It is possible to create a model to calculate flow rates. You need to go to modelling all the components, the forces between them and the air itself in the barrel and cylinder. This isn't easy but I did it about 10 years ago. There is no simple formula you can use.
If the transfer port gets too small, the flow between the cylinder and barrel chokes as the air can't go supersonic (based upon its properties in the cylinder). This is a bad thing as the flow levels off and the pressure in the cylinder goes up a lot. The higher pressure air in the cylinder pushes on the front face a lot harder and makes the forward recoil push horrible.
The pressure difference between the barrel and cylinder causes the choking. Smaller diameter ports have more resistance so make the pressure difference higher, which leads to choking. So do longer or rougher transfer ports.
BB
There has been a number of findings from experiments conducted shared with us over the years and, if I remember correctly, the short answer usually boils down to no fancy shapes / bell mouths etc required. Just a nice, parallel, smooth port with no sharp, jagged edges. I think one of the links that T20 posted onto the thread about HW80 transfer port size to the Pyramydairblog shows a few experiments with differing layouts and I'm sure I've seen plenty of others in the past. That old thread is now on page 1, by the way.. Jim has provided some brilliant information over the years on the subject in Airgun World.
As has been said up above, all very interesting and intriguing stuff.
THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
NEXT EVENT :- May 2025.........BOING!!
Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in
3.0 x 15mm, and it's too small, even when the bore is reduced down to 25mm. TOok that one out to 3.2, but the accelerometer data indicates it shoudl be opened up a little more.
I'll be doing some experiments with my FAC one next year - I don't have all the tools and equipment with me on this trip - or even a shrono. Will also bring a machined lighter piston. However I have no doubt that 3.0 will be too small for a 30mm tube... 3.4 woudl be my guess..
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.
The first British built Magnum springer of the 1970s, the Webley Vulcan, had a 2.5mm transfer port.
I've used 2.5mm ports on a number of guns without problems.
I go 3.5 mainly due to the fact that’s the size drill bit I’ve usually got lying about 🤣🤣
"But we have our own dream and our own task. We are with Europe, but not of it. We are linked, but not comprised. We are interested and associated, but not absorbed."
Winston Churchill 1930
Crikey, can't resist this. When I was knee high to Ronnie Corbett I had an air rifle ... well maybe I should say 'we had' because my friend and I both used it. No idea where it came from or what it was, maybe a Wobbly Jaguar or a Diana 15/16? I remember firing nails, cocktail sticks and potato bits and ..... don't know how, but using it as a water pistol. It wasn't very good. I can't remember how we loaded it. Maybe trickled water in as we cocked it? We did suss that it needed oiling after a few shots though...
Oh the things you do in the innocence of (very) youth.
Cheers, Phil