All three versions of the stargate have something that moves - with the Milky Way gates, it's the inner ring, the Pegasus gates move lights, and the Universe gates rotate the entire gate. I've noticed that sometimes the gates will rotate directly to the symbol (or chevron, in the case of Pegasus gates), but sometimes they'll do a complete rotation before locking the symbol. My question is, is there a rule for when the gate will make a full rotation before locking, or is it random?
In the original film, the first time they dialed:
We can observe this sequence of rotations (each number is the direction of rotation before that chevron is shown locking):
- Chevron is not shown to lock.
This is not the same as "whether it makes a full turn," but we are not given that information explicitly. Based on the speed at which the ring is rotating and the amount of time that elapses between chevrons, I estimate that all of the chevrons, except perhaps for 4, get at least one full rotation. By my count, chevron 4 is not shown to lock, so it's possible that it locked immediately after 3 while Daniel was giving exposition and distracting the camera.
Also, they claim that "chevron 1 is locked in place" while the ring is still rotating. It then stops, presumably locks a chevron, and reverses direction, and that presumed chevron is not counted in subsequent dialogue. So I assume this was just a timing glitch and that chevron was intended to be chevron 1.
The film is not usually regarded as being in continuity with the TV shows, although to my knowledge they are not explicitly stated to be set in separate continuities.
I have not been able to find comparable clips from the TV shows. There are a lot of unofficial fan videos containing recreations of various Stargate dialing sequences, but I reject these as unreliable sources. In later episodes, they unfortunately figured out that the vast majority of the audience does not enjoy watching the gate dial, so the viable source material is mostly the film and early SG-1, which also rules out most information about Pegasus and Universe gates. Even the first time they dialed Atlantis, the full dialing sequence is almost entirely taken up by shots of things other than chevrons encoding or the ring spinning. However, if someone can find a clip from Atlantis or Universe, I would be very happy to be proven wrong on that point.
Out of universe: The total length of the dialing sequence is long enough to fit in any expository dialogue the writers need, and otherwise of whatever length the episode requires. So if the episode is running short, they can run all seven chevrons and make sure each one takes at least ~5 seconds to spin (by forcing a full turn whenever it would otherwise be too short). Together with the actual locking and other extraneous cuts (to the computer, to the characters' faces, etc.), this can easily add up to multiple minutes of padding. On the other hand, if they are running long, they can cut most of the dialing sequence entirely since it has little to do with the actual plot. The point is that they only have to decide which to do in editing and post production, which means they have greater flexibility in constructing and timing the rest of the episode.
After watching a few more videos (watching a handful gave me this question), it seems to actually have a concrete answer.
The rule in the Pegasus galaxy seems to be that the lit symbol must travel past chevron 7 before it is able to lock. I was unable to determine whether or not the 7th symbol locks the first time it encounters chevron 7, given that all of the dialing sequences on YouTube either didn't show the gate during this time, were interrupted by an incoming wormhole, or didn't go in the proper direction. If we ignore the last constraint, the 7th symbol locks upon the first pass of chevron 7, however I expect that dialing following all the other rules would require the 7th symbol to pass chevron 7 once before locking.
The Universe gates seem to require that the last symbol travel past the top of the gate before the next symbol can lock. I couldn't find any clips on YouTube showing the final lock, but I believe it's safe to assume that the pattern continues since the Universe gates lock symbols rather than chevrons.
The SG-1 gates are the hardest of the three to determine, given both that the symbols are hard to see on all the clips I could find and that most of the shots during a dial don't have the gate anywhere in them, but watching the speed at which the ring rotates and listening to the amount of time between each chevron locking in multiple clips, I believe it is safe to say that the dialing program used by the SGC simply moves the necessary symbol to chevron 7 without any extra rotation involved.
The gate in the movie is inconsistent (as Kevin noted), which I expect was done to create suspense, but makes this question nearly impossible to answer. However, given that the movie gate and the SG-1 gate are supposed to be the same device (minus the retcons), I believe that it is safe to say that the dialing program simply moves the symbol to the proper chevron and locks it without any extra rotation.