I've been seeing some reports about the 4A-300. Reticle - that it's a revamped TDS reticle. How different is the 4A-300 vs the BRH. Swarovski websites really don't seem to provide much explanation on the difference.
any insight would be greatly appreciated
The 4A-300 is a largely discontinued reticle (at least in North America), having been replaced by the BRX/BRH. It is still available in the Z4i scopes, which are not generally sold outside of Europe.
The 4A-300 only has three subtension lines, while the BRX/BRH has five primary subtension lines, and four intermediate "dots" for a total of nine points of impact (not including the center of the crosshair). The spacing of the subtension lines are different between the reticles - entering the same load data, with the same magnification, and the same scope with yield different point of impact values.
In short, the 4A-300 was developed for the European market, for calibers that are less flat-shooting (i.e. 30-06), at shorter distances. As an example, using the 4A-300 reticle will give you a point of impact value of under 400 yards, while the BRX/BRH will give you a value very close to 600 yards, for the last subtension line, all else being equal (scope, magnification, caliber, and load data).
Hopefully that helps.
