There are quite a few different options when it comes to picking what steel alloy your 300 blackout barrel will be made of. First, you’ve got to pick between Chromoly and Stainless alloys, then you have subsets of those alloys that will determine barrel life, accuracy, and cost. For 300 Blackout, the quality of your barrel steel matters more than with any other AR 15 cartridge.
After all, you are taking an existing platform designed to sling .22-caliber rounds and shoving a large .30-caliber round through it. One that can go as slow as subsonic, and as fast as those .22 rounds. There’s a lot of ballistic variance and abuse happening inside the chamber and barrel of your 300 Blackout AR, so let’s go over why certain barrel steels are better than others for this platform.
To break all this stuff down, we have to clear up what the hell all those steel numbers (1060, 410, 4150) mean. The first number in the 4-digit sequence is easy, it’s the SAE classification of the alloy: 1 is carbon, 2 is nickel, 3 is chromium, and 4 is the stuff us gun owners care about, molybdenum. We’ll use our favorite top-tier barrel steel, 4150 Chromoly, as our example.
Barrel Steel by The Numbers
So, in 4150, the “4” refers to the fact that it’s a Molybdenum alloy. The second digit, “1” refers to the presence of other strengthening elements in the steel – in this case, that would be Chromium and some other elements. The last two digits, “50”, is actually just one digit. It refers to the actual, nominal percentage of carbon content present. Rather unsurprisingly, that means 0.5%.
What About “Mil-Spec” and “CMV”?
There’s a common misconception surrounding “mil-spec” gun steel, sometimes called CMV (Chromoly Vanadium). You might have seen a number on gun makers’ websites that reads something like, “Mil-Spec B-11595E”. This is not a different, special, magical kind of barrel steel. In fact, it’s the same exact stuff as 4150 Chromoly steel – it just underwent a testing process that certifies it contains the right stuff.
You may see forum threads or posts from AR 15 owners claiming “11595E” is different from 4150 Chromoly steel. It is not. That’s not to say that all 4150 steel can be 11595E certified. This certification simply means you have a quality 4150 alloyed barrel that contains the right elements and non-metallic additives the military deems best for their service weapons’ barrels.
And 416R Stainless?
There are generally three tiers of stainless alloys when it comes to gun barrels: 410, which is good, 416, which is better, and 416R, which is best. Now “best” can be defined by different measures of properties, but if you’re investing in a stainless barrel, you’re looking for accuracy – and the best accuracy will come from a 416R stainless barrel.
Basically, 410 stainless provides moderate qualities at a discount. It offers good heat resistance and it resists corrosion quite well, though it is not as easy machined as 416 alloys, making rifling and finishing processes less “polished”.
416 stainless provides the highest machinability of any stainless steel. It is slightly less resistant to corrosion when compared to 410, but its ability to provide superior accuracy outweighs this minor drawback.
416R stainless is a proprietary stainless developed specifically for firearms. It features the addition of Molybdenum, like non-stainless Chromoly barrels feature, which improves barrel life and corrosion resistance without affecting machinability or accuracy. 416R also features a lower sulfur content, which reduces rifling wear.
Why 4150 and 416R are Best for 300 Blackout
Put simply, these two steel alloys provide the most hardness and the best barrel life. Like we mentioned in the beginning of our discussion, slinging a large, fat, fast (and sometimes slow) .30-caliber round through the AR 15 platform will result in greater wear and tear, and a greater need for a better quality barrel.
Both of these barrel steels undergo a heat treating process that hardens the finished barrel’s rifling and steel, ensuring you’ll get the best accuracy with any 300 BLK load, and you’ll enjoy improved barrel life.