I agree J frames can be sufficient as a BUG gun, if I did not, I wouldn't carry one for that role. I like mine a lot, it fills that role in much more comfortable way than anything else I've carried.
I also agree with the 21-30 foot rule. It does not take but once seeing a human being gutted, this time with a box opener, trying to hold their guts in to convince you of the threat a knife constitutes.
That said, I'm reading some of the 30 plus yard J frame comments with a twinkle in my eye...maybe you can, maybe, in a perfect environment. The 7 yd comment is aimed at the stress filled environment you are in when confronted with a deadly weapon and jerking the trigger rather than slowly engaging it as is done in stationary practice. I have seen a crime scene where two officers who were "good shots" in qualifying and a BG dropped 70 plus used pieces of brass within 4-7 yards of each other and no one got a scratch. The moral being adrenalin makes us all lesser shots.
Me personally, I want all the accuracy and round advantage I can get, which is why a J frame will never be my sole carry unless it's impossible to carry anything bigger.
As others have said, the more realistic J frame scenario is your J jammed in the guys gut going off while you are defending with your other arm. The guy's going to have 30-60 seconds of 100% performance unless Head or CS shot before your rounds affect him. Plan on defending and minimizing the damage, not dropping him and walking off untouched.
JMO YMMV.