Hopefully, you will excuse my artwork, but shown here is a typical rigging for a shotgun logging system. The carriage is simple and needs only rider blocks for the skyline. These are the 'wheels that sit on top the skyline cable. The main weight of the carriage is underneath the skyline thereby keeping the wheels on top. These 'wheels' may be with groves just the diameter of the cable, or they may be several inches wide and tapered to a 'V' in the middle. This latter design is called a 'Tommy Moore' and is utilized to the blocks will pass splices or even shackles in the cable. The chokers will be connected to the carriage on one end and will have a slip knot to 'choke' the log on the other end.
The 1,000 to 4,000 lb carriage whistles down the skyline at a very rapid rate until stopped by braking the skidding line. To hook, the skyline is then slacked lowering the carriage to the ground. The chokers are connected to the logs, the skyline raised, and the skidding line wound in. At the landing, the logs are landed by once again slacking the skyline, and if the yarder is a swing machine, by swinging the boom to the side so as to drag the logs to the side where the loader can reach them, where they are 'hot decked' until the loader sets them out of the way or loads them on a truck for transport. Some yarder designs are only a tower which does not swing. In this situation, the tower must be set back from the edge of the hill far enough to permit landing the logs in 'front' of the tower. This means that a tower is unsuitable for working down a road with a sharp drop off at the edge of the road because there is no place to set the logs while they are unhooked which is where they can be reached by the loader. By contrast, the swing machine can be driven right up to the very edge of the cliff (actually a preferred location) with the boom actually extending out over nothing in particular, and the logs can be piled off to the side. This ability to work 'close to the edge' provides better ground clearance for the lines for any particular machine height, and this is what cable logging is all about.
We have all heard the saw in real estate that the three most important things are 1) location, 2) location, and 3) location. Well in cable logging, the 3 most important things are 1) deflection, 2, deflection and 3) deflection. Deflection in cable talk is the amount of sag in the wire. the skyline needs to be tight enough that it doesn't lay on the ground but needs sag---the more the better, but 10% is what it ought to be--minimum. This means that if you are out 500 feet you should have 50 feet of sag in the line. If you think about it, and contemplate this rule and compare it to the fact that the Washington TL-6 for example holds 800 feet or so of wire and has a 40 foot high boom, you know why loggers like to park them at the top of a 40 foot cliff. It also helps explain why logging roads always head for the high ground. Public roads and highways are often built along streams to obtain the benefits of favorable grades, but logging roads, given their preference head for the high ground as quickly as possible. It minimizes the environmental impact to the streams, while providing a preferred location for setting up the harvesting equipment.
For all kinds of good reasons (mostly forgotten), the 2 drums of a double drum yarder will not be the same. One is intended for a main line, and the other for a haulback which is what you would have if you were doing highlead logging. The forward drum will have the larger cable on it and will be the lower line on the boom. It will be geared slower than the haulback.
Of all rigging methods the shotgun system is the one preferred by loggers. It is fast because the carriage goes out at amazing speeds. It is efficient because you don't have to rev the yarder motor on the out haul. You just kick raise the skyline and let 'er fly. There is no distance that is really 'too far' for this system. The real question is whether you can get the deflection. You need to maintain the deflection and the skyline must clear the ground. Obviously, it must also be down hill all the way so the gravity system cannot be used at a greater distance than the hill is high. Beyond those criteria the question is whether the skidding line will fit on the drum or not. The main drum need not even be capable of holding all of the skyline, since you don't take it up during operations, and you can hang the extra wire on the side of the machine on hooks during transport.
The skidding line is the line which is usually the haulback line in a highlead configuration which means that it needs to be twice as long as the main line, however this is usually managed because even though the drums are often the same physical size, the haulback wire is usually a much smaller wire. For example, the TL-6 is fitted out with 5/8 Inch skidding line and a 7/8" main line.
Two variations add to the flexibility of the shotgun system. In the basic system your lateral reach is limited by the length of the chokers. That addition of a manual or motorized slack pulling carriage will add side yarding capability. Secondly road changes can be expedited by substituting a large bull dozer for the tail stump.