Speedball has a full shot cup like all hevi ammo,the cork ball creates compression its how speedbal attains 1650fps and the flax seed is used to pack the pellets tight/filler with crimp instead of a buffer.
Last couple years Hevi Shot had rebates,$5/box and $50/flat (10boxes) for hevi metal which makes it more affordable@$20/box plus the rebate,but yeah regular hevi shot is way spendy@$4/shell and no rebate.Hevi Shot tells me the chinese are buying up all the precious metals they can for the military driving up the cost of tungsen iron and other metals.
Hevi Shot pellets are made up of "tungsen iron,nickel and tin" hence the heavy dense pellets 10% heavier than lead pellets and heavy price tag,but man does the load give ya lethal range,so much so I rarely shoot 3.5's other than shooting the 10ga.Today I use what hevi shot I have left for the 3rd shot if needed,hevi metal does a great job as it is at $.75/shell.
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remm,3" hevi metal in 11/4 oz loads is 1500fps,I do know when we tracked target loads crossing shots@ 40yds the difference in lead between 1200fps and 1350fps was 1",so not a big diff at all if you compare 1500fps to 1650fps.