That is not AT ALL true, I have muscled, and plugged black tar heroin, and I can say that IM is very close to IV if you do it in the right spot, like the peak of your deltoid or your ass. IM'ing BTH has a much faster onset too, I could feel an IM shot of BTH within 3 minutes, sometimes within 1 if I was sick enough.
One thing that I disagree with in the OP is about boiling the tar. I have cold cooked shots and I have cooked my shots until they bubbled, and honestly never noticed a difference in terms of potency. Now it goes without saying, you don't want to boil the fuck out of it, but if you heat it till it bubbles, that'll be fine, then stir it up. Also, boiling it makes it a lot safer for IM injection as you're more likely to kill any bacteria. At the same time you're also melting down insoluble cuts, but shit, you're already shooting black tar heroin, and chances are that in order to get all your heroin drawn up you're going to have to melt down all the inactive shit too. I read somewhere though that there are more incidents of HIV and HEP C on the EC from IV drug use then the west coast, I wish I could remember where I saw that but it was attributed to the heating of the tar heroin. I kind of find the HEP C part hard to believe though because in NY almost no one had it that I knew and then in Oakland it seemed like even the non drug using population MUST have it lol.
IM is very different from SC injection I should mention, it is not the same as a missed shot either, it has a much higher BA, a missed shot would just barely get me well, a muscled shot would get me high. It's key when you're muscling NOT to share needles with someone else, and to use clean equipment every time. I've had one abscess and its because I let a friend use a cooker of mine and he had a staph infection. While this might seem like an obvious to some, for people shooting heroin on the street, it is not, people quickly stop caring, especially when there sick, even if they are educated in harm reduction.
I personally think it's better to learn how to hit your deep veins (but learn it right) than to resort to IM'ing BTH, but either way its kind of playing with fire. However it's good there is a thread on the topic because people will do this regardless. I've seen a million people swear that they would quit the day they couldn't hit a vein and would never muscle, and did they quit? No, they muscled, or started hitting their femoral vein, Deep Brachial, or jugular. This is one of those kind of touchy subjects, but it's important to discuss, and to realize the reality is almost 100% of drug addicts are not going to give the needle a break once they start having difficulty hitting. I personally muscled the shit for about six weeks until I got an abscess and then I learned how to hit my femoral, which for the next year and a half until I quit never failed me, though it's not the kind of thing you can teach yourself, you need to have someone show you, and do it for you and then be around you when your doing it by yourself the first few times.
oh and I find plugging tar far more painful than muscling it. same with insufflation.