Why do I get only a 5.3% reduction in velocity with a 40.1% increase in pellet weight when going from .177 to .22 caliber barrel? (when switching from 10.2 grain .177 JSB heavy to 14.3 grain .22 JSB express)
Here's the data for those mechanical physicists out there (most spring piston people)
Whiscombe JW75 MKII (same rifle used but swapped barrels of equal length)
Chrony used indoors, 70 degrees F. 10 shot average 3' muzzle velocity strings. Pellets lubed with Whiscombe honey.
Assuming that the brinell hardness of JSB pellets are the same, I was initially thinking that it has to do with friction coefficient - either I have a smoother 22 barrel or pellet bearing surface for the .22 is less (due to shape difference or diameter over bore).
Then I thought - could this be due to some pressure/volume theory that I don't understand? The swept volume from the piston is exactly the same from the rifle, but swapping equal length barrels would increase the volume in the .22, decreasing pressure, hence lower velocities...
OK I know I should just shoot and enjoy, but this is puzzling me. Any thoughts?
My guestimate is that possibly the caliber of the barrels and pellet sizes are different..
March 28 2009, 7:07 AM
meaning that may be pellet for the .177 barrel may not be as close of a match in diameter (4.52mm, 4.53mm?) than the pellet for the .22 barrel. Therfore when shooting .177, more air may escape around the pellet and less efficient?
Or may be the choke acts differently in each barrel, if there is one inside.
because the flow rate is too restrictive in the .177 for the amount of air being forced into the bore,causing some of the energy used more efficiently in the .22 cal.,to be transferred back to the spring in the .177.
If this is true, though, it might imply that I should technically avoid shooting the light .177 pellets through the Whiscombe (opposed piston) design as it would be harder on the pistons as the .177 pellet would have left the bore before the pistons complete their stroke volume.
Very likely an efficiency proposition. A big energy drain is shock waves that develop where the cylinder transitions to port/breach. I think the larger bore of the .22 reduces the shock wave formation somewhat, allowing that energy to be used to propel the pellet.
I think the larger bore of the .218 allows the expanding air mass to expand with less resistance while working against the larger pellet base. A more definitive experiment would be to produce or procure projectiles of identical weights in both calibers. Take a garden hose and send a finite volume of water through it at a proscribed pressure. Take the same hose and the same volume and and pressure and now restrict it with a smaller orifice. The restriction produces an even higher pressure at the nozzle but lower flow rates in the hose. The identical volumes of pressurized water will take a different amount of time to pass through the hose because of the different restrictions of the outlet. Water can be pressurized but not compressed. The same is true for the compressed air moving through the different sized barrel tubes. All of this is regulated by Boyle's law.
My brain has been puzzling over an experiment I did recently with my .22 Diana RWS 52, to wit: At a range of about 60 feet I sighted in my scope using different brands of light-weight pellets at or about 14 grains. The accuracy was not great with occasional flyers, so I switched to Beeman extra heavy Kodiaks at 21 grains, hoping for improved accuracy but fully expecting a lower POI due to the increased pellet weight.
Result: I missed the 8-inch target completely (no surprise), but it turned out that the POI was now way too high, not too low, which was a BIG surprise to me! Once I got the scope re-zeroed the accuracy was mucho better, and I was very pleased. But what about that higher POI when it should have been lower (or so I had reasoned). The claimed velocity for a Diana 52 in .22 caliber is 900 ft/sec, by the way. -- Can anyone explain to me (who just squeaked thru high-school Physics 101 many years ago) what happened here? -- Thanks in advance.
Shot#3 2,300 PSI 656 FPS 287 FPE Bullet 4" higher than first shot.
Shot#4 1,800 PSI 562 FPS 210 FPE Bullet now 1.5" above the first shot.
I confess, I was sitting in the back of my truck with the rifle resting on a pad
flung over the bedwall. I was holding the foregrip tightly and found that the "climbing phenomenon" was much less pronounced than it had been 2 weeks before when I was shooting from a Caldwell rifle rest and not holding onto the foregrip at all. On that day the POI was lifting roughly 10" from the fastest and lowest shot to the slowest and highest shot. It was foggy, I was cold, Kim was ready to head home and I thought me eyes were playin' tricks on me!
On that day at the range my DAQ Exile .308 did not seem to change POI in 4 shot groups. I was also letting it sit on the benchrest with no hold on the foregrip. It makes about 1/2 the FPE of the LA .457.
I will definitely try this with another LA .457 with longer and shorter barrels, as well as some less powerful big bores.
The heavier projectile remains in the barrel longer and is more effected by upward effect of recoil. Example: A 38 fired in a 357 revolver will strike higher on the target than the 357. Same effect when when 38 is compared to 38+P.
First of all, it's kinetic energy at the muzzle, which proportional to velocity squared...
March 28 2009, 10:40 AM
...that you might expect to be conserved if the two calibers were equally efficient. The reduction in muzzle energies you measured between the 10.2 and 14.3gr JSBs was about 20fpe vs 25fpe - about 20%.
Some of this may come from the difference in bore volumes in large caliber vs small, but you have that relationship backwards. A larger barrel bore usually produces more muzzle energy than a small one for exactly the same reason that a longer barrel length usually does: both have a larger volume which allows a more complete expansion of the propellant charge. This allows a more complete transfer of the charge's energy to the pellet.
Does it mean the spring in .177 could be tuned lower and still have same velocity?...
March 28 2009, 11:31 AM
It seems, if I understand it correctly, more of the spring power may be wasted in the .177 vs .22. Would the spring can be tuned lower to its optimum in .177 while still able to produce almost the same velocities? If yes, would we be able to reduce some of the recoil, cocking effort, noise?
I always figured it was the greater area at the base of the pellet.
March 28 2009, 12:35 PM
relative to the area of contact in the bore. Since the cross sectional area goes up with the square of the radius while the friction creating area in contact with the barrel only goes up proportionately with the radius it always seemed to me that the larger you made the pellet the greater the ratio of force to friction and hence the greater it's theoretical efficiency.
I never really paid too much attention to barrel length or volume with springers, but I suppose that in the case of a .177 Whiscombe that it's a definite factor.
There's a lot more surface on the .22 than the .177 and that's the route the gun has to use to transfer energy to the pellet. The same pressure can do more work with a bigger piston to push on. This is a prime reason CO2 'prefers' .22 over .177. The ratio of area to wall also goes up, favoring the fight over friction.
Simply put there's less pounds of lead per sqaure inch to push and relatively more area to fight the wall friction.
It all gets tied up with available energy of course, but I bet that the bigger bore is the key.
The .22 is more efficient than the .177 in the same power plantas discussed above. Think in terms of energy (pellet weight in gr X velocity x velocity x .000002221) and you will see a real difference.
In the UK, where they have a 12 fpe limit, Theoben makes differently powered gas rams for different calibers. If you use a .177 gas ram in a .22 rifle you get more than 12 fpe. For my BSA Lightning gas ram conversion, I used a .177 gas ram to get 14 fpe from my .22 rifle.
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