Pharmacology - Bioavalibility Question - Opana

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SWIM has used opiates for a long time and has a very high tolerance. He recently came across some Opana 20mg ER and simply chewed them up like he has done oxcontin in the past. After not getting any effect from them, he researched and found that they only have about 10% BA. This bein the case, why are these pills so popular and highly sought after ? How can a pill with such poor BA have such good pain relieving qualities ? I now know now that the preferred route of administration is snorting but what about all the people out there that take them as prescribed, do they just not get much reief ? Just because a medication has a low BA does that make it "not as good" or not as effective for management of pain ? Any enlightenment would be greatly appreciated.