The .458 SOCOM ammunition is a specialty product originally designed for use by US Special Forces like the Navy SEALs and Green Beret operations. However, they're widely used now for hunting and self-defense thanks to the combination of high velocity, high power, and compatibility with many different short-barreled rifles like the AR-15. Since this type of ammunition has only been on the market for less than 20 years, many gun enthusiasts are unfamiliar with its requirements for optimal barrel length. Choosing the right barrel length gives you maximum velocity and power with each shot, so there's a relatively tight limit to the barrel lengths that work best with .458 SOCOM ammunition.
Minimum Barrel Length
Since this ammo is widely used in pistols and short-barrel rifles, performance was tailored to a surprisingly short minimal length. Some companies sell barrels for .458 SOCOM use that are as short as 7.5 inches, and the ammunition is optimized for performance at around 9.5 inches. However, there are two limitations to how short a barrel you choose. First, there are many state and federal legal limits on how short a barrel can be on a rifle. Secondly, shorter barrels also limit your options for fluting and venting for increased performance and reduced kickback. Check the legal limits for your area before choosing a very short barrel since it can turn a legal short-barreled rifle into an illegal one without the right paperwork.
Maximum Barrel Length
It's technically possible to use nearly any length barrel with .458 SOCOM rounds. However, there is a definite point at which a longer barrel no longer increases velocity or distance, and therefore a longer barrel is just more awkward to carry and adds unnecessary weight to the gun. This point is around the 16 inch length, although 20 inch barrels are also manufactured for use with this particular ammunition. A barrel length beyond that length is pointless, so they're not available unless by custom order.
Other Length Options
It's possible to find barrels at practically any length between the 20 inch maximum and 7.5 inch minimum. 12 inch, 10.5 inch, and 14 inch barrels are particularly popular for short barreled rifles that can fire the .458 SOCOM rounds. Each length offers a different velocity and speed balance, but the differences between each size are relatively small when compared to the difference barrel size makes to other types of ammunition. You're free to choose a barrel length that is convenient for you in terms of size and weight without worrying about compromising power or velocity.
Contact a company, like Tromix, for more help.