Look.... I don't care about a ton of things and I super don't care about bullshit like what your SEAL friend says. I'm intending to get a PRACTICAL discussion of these two 30cal options. I know this isn't a great forum for PRACTICAL advice because some of you lunatics spend $1000 on a bipod 
But....
I wanted to buy a 12.5" AR-10. Something like a POF Revolution/Rogue, or build something up for 100% suppressed use. I know enough about tuning gas systems that I think I can make this happen and end up with a 1-8x scoped gun that can hit pretty hard at 200-300m and in. Nothing fancy. For hunting and defense.
BUT THEN... I read a post here where people discussed the 135gr loads they were using in their 12.5" 308s, and how they were dirty and loud even suppressed, still heavy guns, expensive, just generally unlikable assholes that make barely acceptable niche guns.
In terms of practical discussion (do you see how I keep refering to that?)... Right there, that seems bad. If nothing else, the special ammo and how it was still very dirty are bad signs to me.
1. If you didn't consider it very practical to make special ammo per firearm (which I do not)... Is there a sensible round that should work well in both those sizes, something I could roll for a 16" bolt gun and a 12.5" gas gun? I wouldn't want to shoot 135gr from a 16" bolt gun necessarily, as 168-175gr seems better there. But that heavy a round in a 12.5" gas gun seems like yea maybe it is dirty and loud with lots of unburnt powder.
2. A 16" AR15 is 2" longer than a 12" AR10, just difference in the receiver and a large weight difference. But, does a 16" 300blk get anywhere near the same energy as a shorter 308 in some balanced load? I guess this comes down to how much powder you're willing to use and leave unburnt?
3. It seems to be that an 8" and 16" 300blk have more ammo compatibility per barrel length sets than the 308 guns. But does this just circle back to special ammo? Where yea, you can put a 125gr round in both, but it'll be excessively loud and dirty in that 8" 300blk gun. I sort of imagine there is no free lunch, but, in terms of PRACTICAL ownership and use, maybe it's closer to ideal than 308?
I briefly considered something in greater than 30cal range but I'd rather not get into that. Most options like 350 Legend were dead on arrival. Focusing mainly on AR platforms because of again, practicality.
Am I overlooking anything? Because I'm sort of on the line of a 12.5" 308 or a 16" 300blk. Pros on the 308 is that it can definitely hit harder, Pros on the 300blk are almost everything else including ability to cycle subs for real.
RESOLUTIONS / REALIZATIONS / EDITS:
A. So... no. There doesn't seem to be any 308 ammo that would be clean, fireball-less, efficient, quite suppressed, not gassy when fired from a 12.5" 308 that you would also want to use in a 16"-20" 308. Ignoring twists, it just seems like the only application of 12.5" 308 is battle rifle using mil/surp ammo. I don't really want to stock milsurp or reload a 130-155gr round specially for this. As to 300blk, no one questions that it covers short and longer barrels better. However, it appears to be AT BEST between 12.5" 308 and 16" 300blk super, a 50% energy difference if you don't mind the above issues with shorty 308. If you where going to kick in doors, or want ethical shots on game without a long gun, a 12.5" 308 does seem to have specific uses. Overall this discussion was about PRACTICALITY, and for me at least, it's not there.
B. Something I hadn't considered was a bullpup. Sure enough, that could work. I wouldn't mind bullpup issues here, and you COULD get a 16" 308 in a physically very short package. Sounds great! Except... The market seems to suck for options here. There is a horrible M17S that has managed to crawl out of the 80s. The Keltec RDB with it's FAL mags and extremely finicky function. The Desert Tech MRDx which was plagued with far too many issues on release and now while there is a new version, who knows if it's worth giving a shot, that said as far as practicality, they have a single gun that can be as small as a P90 but as an 11.5" 556, a normal 16" 556 or 300blk that's smaller than an 10.5" AR, and a 20" 308 that is considerably shorter than a 16" AR-10, seems great if only they could get it to work! Then finally there is the TAVOR-7 which is the only gun that possibly has any proven reliability via it's military application, but is a 2x6 in the hands and struggles to get 3MOA accuracy, this could be largely the trigger. So.... It's a pretty slim market here. Best practical solution is if Desert Tech could unfuck their gun, but I'd still expect no better than 2MOA which would be fine for me.
