The thing about Morphine in an extended-release form is that you aren't getting all 15 mgs. at once. It is spread out over the course of a 12 hour period. You are essentially getting less than 1mg. per hour of morphine per pill, unless you somehow beat the time-release matrix. It's really hard to do that because the matrix is wax and the medication is in little bits, all bundled up in wax casings, pressed together. Easiest way is melting it in boiling water and skimming the wax off, then chugging once cooled. Even then, it's still really hard to get enough out of such small pills in the ER form to even feel it.
It does tend to build up, you don't eliminate it as fast as you absorb it, but you probably have, at most, around 4mg. in your system at any given time, per 15mg. pill you take.
When I used to take morphine ER, I needed 150 mg. of straight Oxy to feel it, and I usually had to do no less than 60 mg. Morhine ER to just stay off sick, never mind feel it. If I took enough to feel it, I had to take 180 mg. at once if I didn't break them down.
They really aren't good for recreational use. They are a superior pain killer, in high enough doses, just not a lot of euphoria associated with them. Even at 180 mg. ER, I would usually get pretty nauseous before throwing them up and losing them and I can do 150 mg. Oxy on an empty stomach without it even bothering my belly. Not worth it IMO, not a decent high and too expensive to bother with for such little benefit. I did them for a while just to stay off sick but if I wanted to get a good high, I had to use something else instead. I wouldn't even bother taking them if they were free, except to boost my high from something else, or to keep the withdrawal monster at bay until I could score my DOC. and yes, I agree, the buzz you can get if you do enough is more of a body high, not much in the head to speak of.
I'm in no way advocating such high doses, just sharing my experiences so you don't go through the same frustration. I only used them because they were free to me. If I was paying, I would have found some dope or Oxy.