system
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human
[Star Trek] Why are transporters not used as weapons?

As I start my tactical training at the academy I can't help but wonder at the oversight of the tactical utility of transporters.  

* Methods of transporting even antimatter have been known since the days of Kirk, and lower-grade explosives and weapons are routinely transported.

* Even with the supposedly big bad Borg, our ability to routinely access their vessels via transporter, even to the point of carrying out assaults and kidnapping their leader during that Locutus thing... for as adaptable and scary as the old timers say they are, I don't believe I heard they ever adapted to this.  So why didn't the Enterprise simply beam over their compliment of photon torpedoes, the antimatter storage, or even the warp core?  Or override protocols and beam matter into the enemy's antimatter storage or weapons systems?  

* If Borg can be unwillingly transported why not repel Borg (or any) invaders on a ship by simply slapping on a pattern enhancer and dematerializing?  

* Sure, most of their doings are way to classified for the clearance level of a cadet, but I heard something about one of the early enemies Voyager faced... the Kayzon or something like that?  Anyway, space-fairing race that couldn't even replicate water???  (Or even harvest hydrogen and oxygen enough to manufacture it?  Sheesh!)  So clearly no understanding of any of the principals of replication/transportation, yet were somehow a tactical threat to an Intrepid class vessel?  They want water, exchange all the air in their ships' haul with it!  

I can't wait to to sit up front and ask Admiral Shelby about this when she does her big Borg guest lecture in a couple weeks!
gpt
Transporting is more difficult than you seem to know - looks like you'll be spending a lot of extra time reading your engineering handbooks!  First off, transporting requires transponder signals - a physical beacon - in most cases to be done easily.  Starfleet officers have these embedded in their communicators.  Transporting is possible without them, of course, but if you don't have one, transportation engineers must spend a lot of time and effort correctly calibrating the transporter for each transport, which is difficult enough when there is no high-stress situation.  In combat, much more difficult!

Next, most shields block transporters.  The Borg were actually uniquely weak in this regard, since Borg vessels didn't use conventional shields, instead relying on their adaptation technology.

Also, transporting must done between two locations that are stationary relative to each other.  During combat, that's very unlikely, and ships in combat are moving fast enough that while you might TRY to transport a photon torpedo into the enemy ship's engineering, by the time the transport is completed, the enemy vessel may have moved from that location.  In order to accurately transport something onto a ship that isn't stationary relative to yours, you'd have to correctly guess where the ship would be, and "lead" it, so to speak.  But that assumes that you'd know exactly how long the transport would take.  Transport times can vary wildly, however, depending on certain kinds of interference - that are very likely to be present during a battle.  And if the transport fails?  Well, now you have unstable antimatter or an armed torpedo in your OWN transporter bay.

So while there are some tactical applications to transporting, they generally apply to sabotage, not to actual combat.