Why do PCP’s-Springers prefer smaller-larger size pellets?

[Albert T]

Recently I tested several PCP and springer rifles to see what size pellets they liked best. The PCP rifles (Walther LG300 and Hammerli AR20) both preferred the smaller head size (4.48 and 4.49mm) pellets, while the springers (FWB 300S Universal Gen. 2 and 3) preferred the larger head size (4.50 and 4.51mm) pellets.
Who can give me a technical explination for the difference in preference for the pellet sizes?

Albert T
(The Netherlands)

[Rover]

There probably is no preference, merely the lot# of the pellets you tested. The results may be reversed with the next batches you tried.

Even ten shot groups will not give you a reliable indicator of true pellet accuracy.

If I'm wrong, though, I'd like to hear about it.

[kevinweiho]

FWB used to make precision barrels for their older airguns vs. the Lothar Walther barrels used in Walther and Hammerli airguns. Different barrel blanks and the actual condition of the mandrel imparting the rifling during manufacture may also affect the preference in pellet head diameters.

[Thauglor]

I'm reaching a bit with no information to support my claim, but is the pressure curve on the springers slower to build up? If it is slower to build to peak pressure in the barrel, it could need a larger pellet to more effectively seal the barrel.

[Andre]

I'm reaching a bit with no information to support my claim, but is the pressure curve on the springers slower to build up? If it is slower to build to peak pressure in the barrel, it could need a larger pellet to more effectively seal the barrel.

The theory is that when air hits the skirt of the pellet, it expands and seals with the barrel. In theory head size shouldn't matter if this is true in practice. I think it's all just in the barrel, some brands like different pellets.

[Albert T]

In my test I used 7 different head diameters (8.2 grain rifle pellets 4.48, 4.49, 4.50 and 4.51mm; and 3 7 grain pistol pellets 4.48, 4.49, 4.50mm). All were RWS R10 Match.
Searching in other forums I found older threads that stated that the FWB300S likes the bigger diameters better (4.51 and 4.52mm).
If we assume that the info in the book “from trigger to muzzle” about springer air rifles is correct, the seated pellet starts to move when the pressure is at its peak and the piston is slammed fully forward (see diagram in the book). A PCP also starts witch full pressure against the seated pellet, so there should be no difference in pressure build-up in a PCP or springer. The difference is that in a PCP the hammer block bounces back on the valve several times, creating several pulses of air behind the speeding bullet in the barrel (see slow-motion films on Youtube).
The only thing I can think of is that in a springer part of the air that is compressed by the piston leaks past the side of the skirt of the pellet while the piston is moving forward (pressure build up takes some time and is not instantanious as in a PCP). When a small amount of air leaks past the skirt there is less friction between chamber and pellet, causing the pellet to move slightly forward in the chamber. As the larger head diameter pellets seal the bore better, less air leaks past the pellet, creating a better seal, more friction and more constant air pressure behind te pellet.
Also, in the 300S the pellet is seated by the hand/thumb (slightly different with every pellet), while in the LG300 and Hammerli AR20 the pellet is seated by the bolt sliding forward (exact same distance with every pellet). Could all of this make the difference in pressure drop/build-up?

Albert T
(The Netherlands)

[kevinweiho]

A PCP also starts witch full pressure against the seated pellet, so there should be no difference in pressure build-up in a PCP or springer.

A springer only produces a tiny amount of air at peak pressure; CO2 and pcp’s have a greater pressure build-up, allowing faster lock times.

Also, in the 300S the pellet is seated by the hand/thumb (slightly different with every pellet), while in the LG300 and Hammerli AR20 the pellet is seated by the bolt sliding forward (exact same distance with every pellet). Could all of this make the difference in pressure drop/build-up?

As long as the pellet is seated flush within the breech and there is no damage to the pellet skirt, both loading systems are reliable, but we’re talking about pcp’s and springers, so there is a difference in the pressure drop/build-up.

[Albert T]

So, what is the difference in pressure build-up? Can you show me this in a diagram/drawing?

Albert T

[yana]

They dont. Its usually the other way around; AP prefer smaller, AR larger.
My Walther LG300 seems to like 4.51 for instance.

[Albert T]

Yana,
I think you are confusing weight of the pellets with head diameter. Air rifles like the heavier pellets, air pistols prefer the lighter pellets. This because of the slower V0 of air pistols and the shorter barrels.
A change of 0.02mm in head diameter does not mean a large increase of weight, but certainly makes a difference in friction and air leakage between barrel and pellet, which in turn influences the barrel time and therefore the time and position/direction the pellet leaves the muzzle (barrel harmonics and tuning).

Albert T

[Rover]

In my testing and experience, weight seems to be irrelevant, as Don Nygord told me.

[Andre]

In my testing and experience, weight seems to be irrelevant, as Don Nygord told me.

I have some R10 light pistol pellets, that I tested in both my LP10, Challenger, and FWB 700. They shot terribly from all three in a vise. Your mileage may vary (they make pistol pellets for a reason!) but my guns like heavier pellets.

[kevinweiho]

So, what is the difference in pressure build-up? Can you show me this in a diagram/drawing?

Albert T

The BIG difference is that pcp’s and CO2 guns uses more volume of air/gas than a springer can generate. Sorry, I don’t have a diagram/drawing to show you.

[robf]

Head size is a myth on pellet tins. Go and measure some and see just by how much.

[Rover]

It just came to me that during my testing with some springer air rifles, the accuracy was proportional to the power. In other words, low power guns "liked" lighter pellets; high power guns "liked" heavier pellets. It don't know what the hell that means.