It's a balance thing as the link to the excellent post above alludes to. You also need to learn the gun! I also shoot heavy (no greater than 10.5's) iin several .177 guns. The spring wire diameter if it is less than .128 dia is not heavy enough to take the abuse. Especially in .177 bore , or so it is said. The thing is, there are no medium powered springers that have springs with wire that heavy. In fact there are hardly any springers that do . Even guns that are referered to as "magnums ". Then there is the issue of spring and gun quality. Personally I shoot what is most accurate in my guns and base that on shooting groups of ten shots and through the use of a cronograph. If a .177 can give velocities in the mid 700fps with a 10.5 grain pellet with better accuracy than a lighter "normal" wt. pellet, I will use it. I will list a recent example. I just tore down a delux HW, R-10 bought new by me in 1986. This gun had a slightly hotter spring in it that gave an honest 1000fps with 7.0 grain pellets. (spring wire dia. of the old factory spring is .118) It also shot poorly with them. It was also a gun reported to have a "brittle" spring prone to breakage, according to the experts. So, I've been using kodiaks , and later when they became available, CP 10.5 grain pellets in it for the last 23 years. Probably something like 20 to 30 thousand heavy pellets in all. Why? Because they shot the best, and still gave decent velocity. Recently the accuracy dropped off, so I tore it down. Seal finally went bad. The spring exhibited some cant , but was not broken. Put in a Maccari hornet seal, spring and lube tuned it. Now it likes the wonderful JSB Exacts best, a normal weight pellet (?), so that is what I will use now. Of course they weren't available when I started shooting springers, nor were there any expert tuners or forums filled with do's and don't either.
