After avoiding .25 airguns for over 50 years, Jim becomes temporary custodian of a .25 BSA Superstar
Jim Tyler shortens the spring on his friend’s rifle to make it a true ‘Superstar’
Let’s start off on a positive note, and consider what .25 pellets have got going for them, which is momentum. Momentum is defined as ‘quantity of motion’, and is best thought of as how much more difficult it is to arrest the motion of whatever it is that possesses the momentum; in our case, a pellet.
Momentum is the product of mass (think pellet weight) and velocity, and is usually expressed in kilogram metres per second, though to save a lot of conversion of the units we airgun enthusiasts are familiar with, momentum can also be expressed as grain feet per second (grain-feet/ sec). An 8.4 grain .177 pellet traveling at 780fps, for instance, has momentum of 8.4 x 780, or 6552 (grain-feet/sec).
To put .177 pellet momentum into context; a 15 grain .22 pellet with the same muzzle energy will be travelling at 583 fps, so will have 8745 grain-feet/sec, 133% that of the .177; and a 25 grain .25 pellet with the same muzzle energy would have 11300 grain-feet/sec. 129% that of the .22, and 172% that of the .177.
What does this mean in practical terms for airgun pellets? It means that when the .25 pellet hits a movable object, it tends to move the object more than the smaller calibres. A grey squirrel in a tree, for instance, would not only be killed, but also usually knocked clean off the branch. I have known .25 pellets to hit field targets so hard that they are rattled enough to fall over from a hit to the faceplate. What happens is that the face plate collides with the paddle, and the two bounce off each other, to the extent that the paddle reaches its tipping point and pulls the face back.
Larger calibre pellets tend to have greater sectional density, making them less prone to distortion when hitting a target. If we take 8.4 grains as a medium weight for .177 and scale that up by cross sectional area, a .22 would be 13.12 grains, and a .25 would be 16.94 grains, both of which would actually be considered lightweights, with most .25 pellets tipping the scales at nearer 25 grains. Because of this, the .25 uses less of its kinetic energy in distorting
itself, leaving more to displace whatever material is in front of it, and giving a deeper wound channel.
The high momentum of .25 pellets makes them a good option for those who possess firearm certificates and are able to shoot well above 12 ft.lbs., but for those who shoot sub-12 ft.lbs. airguns, the momentum of the .25 comes at a cost, and that is the amount by which the pellets fall over distance, in comparison with lighter, smaller calibres with equivalent muzzle energy. The .25 pellets fall at the same rate as .177 and .22, but they take longer to reach the target, and so fall more, which means that that the shooter has to aim off more.
A 25 grain .25 pellet exiting the muzzle with 11 ft. lbs., and zeroed at 30 yards using a scope in medium height mounts, would need around 1.5” of hold under at 18 yards; the apogee (the point in the trajectory when the pellet is at its highest above the sight line), and an inch of hold under at 12 and 25 yards. You’d need to be almost yard perfect in range estimation to hit anything.
Bring the zero range down, and things start to change. With Bisley Superfield pellets at 450fps, zero at 19 yards and you will be rewarded with a continuous PBR (Point Blank Range) of 0.5” from around seven to 21 yards, so there will be no need to hold under at any range, only to hold over at ranges beyond 21 yards; by one mil dot at 26 yards, two mil dots at 31 yards, three mil dots at 36 yards, four mil dots at 40 yards, and five mil dots at 45 yards, which is nice and easy to remember. Those longer ranges need too much aiming off for pest control for my liking, and the lengthy flight time at longer ranges would increase the opportunity for the target to move before the pellet gets there so, if I used a .25, I’d probably limit my pest control to around 26 yards. This, to my mind, tells us where a .25 sub 12 ft.lbs. rifle reigns supreme, which is in short-range pest control.
Plinking is all about having fun safely. Seeing a reaction to a pellet strike is part and parcel of that fun, and .25 pellets excel at knocking some types of plinking targets about. However, plinkers generally tend to shoot more pellets per session than hunters, or competitive target shooters, and the .25 is rather more expensive than the smaller calibres, so the extra fun comes at a price.
For outdoor target shooting, I have only ever known two people to use a .25 for HFT, neither of them really serious competitors, and they did it for fun, rather than in any
expectation of a high score. The very marked pellet drop at range, and the need to be almost yard perfect in range estimation, would put anyone trying HFT with a .25 at a huge disadvantage.
Due to its low velocity, the .25 pellet takes longer to travel up the barrel, so it will generally achieve higher muzzle energy than smaller calibres in PCPs – provided the valve releases enough air to accelerate the pellet the full length of the barrel – but the situation in spring/piston airguns is not so clear cut.
