Changeable air transfer ports
by B.B. Pelletier
This subject was raised by Frank B., I believe, from our conversations regarding deep-seating pellets in spring guns. Someone asked if the transfer port of the Hy Score 801 was particularly short, which he felt explained why seating pellets deeply would show a velocity increase.
I have to tell you that it isn’t that simple. The air transfer port conducts the high-pressure air from the compression chamber to the back of the pellet sitting in the breech. While it has a simple job to do, the transfer port is another factor in the overall performance of the gun. In that capacity, the tune of the gun relates directly to the length and shape of the transfer port. Yes, I said the shape, too.
If you have a .22 caliber Beeman R1 humming along at 22 foot-pounds and you alter the size and shape of the transfer port, don’t count on the gun delivering the same power afterward. In my experience, and from what limited testing I did with the set of ports I’m about to show you, the power usually drops when the port is altered.
Changing port dimensions and shapes was all the rage back in the mid-1990s. Jim Maccari did a brisk business altering ports for customers. And he came up with some observations of his own while doing it. If you want the rifle to continue to function over a broad range of power, based on changing the state of tune (without altering the transfer port), he found it was best to leave the port as the factory designed it. Let me give you an example to illustrate the wisdom of that.
The Beeman R1 used to come in all four calibers (.177, .20, .22 and .25). But when Weihrauch produced the R1, they made the transfer port the same size for all of them. It would have been a costly management nightmare to make a spring rifle in different port sizes according to the caliber. So, all R1s came (and still come, I believe) with a transfer port that’s very close to 0.125″ in diameter. The actual size is metric, but that’s what it measures on an inch scale.
Now, say you’re the owner of a .177 R1 that you want to rebarrel to .22. All that’s needed is a new barrel and cocking link. Everything else on the rifle is the same between the two calibers. But if you altered the port for enhanced performance in .22 caliber, you might find it next to impossible to get decent performance out of it in .177. And, when you altered the port for optimum performance in .22, that’s just for one or two pellets. You generally lose performance with other pellets when you make changes.
Because the transfer port is such a permanent part of the spring tube, any changes that are made can be permanent. Yes, I know of several ways to bush the spring tube so you can start all over, but is it worth the effort? Jim Maccari apparently didn’t think so, because he donated a ruined spring tube to me for an experiment. Dennis Quackenbush made a set of transfer ports that slid into the hole and were held in place with a setscrew.
Another thing to think about is the shape of the transfer port. Many people suggested an air venturi. That would be a smaller hole with a bevel on either side. A properly designed venturi should speed up the flow of compressed air because it’s made to pass through a tunnel that changes shape from large to small. But I never recorded any advantage from a venturi-shaped transfer port, perhaps because the machining was too rough.
Before you start your drill press, please know that I was never able to get this setup to shoot as well as an unaltered R1, but it was good enough for a few experiments. In a nutshell, here’s what I learned:
1. Transfer port sizes from 0.120″ to 0.145″ give the same results for a .22 caliber R1 tuned for maximum power…in this gun, which was about 19 foot-pounds. When the size drops below 0.120″, the velocity slows. When it gets above 0.145″, it slows and the gun acts like it’s being dry-fired. Lotsa dieseling, etc.
2. Exotic shapes such as venturis don’t seem to affect the performance within the optimum size range and the targeted caliber.
There’s more to the study of transfer ports, like ports that are centered in the compression chamber, versus ports that are offset to one side. But this should get you thinking.