I think that its variations in the stuff. I get way fatter rushes from stuff that is d,l-methamp (this is the stuff that comes in small chunks and powder, although d-amp can be powder as well). I think it also has to do with the fact that alot of the times the largish shards one might buy, while they are d-methamp they can very easily be cut with MSM and look just as good if not even more "quality" if you dont know what you are looking for. While I've found that with d,l powdery "crank" type stuff, alot of the times its almost completely uncut, dissolves instantly and it gives a huge body rush while the d-methamp is completely a mind rush in comparison to physical effects.
If you dont feel a shot then take another one, sometimes when I take a shot it gets me "on that level" of feeling good, and then I take another one and I get some rushing effects. You can get rushing effect from d-methamp but the d,l stuff is the one that knocks your socks off.
Good dope with little to no MSM, I have noticed that when preparing a shot, the crystals should dissolve really fast like just mixin em aorund gently and crushing them a bit by pushing down should be able to be done with not much force. The MSM dope ive found it sometimes splatters when I push down to crunch it up, and it crunches into a bunch of different tiny little shards that I have to keep crushing down so that they dissolve. Also stuff that has alot of MSM will be way more "oily" type of solution, meth hcl in water gets "thick" but its not the same type of "oily". Bad description I know, but if you preped a few shots/diff products youd know what im talking about.
I have had certain dopes that just blew me away physically, but left me not as high after the rush was over. Very strange, I dont know if perhaps they add some other chemical that gives you that type of peripheral effect or if it is maybe like, 20:80 D:L or 30:70 or whatever. Maybe it was just me taking bigger shots of this certain batch when I had already been up and using for a long binge.
Also if you blazed or drank beforehand then the rush can be increased in my exp.
I have been told that if a shot "doubles up" ie: it goes from 0,2 mL water to 0,4 mL water + dope, then its good shit. However its just a rule of thumb, i think they mighta thought that hcuz they usually got really cut powdery shit and when they got shards of quality d-methamp cut 50/50 with MSM it was way stronger than their usual stuff. The reason for this - meth HCL readily forms a solution with H2O, and adding solutes to a solvent doesnt increase the volume. However particulate matter that doesnt 99%+ dissolve right away like meth, stuff like MSM that tends to need to be crushed down to smaller and smaller litttle shardlets til they are not visible to the naked eye, some may be dissolved but i think alot of the MSM remains as particulate matter which will increase the volume of the liquid carrying it (in this case H2O)
Lastly, the rush is increased the less water you use. I use somewhere between 20-25 units usually. I have seen people use 10 units for big shots, but sometimes i lose a drop when preparing it and I wouldnt like to waste twice as much dope when that happens.