A set of straight-line dies is worth the added expense if you’re shooting long ranges where you need accuracy of the highest order.
Straight-line dies are the oldest type made for reloading ammunition, invented long before the modern reloading press.
I can hear the first complaint already: they are slow to use! True, but if the quality of the ammunition produced might lead to tighter groups, would you still complain?
This type of die has two advantages over the normal two-die set.
Firstly, there’s no expander ball that might pull the neck of the case slightly off line when the case is pulled over it.
Thus, the size of the neck can be precisely controlled via a removable bushing. The size of the bushing used is nominally .002 inches less than the loaded round. Currently, there is some experimentation going on about neck sizes per se. Necks are reduced in size and then expanded with a very accurate mandrel, cut to four decimal places!
Secondly, with the seater die all components are kept in a line during the process; the projectile cannot enter the case mouth at an angle, hence less runout.
When seating projectiles with normal dies it is possible for the projectile to initially enter the case well offline. In extreme cases the neck of the cartridge may be damaged.
I first used straight-line dies around 2010. I had a very accurate Savage Long Range Precision Varminter in .22/250.
Initially, I used normal dies for reloading, then obtained a set of LE Wilson straight-line dies. Groups immediately tightened up and I have been using straight-line dies for all of my long-range cartridges ever since. Wilson dies and components are available locally through BRT Shooter Supplies.
The other company that provides a sleeve-type benchrest seating die is Forster. I have several sets of these and they also keep everything aligned when seating projectiles.
By keeping all components square during the seating process, runout is reduced, a known accuracy aid.
This particularly applies to longer, heavier projectiles often used at long range with good effect. The speed may be less but the ballistic coefficient is often greater and the retained energy a much higher value. If they are crooked within the barrel, your potential accuracy is reduced.
Other than the so-called varmint calibres, the use of inline dies for hunting cartridges might also be of benefit.
Depending on your particular target, anything that will assist in making accurate ammunition has to be considered.
Yes, a certain amount of velocity is a requirement but any such velocity is meaningless if you cannot hit what you are aiming at.