Whilst the .22 is usually more energy efficient than .177, anecdotal evidence appears to suggest that .25 does not necessarily continue that trend and, if that is the case, the reason would probably be that the rifles concerned lack sufficient swept volume and/or piston stroke. Too little air mass would mean that, even with high internal kinetic energy, the too few molecules would lack the momentum necessary to get the heavyweight pellet up to speed, whilst too little available piston stroke would fail to accelerate the pellet for long enough to get it up to speed.
If the above thoughts on air mass and momentum are correct, then lighter weight .25 pellets (in the region of 19 grains) would give much higher energy levels than heavier ones if the swept volume were too low. This theory cries out to be tested, because it could help to reveal more of the secrets of the spring airgun, but it would need to be based on one action tested with all four calibres, and a rifle with swept volume to spare.
The BSA Superstar is a real curate’s egg of an airgun, because parts of it are excellent, and then there’s the trigger unit, which simply can’t live with modern, four-lever groups. The rifle has been reviewed many times, so I’ll concern myself with an issue specific to this rifle, but probably fairly common.
The only pellet that fed easily into the breech without needing some implement to push it home was the Milbro Rhino which, at 19 grains, is a lightweight amongst .25 pellets, and was the owner’s preferred pellet. The tightness of the breech made me suspect that the barrel was actually .243, a calibre in which BSA made centrefire rifles like the beautiful Stutzen that I briefly had on loan from the company in the 1980s. I’m informed that today’s BSA barrels are true .25, and that information came direct from the gentleman who makes the mandrels on which the barrels are formed, so he should know.
The problem with the rifle/pellet combo was that the muzzle energy was uncomfortably high, and the owner asked me to take a look
“more energy efficient, which in turn requires less spring force, for less recoil”
inside and make it safely legal with the Rhino. Inside, I found a crudely cropped, unfinished and heavily greased, unidentified aftermarket mainspring, of 21mm OD, 3.25mm wire, 24.5 coils and 40mm of preload with no packing washers.
I did the sums – calculating how much energy the spring made available to the piston, the energy efficiency, and what length of spring and preload would give the desired muzzle energy – which suggested that reducing preload, by shortening the spring, to 27mm would give around 11 ft. lb. Because the end coil was unfinished, cutting off just threequarters of a coil and flattening and dressing the end coil gave the 13mm reduction in preload, and the muzzle energy dutifully reduced to 11 ft.lbs. to 11.2 ft.lbs. Perfect.
Unfortunately, the trigger weight was so high that I was unable to record the shot cycle without the action of pulling the trigger blade affecting it, so I had to settle for the oldfashioned way of shooting the rifle and subjectively judging the recoil cycle by feel.
The shot cycle with the rifle set to 11 ft. lb. with Milbro Rhino pellets was, in a word, delightful, so much so that I can at last appreciate why some people choose to shoot sub-12 ft. lb. spring airguns. The best I can describe it is like the difference between .22 and .177 and, I suspect, for the same reasons.
The .22 variant of a pellet tends to have a lower start pressure than the .177, which means that it starts to move earlier in the stroke, and benefits from the energy of the piston for a longer period of time. This makes it more energy efficient, which in turn requires less spring force, for less recoil. At the same time, the volume of air behind the pellet as it travels up the barrel is greater, leaving less in the cylinder to drive piston bounce, so the surge is less. The same principles apply for .25 versus .22 so that, for a given muzzle energy, the recoil cycle from the larger calibre is markedly better.
In my opinion, a light- to medium-weight spring airgun in .25 would seem to have the makings of a superb, short-range, pestcontrol tool. I
Reducing spring preload by 13mm put the muzzle energy right where I wanted it.
At 19 grains, the Milbro Rhino pellet is a lightweight for a .25, but gives the best trajectory.
A length of Acetal rod is quite sturdy enough to take the preload force.
Inside, I found a crudely cropped, unfinished and heavily greased unidentified aftermarket mainspring.
To remove the mainspring, you need to make a tool to bridge the cross pin and take the force from the mainspring.
After fruitlessly trying to lever it out, I used a neodymium magnet to remove the steel plug.
To the rear of the cross pin is a steel plug, held by the main stock screw and the scope rail, Allen head bolt.
With the stock off, pull off the plastic end cap, and the steel plug will be revealed.
I was unable to reduce trigger pull weight for serious recoil testing.
If I had to sum up .25 in a word, that word would be ‘clout’.
With its shortened spring, the Superstar is running sweetly at 11 ft. lbs.