But....I wanted to buy a 12.5" AR-10. Something like a POF Revolution/Rogue, or build something up for 100% suppressed use. I know enough about tuning gas systems that I think I can make this happen and end up with a 1-8x scoped gun that can hit pretty hard at 200-300m and in. Nothing fancy. For hunting and defense.
BUT THEN... I read a post here where people discussed the 135gr loads they were using in their 12.5" 308s, and how they were dirty and loud even suppressed, still heavy guns, expensive, just generally unlikable assholes that make barely acceptable niche guns.
In terms of practical discussion (do you see how I keep refering to that?)... Right there, that seems bad. If nothing else, the special ammo and how it was still very dirty are bad signs to me.
1. If you didn't consider it very practical to make special ammo per firearm (which I do not)... Is there a sensible round that should work well in both those sizes, something I could roll for a 16" bolt gun and a 12.5" gas gun? I wouldn't want to shoot 135gr from a 16" bolt gun necessarily, as 168-175gr seems better there. But that heavy a round in a 12.5" gas gun seems like yea maybe it is dirty and loud with lots of unburnt powder.
2. A 16" AR15 is 2" longer than a 12" AR10, just difference in the receiver and a large weight difference. But, does a 16" 300blk get anywhere near the same energy as a shorter 308 in some balanced load? I guess this comes down to how much powder you're willing to use and leave unburnt?
3. It seems to be that an 8" and 16" 300blk have more ammo compatibility per barrel length sets than the 308 guns. But does this just circle back to special ammo? Where yea, you can put a 125gr round in both, but it'll be excessively loud and dirty in that 8" 300blk gun. I sort of imagine there is no free lunch, but, in terms of PRACTICAL ownership and use, maybe it's closer to ideal than 308?
I briefly considered something in greater than 30cal range but I'd rather not get into that. Most options like 350 Legend were dead on arrival. Focusing mainly on AR platforms because of again, practicality.
Am I overlooking anything? Because I'm sort of on the line of a 12.5" 308 or a 16" 300blk. Pros on the 308 is that it can definitely hit harder, Pros on the 300blk are almost everything else including ability to cycle subs for real.
RESOLUTIONS / REALIZATIONS / EDITS:
A. So... no. There doesn't seem to be any 308 ammo that would be clean, fireball-less, efficient, quite suppressed, not gassy when fired from a 12.5" 308 that you would also want to use in a 16"-20" 308. Ignoring twists, it just seems like the only application of 12.5" 308 is battle rifle using mil/surp ammo. I don't really want to stock milsurp or reload a 130-155gr round specially for this. As to 300blk, no one questions that it covers short and longer barrels better. However, it appears to be AT BEST between 12.5" 308 and 16" 300blk super, a 50% energy difference if you don't mind the above issues with shorty 308. If you where going to kick in doors, or want ethical shots on game without a long gun, a 12.5" 308 does seem to have specific uses. Overall this discussion was about PRACTICALITY, and for me at least, it's not there.
B. Something I hadn't considered was a bullpup. Sure enough, that could work. I wouldn't mind bullpup issues here, and you COULD get a 16" 308 in a physically very short package. Sounds great! Except... The market seems to suck for options here. There is a horrible M17S that has managed to crawl out of the 80s. The Keltec RDB with it's FAL mags and extremely finicky function. The Desert Tech MRDx which was plagued with far too many issues on release and now while there is a new version, who knows if it's worth giving a shot, that said as far as practicality, they have a single gun that can be as small as a P90 but as an 11.5" 556, a normal 16" 556 or 300blk that's smaller than an 10.5" AR, and a 20" 308 that is considerably shorter than a 16" AR-10, seems great if only they could get it to work! Then finally there is the TAVOR-7 which is the only gun that possibly has any proven reliability via it's military application, but is a 2x6 in the hands and struggles to get 3MOA accuracy, this could be largely the trigger. So.... It's a pretty slim market here. Best practical solution is if Desert Tech could unfuck their gun, but I'd still expect no better than 2MOA which would be fine for me.
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then keep on rocking that blaster.
that would be pretty neat. Could get a decent sized tube under an Aero type handguard. Cut barrel to sufficient length, add some porting...... your ammo worries would be solved. Full speed from bolt guns, ported barrel helps slow some gasses, 4-6" of a 50° cone stack, maybe a diffusion wall? 
