How do I identify my Olympic Arms barrel?


First off, every barrel is stamped with the caliber. Since we make over 20 different calibers that are interchangeable on any standard Olympic (or mil any spec) .223 caliber lower, this should be the first thing you check. The standard 5.56 NATO caliber rifles are marked 5.56 and .223 REM is marked .223 while the alternative calibers are marked appropriately (9mm, 10mm, 40 S&W, 45ACP, WSSM, etc).

Note:  You can fire both 5.56 NATO and .233 REM ammunition in a barrrel marked 5.56.  You Can ONLY fire .223 ammuntion in a barrel marked .223.


The next marking, or lack of it will determine the type of barrel material; in other words, what the barrel is made of. Since we know what to look for to determine the caliber, lets go under the assumption that our barrel is a .223 caliber (5.56), but this same system applies to all calibers. If the only barrel stamping is "5.56", then the barrel is made of 4140 chromemoly steel. Chromemoly is a type steel that incorporates chrome right into the barrel metal itself. This barrel material is corrosion resistant, but not as much so as a stainless material. Using the 4140 chromemoly material allows us to give the consumer a barrel with similar benefits of a chrome lined barrel, but maintain consistencies in the bore that allow for superior accuracy. Our studies have shown that without a doubt chrome lining of any barrel will deplete it's accuracy.

If there are any stampings other than the caliber, they will answer other questions that we will address later.

If after the "5.56" markings, if you also see "SS", then your barrel is manufactured from 416 stainless steel. This material is very resistant to corrosion, and is universally accepted as one of the best barrel materials available. Even though this barrel is stainless steel, because of it's carbon content it is slightly magnetic. So if you have just tested your barrel and found that a magnet sticks, don't panic, this is normal.


Every barrel that we manufacture is of Match grade quality. In other words, with all of our rifles, regardless of type you should be able to around achieve 1" groups at 100 yards, using a good factory Match ammo or custom hand loads. This even includes our 16" guns.

Our standard barrel is button rifled. If you barrel is button rifled, it will have no special stampings to indicate such. It will simply have the normal stampings as stated above, and is chambered in 5.56 NATO. If your barrel is other than buttoned, the stampings will indicate so. All buttoned barrels have 6 lands and 6 grooves in the bore.

These barrels are our most accurate barrels, and quite frankly, these are the most accurate AR-15 barrels on the market. These barrels have Broach-cut Rifling. What is that you say? Well, broaching is the method used to create the lands and grooves. For these barrels, we use the stamping "SUM" which means Stainless Ultra Match. SUM barrels are all chambered with minimum SAAMI 223 Remington reamers. These barrels can also be identified by looking down the bore. All of our broach-cut or Ultramatch barrels have only 4 lands and 4 grooves.


This is probably the single most important factor when purchasing a rifle or rifle barrel. The twist rate that will best suit you is determined by the weight of bullets that you will be shooting. The wrong twist rate for the particular bullet weight you are shooting can be disastrous to your accuracy. Many articles , and tons of information has been written about twist rates and what suits the bullets you plan to use best. We won't discuss that here. Your best bet is to call the bullet manufacturer for their recommendation.

We have 2 standard twist rates. One for the buttoned barrels (standard match), and one for the broached barrels (Ultramatch barrels). If your barrel is of the standard twist, no indication will be made on the barrel as to the twist rate. If the barrel has an alternate twist rate, that will be indicated by the appropriate stampings. In other words, anything other than the standard twist will be stamped on the barrel in the formula of 1 rotation of the bullet per number of barrel inches. Examples would be 1x7, or 1x12. These would indicate 1 bullet rotation in 7" of barrel, or one bullet rotation per 12" of barrel. The lower the number, the faster the twist. The higher the number the slower the twist. Generally speaking, a lighter bullet requires less rotation to stabilize itself than a heavier bullet.

Our standard twist rate for these barrels is 1x9. One rotation of the bullet per 9" of barrel length. This is generally accepted as the best twist for all-around general plinking and shooting. It will stabilize the greatest variety of commercially available bullets. Optional twist rates for button barrels include; 1x7, 7.5, 10, 12, 14, and 16.

Our broached barrels have a standard twist rate of 1x10. Alternative twist rates on these barrels include; 1x8.5, 10, and 14.


There are three possible coatings for our standard barrels. These are black oxide, parkerized, or a bead blasted stainless steel. As a standard feature all of our AR-15 barrels are coated black regardless of the barrel material.

Carbon steel barrels like the 4140 chromemoly barrels we make are parkerized. Parkerization is a coating method that uses a hot dip chemical process using zinc as the primary active ingredient. A barrel in the white (non-coated) is plugged on both ends using corks, and dipped in a hot parkerization solution and boiled to a dark black finish. During this process the surface molecules of the chromemoly barrel react to the zinc in the solution causing the surface of the barrel to turn black. This finish is rough, dull, and very durable as it is part of the barrel steel that was turned black, and not just a coating. Parkerization is very resistant to corrosion as long as it has a light coating of oil. This does not mean that you should soak your barrel in oil, it simply means that as with most coatings on carbon steel, it will perform best if a light coating of oil is applied during general cleaning and maintenance.

If by using the information above you have just determined that your barrel is stainless steel, yet as you look at the barrel it is black, again have no fear. Since stainless steel will not react to the parkerizing solution in the same way that a chromemoly/carbon steel barrel will, black oxide was developed. Turning a stainless steel barrel black is done essentially the same way, except the we use a different chemical mix. With black oxidizing the main active ingredient is sodium hydroxide. The result of this method is basically the same looking finish on a stainless steel barrel. These finishes look the same, but you will find the black oxidized surface to be smoother than a parkerized surface. Additionally, because the black oxide coating is just that, a coating, it will not be as durable as a parkerized surface. You will need to take greater care with your stainless steel blackened barrels to keep the finish as new. Knowing this bit of trivia, will allow you a point of reference if you are perhaps making a deal on a used rifle. If it does have a stainless steel blackened barrel, and the finish is superior or like new, the weapon was well taken care of.

Having this information at your fingertips will allow you to identify any barrel that Olympic Arms has manufactured for AR-15's. If the barrel has different stampings than what is stated above, or none at all, chances are we did not manufacture it. The best advice I can give on this issue is, check the barrels before you buy, and use this information to your advantage.

NOTE: Olympic Arms, as with any firearms manufacturer, cannot be held responsible for the fit, feed and or function of firearm(s) not fully assembled at the factory. As with any mechanical device, some fitting may be required. Some barrels may require headspacing depending on the combination of parts being used. If you are not an experienced gunsmith, do not attempt the assembly of any firearm without professional assistance. If you require technical assistance in assembly, please contact us for technical assistance.