The reason you can drop down too 7s with Hevi-Shot type loads is because of the physical weight of the metal the pellets are made of(tungsten iron mix). Size wise they are sized like lead, but because they weigh more (than an equally sized lead pellet) they result in more knockdown and penetration when driven too equal velocity. YES the 7s will kill like lightening! They do have the knockdown of lead that is 1-1.5 larger sized lead. Since the shot is smaller you can get more pellets in a equal weight load compared too lead (like 2oz of lead 6vs 2 oz of HS 7s) but because of the shots heavier metal you don't give up knockdown.

Nitro Ammunition (Mtn. Grove, MO) also makes a straight 7 load. I stock them and have them from 20ga too 10ga in every size shell. They are expensive tho, $5-$6 a piece! But for those who use them, you know there is NO substitute!!!

LOL, "Nitro Ray" also makes a 3oz 10ga load with nothing but 7s. Talk about an OBSCENE pellet count. Over 1k!!!
RA