Hey, Bob,
Simple answer is that heavy pellets can (do?) shorten spring life, especially with stock springs. And Bob's cut off is 9 gr. BTW. Jim Maccarri agrees, as far as the stock springs are concerned. He says heavy pellets require a spring wire diameter of at least .128". Or to put it another way, if you want to shoot heavy pellets, you need one of his springs :-). Not to be construed as a slam at JM, it's well known that he sets the standard for springer springs, and knows what he's talking about. My CFX busted it's original stock mainspring after less than 3000 pellets, of which less than 100 were over 9 grains. I had some Beeman Silver Arrows, 11.5 gr. that I tried. Since they shot like crap, I didn't try too many. As I recall, the stock spring wire is .117". Superdomes weigh 8.3 gr. on average, and I know from some of his other posts that Gene shoots Superdomes in almost everything. I guess the question has to be, does your CFX shoot the heavier pellets enough better to be worth breaking your main spring? And do you have a chrony to test what's actually happening? I've never shot Kodiaks in my CFX, but in my Diana 350 .22, the Kokiak is 21.1 gr., the Superdome is 14.5. Kodiak did 673 fps and 21.2 fpe. S'domes were 828 fps and 22+ fpe. I'd expect similar comparisons to the .177 versions. If you're hunting with your CFX, and Predators shoot well, what do you even need heavier pellets for? If you need more power, get a .22, or a PCP. Or both. Anyway, that's the info I have, and the opinion to go with it :-). HTH.
Dave
