It sounds like you know more about gun laws than I do, so your input is appreciated. Still, I'm pretty sure that transporting a disassembled weapon is safer than an assembled one, both due to it being easier to conceal and presumably lesser consequences for getting caught with it.

It would be much more difficult to sway a jury against someone who was carrying a disassembled firearm than one who was carrying a fully assembled one, and I'm sure the authorities know that. While most criminal cases are resolved through plea bargaining, rather than actually going before a jury, the outcomes of those plea bargains are largely determined by how the involved parties think a jury would rule on the case -- and it would be far easier to convince a jury to convict someone for bringing a fully functional weapon into D.C. than for carrying the parts to a weapon that they may or may not have had any intention of assembling.

Sure, they might still ARREST you for carrying that disassembled weapon, but I find it hard to imagine that they would be able to spare the resources necessary to prosecute thousands of such cases (not to mention the difficulty of detaining / moving such individuals). So, either you get your disassembled weapon in with you, or the cops have to waste massive amounts of time searching hundreds of thousands (or even millions!) of people for relatively-easily-concealed PARTS of weapons, just so that they can temporarily detain those people and release them later without charges (and without their weapons, I'd guess, which DOES suck).

Feel free to take my opinion with a grain of salt, though; I'm just a law student, not an actual lawyer. If you're planning to wager your freedom by bringing a "modern" firearm into D.C., you may want to consult a lawyer first (or a kindly lawyer pede could help us out here!).