I think the thing that morphine lacks and heroin has is simply placebo because biochemistry doesn't lie. It is a morphine prodrug. No morphine, no heroin. Call it what you want - diacetylmorphine, morphine acetate, diamorphine - whatever you want to call it, in the end it's all the same it's a quicker acting form of morphine. It's morphine that crosses the BBB, it's morphine that binds to the opioid receptors, and it's morphine that provides the subjective 'high' that we all get when we shoot up "heroin". Heroin exists only in the external form, it breaks down to morphine in the body. 6-MAM is a very, very minor metabolite that quickly breaks down to morphine itself.
I have always understood heroin to cross the blood brain barrier before being metabolised to morphine, and it crosses it quicker than morphine does, this leads to a more intense rush than is associated with morphine, it also seems to cause a significantly less intense histamine reaction where I have often read of the rush of morphine being a "prickle rush". 6-MAM does metabolise to morphine but it is slightly different pharmacologically than morphine and ofcourse is going to lead to atleast a slightly different high before its metabolism. Codeine is metabolised to morphine but I have done countless cold water extractions as well as used morphine on multiple occasions and they do not feel identical at all, although similar.
Heroin is also more potent per milligram than morphine, it is also a great deal more soluble in water and therefore more convenient for injection. You must also consider that street heroin will often contain active adulterants like diphenhydramine for example, or in the case of black tar heroin can contain more 3 and 6 MAM and other assorted acetalysed opiate alkaloids than diacetylmorphine. Illicitly manufactured morphine is not really available, and most people injecting it recreationally do not have access to ampoules, this leads to a more complex procedure involved in prepping a shot and often a noticeable amount of morphine not making it into the shot.
If you are comparing pharmacological morphine to pharmacological diacetylmorphine then there is little difference, but it is undisputable that there is one. When it comes down to end user experience, the overwhelming majority will be using street heroin, and that experience is certainly a great deal different than morphine.
Whilst it is certainly true that most halfway serious opioid enthusiasts that have tried both morphine and heroin will enjoy both, most seem to have a preference with the majority leaning towards heroin, there are obsiously reasons for this. Even if you consider heroin nothing more than a more efficient delivery method for morphine than morphine itself, how is that not enough to mean its significantly superior? IV use is a more efficient delivery method of morphine than oral morphine, and is largely thought to be infinitely superior.
For the record I don't shoot up and whilst I love opiates and have tried a good deal they are by no means the class of drugs I am most experienced with, however I have done a great deal of reading and still considerable experimentation with them and that is just my take on it. Every time I ever used heroin felt different to any time I used morphine, personally.
