First, let me say that I don't own either caliber so I don't have a dog in the fight, however, I have studied the ballistics of both calibers and have experience shooting both as well. The sole reason the 300 BO exists is the M-16/M-4 platform and it's perceived need for a heavy, 30 cal subsonic rd that performs well in a short barreled version of the M-4. Yes, the 300 BO can feed better in an AR platform who's magwell wasn't made to accept the 7.62x39 but that it's single advantage real advantage and many AR lowers are now made to work equally well with both. Further, you don't have to go to a cheap, clunky Commie mil rifle to shoot the 7.62x39 as there are semi-autos and bolt guns chambered in it made by quality rifle makers.
That said, once the legions of fanboys stop drooling all over the AR platform as the end all and be all of semi-autos, the 300 BO reveals itself as ballistically inferior to the 7.62x39 in all bullet weights, either supersonic or subsonic loading. I'm not talking about what someone likes or wishes, I'm talking testable, measurable performance. Fact, not opinion. The fact that the mil-surp 7.62x39 ammo isn't readily available in heavy weight, subsonic loads is only due to no perceived need, not because it isn't doable as 150grn and 200grn .311/.312 projectiles are still being made if their military wanted to load it.
Ballistically, the 300 BO's smaller case capacity requires it to be loaded with magnum pistol powders instead of higher performing rifle powders. This limits it's supersonic performance capabilities. As heavy, subsonic bullet weights are purposely velocity limited, the lower performance of the pistol powders isn't a factor for them. However, it is a fact that the 300 BO doesn't have the velocity capability of the larger cased, rifle powder loaded, 7.62x39 in any bullet weight. It's a matter of performance, not preference as it's capability falls short of the 7.62x39 in all supersonic bullet weights; some as much as 200fps. That translates to range limitations for acceptable performance in the 300 BO well before that of the faster 7.62x39.
Ammo wise, both calibers are available commercially in brass cased, Boxer primed loads from quality manufactures for the same prices so neither has an advantage, however, the 7.62x39 is also available as cheap Commie junk ammo if one is into blasting. While the cheap stuff might sound appealing to some, I don't consider it a reasonable choice as it's dirty, inaccurate, and hard on components but the fact is that some, maybe many, like the option of buying 20¢/rd blasting junk and doing mag dumps. The 300 BO has no cheap, junk ammo option so if that is a consideration, the 300 BO falls short once again.
The facts are that both calibers are easily handloaded at the same cost per rd, and both can be loaded with easily obtainable, similarly priced components from heavy subsonic to medium and light supersonic loads.
Quality 200grn class, 150grn class, 125grn class, and lighter bullets are available for both the 300 BO (.308 caliber) and 7.62x39 (.311/.312 caliber). In addition, surplus 110grn and 150grn class surplus bullets are also available for both calibers while surplus 125grn class bullets are available in .311/.312 caliber only.
Yes, the 300 BO brass can be made from .223/5.56, however, it costs time and money to do it and that same time and money can be expended to process spent 7.62x39 cases to accept Boxer primers. Further, many opt to buy processed brass instead of making their own. Processed brass is available in either caliber at about the same price and using Boxer primed brass cases from same priced factory ammo is also available so arguing that the 300 BO is cheaper falls apart rather quickly when one actually looks at what's available.
