The weight of an ammo will dictate the twist of the barrel used with it. Look at the type of ammo you intend to use in order to make sure that you purchase the barrel most suited for it.
1:9 - This twist is versatile and is commonly found on civilian rifles. Ammunition between 40 and 55 grain can be comfortably stabilized by this twist. Ammo within this weight category includes match grade bullets used for competitive shooting as well as ammunition used for plinking.
1:8 - The balanced, “all around choice.” Truly, a barrel suited for Goldilocks- not too heavy, yet not too light, this twist is just right. The supported range is relatively narrow, best compatible with ammunition grains between 50 and 77. This twist makes up for it with a superb velocity spectrum, meaning that it is compatible with almost any barrel length within reason.
1:7 - The choice of military outfits around the world. This twist is capable of handling some of the heaviest of bullets at the highest of velocities. It is best optimized for ammunition above 55 grain, however it can handle lower grains with only a minor drop in efficiency. It’ll handle almost whatever heavy ammunition you throw at it, all the way up to 77 grain projectiles. If you’re wanting to pack some serious stopping power, the 1:7 is the way to go.
Whereas weight of your ammo will dictate the twist, the type of ammo will end up affecting the chamber you will want.
.233 - This chamber is designed to house .233 rounds, which are the civilian baby brother to the 5.556 NATO. Due to the smaller size and lower pressure, the .233 is capable of handling solely .233 rounds, making it a specialized choice. Generally this chamber will be harder to find due to this.
5.56 - Is capable of shooting both 5.56 NATO rounds as well as .233 rounds. Since the 5.56 NATO is a standardized government round, it is much more easily accessible and can be bought in large quantities for cheap. Because of this, it is a popular round.
.233 WYLDE - This chamber is similar to the 5.56 in that it is capable of firing both 5.56 NATO and .233. However, in terms of construction, the .233 WYLDE has a tighter chamber than that of the 5.56. For someone looking for optimal performance with maximum versatility, .233 WYLDE is a safe bet.
Length from the chamber to the gas port determines the category of gas system length. The four main ones to focus on are Pistol, Carbine, Mid, and Rifle. Each gas system works best with certain barrel lengths, however it is possible to use a shorter gas system on a longer barrel. General longer gas systems should be used with longer barrels as they reduce the “dwell” time of the bullet, resulting in less felt recoil. The length of your gas block will also dictate with fore-end you will want to use with your rifle.
Pistol Length - These gas systems usually measure around four inches long and should be used with barrels under ten inches. Be careful with using a barrel that is too short however, if the gas port is too close to the muzzle it can result in firing malfunction and damage to the weapon.
Carbine Length - The length of the carbine length classified gas system is around seven inches. Any barrel length between 10 and 18 inches fall can comfortably utilize a carbine length gas block. Anything past 14 inches will see a slight rise in felt recoil, but only minimally.
Mid-Length - This length is only a tiny bit longer than the carbine length, measuring in at 9 inches. Despite this, those two inches do make a significant difference, allowing for barrels between a length of 14 to 20 inches. Mid-length works well in this range and helps bridge the gap between carbine length and rifle length.
Rifle-Length - If you have need of a gas system for barrels over 20 inches, then rifle length is what you are looking for. These gas blocks are 12 inches long and can support anything past 20 inches, within reason of course. If you’re looking to build a rifle with a long barrel, this is the key word you want to look for.
The barrel profile is the silhouette of the barrel as viewed from the side. Some will say that profiles are mainly preference, however they can make a difference in the performance of the gun. Some profiles are heavier and thicker than others, resulting in increased weight, but also better heat dispersion.
SOCOM/Government - As you can imagine, this is the standard profile on military rifles. It is a relatively lightweight profile with some added weight near the front of the barrel. The SOCOM profile will not handle long, sustained firefights very well due to it’s relatively thin construction. Despite this, there are still other choices which can reduce the weight of your build even further. Think of the SOCOM as a good balanced option which leans on the lighter end of the spectrum.
M4 - The M4 is the heavier big brother of the SOCOM profile. Another military profile, however this one is generally associated with carbines. It is a bit larger and also is designed to fit add-ons such as a grenade launcher. Not as practical for a civilian rifle, however it has a stunning and timeless look which might just be what you’re looking for.
Pencil - To touch upon lighter builds- pencil profiles do their best to minimize unnecessary material. This results in an incredibly lightweight and thin frame. Unfortunately, this means that it handles heat even less well. If you want a highly portable build which isn’t expected to see use to prolonged encounters, then pencil profile is the way to go.Proprietary - In addition to the three archetypical profiles, gun companies will often come up with their own designs. These include profiles such as the FAXXON Gunner and the Ballistic Advantages Hanson. Generally these profiles are either made for a highly specific purpose in mind, or solve common problems through unique solutions. If you are interested in a proprietary profile, the makers of said profile should have some form of documentation available.