The air transfer port
by B.B. Pelletier
When I was writing the R1 Homebrew series of articles for The Airgun Letter, I did lots of side experiments and research to discover what tuning tips did and didn't work. In the area of air transfer ports, I discovered a lot that wasn't documented at all, and more that was known to only a few people. Apparently, the air transfer port is one of the most modified areas of a spring gun, yet very few people are doing any research on the effects of modification.
What is an air transfer port?
At the end of the compression chamber, there's a small hole that the compressed air passes through after being compressed by the piston. That's the air transfer port. In a spring gun, the amount of compressed air is extremely small when compared to a pneumatic. As early as 1948, the gunsmith and pistol competitor Walther F. Roper surmised that it wasn't the volume of compressed air that makes a spring gun work but the speed at which the air is compressed and released. Three decades later, the Cardews of airgun experiment fame agreed with his observation.
When seen in cross-section, the air transfer port is actually a tunnel that runs from the compression chamber to a point immediately behind the breech, where the pellet sits. The compressed air rushes through this tunnel and blasts into the skirt of the pellet, sending it on its journey down the barrel. How this port or tunnel is made and where it's positioned play a large part in the efficiency of the air blast, which determines the rifle's efficiency.
Don't think for one minute that this design is easily understood, despite being so dirt-simple. Jack Lewis, a famous gun writer and editor of the 1950s and '60s, saw a cutaway like this and thought the empty space he was looking at was a valve. He described that phantom valve in detail in a 1960s article about spring-piston rifles--not unlike the amateur astronomer Percival Lowell describing the canals on Mars. But now you know what it is--the space between the piston seal and the pellet. It's the part that's the air transfer port that interests us.
Air transfer ports are placed where they have to be because of the gun's design and also in order to do their job. Their job is simple--pass the compressed air from the piston to the pellet. Since they're open all the time (being nothing more than passageways), they form a part of the total volume of the compression chamber, even though they're not inside the chamber. Understanding that is important to understanding what comes next.
Concentric ports versus offset ports
The piston is a large hollow slug of metal that's pushed by either a coiled steel spring or by a charge of compressed gas in the case of a gas spring. In front of the piston is a seal that keeps the air from escaping. The compression chamber is in front of the piston, with the air transfer port forward of it. At the end of the air transfer port is the breech of the barrel, which is sealed by a lead pellet. The volume of air behind the pellet and in front of the piston gets compressed when the piston rushes forward.
Where the hole is located plays a large part in the efficiency of the transfer port. If the hole is in the center of the compression chamber, the air flows evenly from all around the chamber and into the port. If the hole is offset, some of the air has farther to go, resulting in a decrease in the intensity of the air blast behind the pellet. That's because some of the air is still flowing through the port after the initial shock wave has hit the pellet skirt and started the pellet on its way.
Some air transfer ports are offset rather than centered or concentric because the barrel is a smaller diameter tube than the compression tube. It sits atop the compression tube, making the transfer port offset. The Feinwerkbau 124 (and other spring guns) has a transfer port opening in the center of the compression chamber, but it also aligns with the center of the breech. To do that, the port is cut on an angle.
Unfortunately, when the port is cut on an angle, it has to be longer, and the extra length adds volume to the compression chamber--remember that? Extra volume means the air cannot be compressed to the same degree, and, if we are correct about the intensity of the air blast being more important than the amount of air compressed, well, then it becomes a very big deal.
The TX200 has a curious humpbacked appearance, because the barrel is lowered to align with the center of the compression chamber. Hence, a light spring can produce great power with less effort. The RWS Diana 48/52/54 is designed the same way and produces similar results. The majority of airguns are not designed this way, so their transfer ports either have to be offset or longer to carry the air where it has to go.
This was just our first look at the subject. Before you can appreciate how the size of the transfer port affects things, you have to understand the design of the transfer port.