There's something in TSS that I find completely unbelievable. It is how Schrenk beats Q to the gun in that showdown scene in Schrenk's apartment.
Let's review what we know about Schrenk's physical condition to that point in the novel. Despite Dr. Steinberg's guarded diagnosis ("he tended to hobble"), outside the apartment Q sees it's much worse, calling Schrenk "half-crippled." Moreover, the motor coordination in Schrenk's upper body is severely hampered; when he drops a whisky glass, he is said to have had to try to pick it up by laborious means, "...his hand swinging like a hook till his fingers connected with the rim; then he put the glass back onto the stool with ostentatious care, though it rattled to the trembling of his hand before he could stop it" [14:126]. Further, we are told that Misha lights Schrenk's cigarettes for him with a familiarity that tells of long practice, implying he would find it difficult to do that for himself. Finally, Schrenk's mental processes are, despite Q's high praise, somewhat disordered: he's depicted as making strange interconnections between Detsky Mir and Lubyanka, and his construction of the story of the woman telling her child to go play with the doll is bizarre in the extreme (although it's one of the most moving and powerful passages in the Q-orpus).
Given all this, I don't see how, if Q were really trying to stop him, that Schrenk would be able to get to the table, open the drawer, pull out a gun, and draw a bead on Q while he was still far enough away not to deflect it. I don't care how well-trained you are: if you can't pick up a glass in the usual fashion, or light a cigarette without your hand shaking, you aren't going to be able to move fast enough to get the drop on someone of Q's superb reflexes (remember, Q's at least as well-trained as Shapiro).
One possibility that I'm finding increasingly attractive is that Q "lies down" in the apartment, symbolically exposing the jugular to Schrenk out of respect to the man he was before being so hideously tortured. In other words, he really doesn't try to get to the gun before Schrenk, but rather lets Schrenk get there first, perhaps to salvage a scrap of pride, and then tries later to get Schrenk to go with him peaceably or at least let him go free. In Q's somewhat idiosyncratic view of justice, there may be a fairness (probably understood only by those in the dangerous trades) in all of this: what he gives Schrenk is a boon by letting him get to the gun first and then leaving it up to him what he is going to do with it. Q's misstep is not in letting him get to the gun, but in assuming that enough was left of the Schrenk he thought he knew to provide a governor that would prevent him from turning against his life and comrades.
Q would probably say he owed Schrenk at least that much.