Re: Forster co-axial press
Conventional presses, with the fulcrum attached to the bottom of the ram, and cantilevered on the opposite side of the ram bearing from the shell holder, by necessity, will advance the cartridge in a vertical arc, or at the least, not a straight line. The alignment of the cartridge to the die is thus a dynamic issue, and must be adjusted throughout the stroke. The ability of "float" to re-align the cartridge with the die is limited while force is being applied due to friction in the floating systems. The user can overcome this by pausing once or twice while advancing the cartridge into the die, allowing pressure to subside, and the floating systems to do their job more easily.
The co-ax supports the shell holder & "ram" between bearings above and below, avoiding cantilevering. The co-ax fixes the fulcrum to the press frame, not the ram. The co-ax linkage applies lateral force to the shell holder in one direction only during the stroke. Thus, all play in the ram is taken out uniformly and consistently throughout the stroke, resulting in a straight line vertical travel, making it easier for the floating systems to adjust the alignment of cartridge to die.
As with many things in reloading, technique has as much to do with accuracy of results as equipment does. Some equipment makes the necessary technique easier to apply.
The Redding Big Boss is an excellent press, but I would consider the Big Boss II variant more desirable, since it has a hollow ram to allow spent primers to be captured more consistently and easily. The co-ax takes that one step further, since it does not have a priming arm in the ram, it never allows primer debris near the bearings of the press while efficiently depositing them in a catch bottle.
As mentioned earlier, the ergonomics of the two presses are quite different. Most users prefer to use the co-ax while standing, making it easier to deal with the handle travel down the center. If you prefer to sit while using your press, this may not be the best for you. I personally like the higher position of the co-ax handle and its range of motion.
As with all things, YMMV, etc.