Sorry, I think you're missing my point that is even if somehow they could flush the entire amount of the illicit substance in time, they probably have other items on hand that would be just as good from an evidence perspective.
For example, you can't flush a scale down the toilet.
If we're talking low-level or mid-level dealers, chances are they have a quantity of pre-packaged amounts of their product on hand and maybe even a firearm or two. Unless they can quickly melt-down and/or liquify all of that stuff, it isn't going down any toilet pipe while the homeowner argues with the cops at the door about the status of a warrant.
A scale with a bit of powder on it does not demand 10+ years in prison. A gun, if registered (and it's not like all dealers have illegal guns) carries no penalty in and of itself.
They (the cops) need something to barter with. You're much more likely to get useful information from someone facing 10+ years than someone facing 1 year (6 months with good behavior) or probation. They want to find everything.
In most of the US, and I suspect for most of the population, there is no such thing as registering guns (that would be akin to registering laser printers, especially those high capacity ones).
Interesting choice of example, as it happens there is in fact a "printer registry", where documents can be traced back to the device that produced them: https://w2.eff.org/Privacy/printers/docucolor/
(it's just that the average consumer doesn't know about it)
To the extent there's any tracking of printers, it would at most tell you which store it was sold from, there isn't any required serial number recording done at checkout (well, as of when I last bought a color printer in 2006).
What you're referring to would be akin to comparing a bullet found at the scene of a crime with one fired from a seized weapon, albeit easier; a comparison system. Much less of a privacy concern, without any required "registration" involved.
Now you're onto something! Your first point is true. If the suspect manages to dispose of even some of the substance, that could hurt the case. Or it may not. There's lots of factors, I was just giving the simplest example I could think of (trying to flush a sufficiently large amount of cocaine).
To elaborate on your own example though, pretty sure that even if a firearm is legally registered you can still get in trouble for having it around, especially loaded, in the context of selling illicit substances.
That is almost beside the point, though. How many dealers do you think are using registered firearms to protect their trade? Of that group, how many are registered to the dealer using it and not some random family-member or whatever?
You'rd right; I can't speak intelligently on the number of dealers using illegal firearms. It's about not leaving it up to chance though. They may know, to a relatively high degree of confidence, that the drugs are there, but they may know little else. You don't want to raid a home and come up empty handed.
I can't imagine a scenario in which finding a small percentage of the drugs wouldn't hurt the case though. If you can't find it then you can't charge them with possessing it.
For example, you can't flush a scale down the toilet.
If we're talking low-level or mid-level dealers, chances are they have a quantity of pre-packaged amounts of their product on hand and maybe even a firearm or two. Unless they can quickly melt-down and/or liquify all of that stuff, it isn't going down any toilet pipe while the homeowner argues with the cops at the door about the status of a warrant.