I traded my well-tuned .22 Sumatra 380cc rifle at the recent Texas airgun show for a stock .25 Sumatra 500cc rifle. Knowing full-well I'd have to tune it to achieve a bell-curve power-band, I hoped the .25 might shoot as well with 34 grain JSBs as the .22 did with 18.1 JSBs, but also return as many competition-worthy shots per charge at around 60 foot-pounds as the .22 did at 45 foot-pounds.
Besides a good barrel cleaning, my tuning of the Korean maxi-blasters primarily entails cutting about an inch of length off the TOO-strong hammer-springs, moly-lubing the hammer and hammer-spring, and a lot of chronograph and accuracy testing. The shorter (or a weaker) hammer-spring also lightens the trigger-pull; then typically within a couple-hundred rounds the guns gain a little velocity as the moly-lubed hammer and spring slickens the firing action.
Today's chronographing showed not only that the sweet-spot fill-pressure of the tuned Sumatra has increased from 2500 to 2800 as the tune 'slicked-in', but velocity standard deviations have tightened considerably. At that happy epiphany I decided it time to reduce the sear-engagement, which further improved the previously-heavy trigger-pull.
By then it was near sunset, and the normally hideous winds at my place cooperated quite nicely by dying completely. That was just long enough to shoot four consecutive six-shot groups at 100 yards before dark; that being a full 2800 PSI charge returning a 24 shot power band that looks like this with 33.95 JSBs- Low= 901, High= 927, ES= 26, SD= 4, Average= 916 FPS and 63.3 foot-pounds.
And the four consecutivesix-shot groups at 100 yards that averaged 1.50" center-to-center looked like this-
SUCCESS!
Though none of the Korean maxi-blasters will return a bell-curve power-band when stock, with some judicious de-tuning to still magnum power levels they can be made to perform as well as most of the finest regulated PCPs out there... albeit in a much less civilized manner!
Besides a good barrel cleaning, my tuning of the Korean maxi-blasters primarily entails cutting about an inch of length off the TOO-strong hammer-springs, moly-lubing the hammer and hammer-spring, and a lot of chronograph and accuracy testing. The shorter (or a weaker) hammer-spring also lightens the trigger-pull; then typically within a couple-hundred rounds the guns gain a little velocity as the moly-lubed hammer and spring slickens the firing action.
Today's chronographing showed not only that the sweet-spot fill-pressure of the tuned Sumatra has increased from 2500 to 2800 as the tune 'slicked-in', but velocity standard deviations have tightened considerably. At that happy epiphany I decided it time to reduce the sear-engagement, which further improved the previously-heavy trigger-pull.
By then it was near sunset, and the normally hideous winds at my place cooperated quite nicely by dying completely. That was just long enough to shoot four consecutive six-shot groups at 100 yards before dark; that being a full 2800 PSI charge returning a 24 shot power band that looks like this with 33.95 JSBs- Low= 901, High= 927, ES= 26, SD= 4, Average= 916 FPS and 63.3 foot-pounds.
And the four consecutivesix-shot groups at 100 yards that averaged 1.50" center-to-center looked like this-
SUCCESS!
Though none of the Korean maxi-blasters will return a bell-curve power-band when stock, with some judicious de-tuning to still magnum power levels they can be made to perform as well as most of the finest regulated PCPs out there... albeit in a much less civilized manner!
