Sorry for the delay on my next ramblings regards pellets and release pressure, studying trigger geometry and what could be improved on the Rekord and CD units after studying the LGV/LGU trigger unit got in the way, then studying the T06 Diana unit and how its a little different put me back another week or so. Now after some .22 TX 22mm testing and seeing just how much effect release pressure can make (on my guns here) I thought it time for the next update. I finished last time explaining the importance of the lead in on the barrel and how the pellet acts as a valve, helping the compression tube build pressure. Now we need to add transfer port diameter into the mix, as between the release pressure of the pellets and the choking effect of the port you can dramatically alter what pellets make good power and what do not, and alter velocity consistency.
As mentioned earlier, JSB made pellets generally have a lower release pressure than other brands such as RWS or H&N, QYS however act more like JSBs so the same rules that apply to JSBs apply to QYS generally. One pellet that responds well to very high peak pressure and fast pressure build is the RWS Superdome, both in .22 and .177 sizes. It can also help diagnose leaks as leaks cause higher peak pressure especially if more input energy has been added to counter the effect of the leak. Eventually, i’m hoping to build a platform to accurately measure release pressure on specific barrels, remembering every barrel will effect the release pressure either up or down on the pellets tested as every barrel seems to have subtle differences. For now though for ease of understanding, JSB/QYS let go and start moving earlier in the piston flight than the other brands. Earlier i mentioned you can turn the rule that spingers are more efficient with lighter pellets on its head , here is why and how.
Now we understand some pellets release earlier in the shot cycle than others, we can now look at how we can control that pressure build and how we can force the gun to favor harder or heavier pellets or favor softer pellets or lighter pellets. Larger port diameter equals higher flow but it also gives a lower peak cylinder pressure. Now consider the heavier and harder pellets like a Superdome have a naturally higher release pressure using a larger port diameter can favor these pellets. Its not uncommon to set a UK spec sub12 springer to push a JSB branded pellet at 11fpe and Superdomes are delivering more energy. Power plant design does effect this also as does the spring rate BUT running that large port can make the gun favor the harder or heavier pellets. Piston stroke plays a massive effect here also BUT that is not for now, keeping the focus on release pressure and port diameter for now 😉
So, thinking we want to shoot JSBs or QYS as they are a more accurate pellet what can we do to make the power plant in the rifle favor them?….reduce the port size!
Choking the port flow by reducing the diameter of the port down allows the pressure to build in the compression cylinder higher but build slower behind the pellet. So one side of the port the pressure is huge, the other side its much lower.
How does this benefit JSB’s?
Well we’re artificially increasing their release pressure, that means when they do let go and start to accelerate down the barrel the pressure behind them is MUCH higher, which in turn means they get a much BIGGER shove by that expanding air, they accelerate faster and as such make more energy and deliver a higher velocity. Now you can do all this with port tuning and not altering input energy at all. You can see an 80fps difference in favor of superdomes be totally eradicated by port tuning, resulting in domes and Exact 8.44s flying around the same speed or in fact Superdomes flying slower, all without adding any additional preload. This effect can be seen when over springing the power plant also but again for now lets concentrate on release pressure and port diameter. Now with a smaller port we are helping keep the pressure behind the pellet to say 300psi while allowing the pressure in the cylinder to rise to 4 or 5 times this, maybe even higher, where with a larger port the pellet is letting go much sooner and the cylinder pressure may only be 2 or 3 times higher at that point…its a subtle difference, but it makes a huge effect.
Port length can have a massive effect on this also, generally im talking about port length of 5 to 10 mm max here, this covers most of the more popular air rifles.
There is one pellet that is more difficult to set up for, this is the 177 JSB heavy, 10.3gr. It should favor larger ports however not always, so you can set up a power plant to shoot Baracuda Match or FT or the tighter fit FTTs etc and JSB heavies still fly slow. They have a low release pressure BUT they are heavy so need a good shove, finding the sweet spot is a little more tricky, but not impossible. As usual different die and batch fly at different speeds and the new Hades variant and new heavy dies have a smaller head size which has made them act more like an 8.44… More on JSB heavies another time.
I personally feel Superdomes have a release pressure almost 2x that of JSB Exacts, FTT 2.5x, Baracuda FT 2.5X etc. This means a Superdome with a large port is able to hold back the pressure to a much higher level in the compression cylinder, they then get a huge shove down the barrel aided by the higher flow a larger port allows. So if you want to shoot Baracuda FT which do act in a similar way to Superdomes you can set up the power plant to do so, BUT at the expense of JSB pellet performance, it will be lower.
Smaller ports for JSB pellets
Larger ports for RWS and H&N pellets
Even smaller ports for JSB made light pellets as their release pressure is even lower!
Next up going to go into piston diameter, and why i choose to run much smaller pistons. They are not without issues however which i will also touch on.
Skip to content