and material on which you plan to open a hole; including aluminum and steel. Click on each picture for better quality.

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