I have critically compared JSB Exacts in 20 cal with CP's in 20 cal, and JSB/Wolverines in .177 with CPL's in .177.
Here's my take:
The CP's have a much better uniformity of weight
see:
http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=10684&posts=5&start=1The CP's seem to be of a harder lead alloy, have a slightly shorter, but thicker skirt. The CPL's in .177 also have a dry lubricant applied. The JSB's and Wolverines (apparently just re-labeled JSB's) are a noticeably softer lead allow with no lubricant and a longer, much thinner skirt. The advantage of the later is that when shot in a springer (but probably not in a CO2, PCP or SPP air rifle), the skirt of the JSB immediately expands to fit the bore, giving a very uniform "fit" and more uniform velocity . . . but . . . the quality control of the CP's is better. They have a much tighter spread of actual weights when individually weighed on a jeweler's scale. So, with the JSB's, you can get some variability of velocity due simply to weight variance, and this may affect the accuracy, particularly at ranges over 25 yards.
IF you take the trouble to weigh and sort the JSB's so that the pellets you are shooting are in fact 8.4 gr, +/- 0.05 gr, then they will out perform the CPL's. My reference springer for the .177 cal was a Mike Melick tuned B26 shooting around 850 FPS, my reference springer for the .20 cal was a Beeman R9 shooting around 710 FPS.
In general, either the CPL's or the JSB's will serve you well in most springers. Because of the difference in the way the pressure is developed just behind the skirt when a springer is fired, vs the pressure from a CO2 or PCP air rifle, only the springers tend to expand the skirt very much. I don't own a PCP, but anecdotally hear many PCP users prefer the CPL's. They perform consistently because they are a much more uniform weight and, having a less fragile skirt, don't tend to get bent skirts from rough handling in shipment. They are also pre lubricated. I know they also perform well in all of my CO2 guns with the exception of the Hamerilli 850, which prefers RWS Superdomes exclusively.
For springers, the Beeman FTS pellets are the better of either the CPL's or JSB's, however. FTS pellets have a much more uniform weight out of the tin, and have a longer, thinner skirt like the JSB/Wolverine pellets, with good down range ballistic qualities. The FTS pellets will usually out-perform either the CPL's or JSB's in my springers when just shooting unweighed pellets out of the tin. They are my favorite .177 pellet.
There are fans of Gamo Hunters out there -- I have tested those as well. The Gamo's have excellent uniformity of weight, but they are swaged rather than die cast, are a softer lead prone to damage, have a shorter skirt and a different center of gravity than the longer diabolo pellets. They also tend to have a lot of small, loose lead fragments in the tin. They shoot with very consistent velocities and perform well out to about 25 yards, but beyond that, at least in my testing, were consistently less accurate than the JSB, CPL, or FTS pellets, holding no better than a 4 inch to 5 inch group at 50 yards.
Bottom line:
General short range plinking - doesn't make a lot of difference, but Gamo Hunters are cheap, readily available, and are a pretty good pellet.
CO2 and PCP's - Probably hard to beat the CPL's out of the box, except for rotary clip multi-shot CO2's like the Hamerilli 850.
Most accurate for springers at long range field target shooting -- FTS or JSB Exacts, but Beeman FTS having the edge out of the tin. If weighed, sorted and inspected for any dings in the skirt, JSB Exacts are the top performing pellet.
But, all that said, individual guns may vary as to the pellet they shoot the best, so no pellet is always the best in any given air rifle.
