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Discussion Starter · #1 ·
I am a bit of a noob, and have been hanging around for a while, and feel like I'm starting to figure things out. Still, there's one thing I haven't really seen a real explanation about: what is the difference between available calibers? I know .223/5.56 is the standard round (with 5.56 being a higher-pressure loading), but what are the strengths and weaknesses of other available options in the AR frame, such as 7.62x39, 6.5 Grendel, 6mm PPC, 6.8 SPC, or even more exotic/involved (.308 requires a different lower due to length?)

I'm not trying to get a 'best' cartridge, because I know that depends on application, and I don't really have one in mind yet (I'm just curious). I'm just trying to get a feel for whats out there, and an honest comparison, as most stuff I read seems to say that their chosen round is the 'best'. Is there a comparison somewhere that I haven't found?

Thanks!
 

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Not sure if this is what you're looking for, but this is the most extensive AR15 caliber charts I've seen posted. And not all of the calibers are in the first post either.

Note, these are all for the AR15, and not the AR10 because I saw you state 308 up there somewhere. 308's won't work in an AR15. The bullet is too long. Theyr'e strictly AR10.

OA2.org :: View topic - How many AR15 calibers exist?
 

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Discussion Starter · #3 ·
Now thats a long list...something like what I was hoping for (thanks!) , but not much actual comparison of the different rounds. I thought I read someone saying they had a modified AR-15 lower for .308, but maybe they just meant an AR-10 lower. I guess a large part of it may be what other firearms or reloading equipment the builder has...

I'm guessing building an AR-15 in another caliber just for the sake of building one in another caliber is basically overkill?
 

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I am a bit of a noob, and have been hanging around for a while, and feel like I'm starting to figure things out. Still, there's one thing I haven't really seen a real explanation about: what is the difference between available calibers? I know .223/5.56 is the standard round (with 5.56 being a higher-pressure loading), but what are the strengths and weaknesses of other available options in the AR frame, such as 7.62x39, 6.5 Grendel, 6mm PPC, 6.8 SPC, or even more exotic/involved (.308 requires a different lower due to length?)

I'm not trying to get a 'best' cartridge, because I know that depends on application, and I don't really have one in mind yet (I'm just curious). I'm just trying to get a feel for whats out there, and an honest comparison, as most stuff I read seems to say that their chosen round is the 'best'. Is there a comparison somewhere that I haven't found?

Thanks!
I don't think there is an actual comparison of the different rounds, just lists of the different variants that fit in an AR-15.
as for comparisons:

  • 5.56/.223 is the base. Out of shorter barrels, it's fragmentation range is severely limited, as it is designed to be shot out of a 20" barrel.
  • 7.62x39: equivalent to a .30-30. Handloading can really improve this. Cheapest milsurp for blasting.
  • 6.5Grendel: Great at long range (400+yds.) with a high ballistic coefficient. Alexander Arms has licensing rights and isn't as free with it as other calibers. Not as easy to come by, rifle or ammo-wise. Supposedly the "best" for 0-1000yds., but that is controversial. Definitely superlative for long range.
  • 6.8SPC: designed by some active-duty army guys, and Remington then took it and screwed it up. Designed to work in the same envelope as the 5.56, but with greatly improved terminal performance. Has taken on a life of its own and has been greatly improved over Remington's botched attempt. Probably the most prolific of ar-15 variants, with close to 14 manufacturers making uppers/rifles for it, and at least 5 ammo manufacturers making loads for it. Excellent hunting round.
  • 6mmPPC: mostly a benchrest round. Don't now too much more about it.
Personally, I have 5.56 and 6.8SPC. If I ever decide to do long range competition, I'll be getting myself a Grendel. My 6.8 will be bagging some deer this week, if I get over this *@$%! stomach virus in time to get out to the field...
 

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if i go for a long range ill get the 6.5 grendel. IMO its the best long range ar-15 caliber. the 6.8 doesnt even compare for a long distance shot. if i wanted a short range weapon i would probably go for the 450 bushmaster or the 50 beowulf. i havnt done enough research between the two to decide which is better, i think i i would get the 450 since it seems like it would be cheaper and easier to get factory ammo and as a plus it uses the same diamiter bullets as a 45 acp it does use heavier ones like 250grain+ but i bet you could load a 230 in it but im not sure.

IMO i dont think the 6.8 is that great at all. for a short range weapon the 450 458 and 50 cals are better and more powerful and for long range (400+ yards) the 6.5 grendel kicks the crap out of it. so i really dont see where the 6.8 excels at anything, it is a little better then the 223 but i dont think its worth it. the only area i can see it being usefull is the 200-500 range but its only better then the 223. there are just better cartridges out there. plus wolf makes reloadable brass case ammo for the 6.5 grendel.
 

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So, for all the rounds discussed above (excluding 308 ) can you build the rifle based on any AR-15 lower, or are the lowers specific to the caliber?

I know there are some manufacturers (like Stag) who sell, say, 6.8-specific lowers. Is there any real difference in a 6.8 lower, a 6.5 Grendel lower and a 5.56/223 lower? Or are they the same part with different markings on them?

And, while we're at it, what about mags. I see 6.8 mags sold, and Alexander sells 6.5 mags. What is the difference between these and a 5.56 mag? Are they the same basic mags with different followers? Or are the feed lips different?

Thanks.

And sorry to hijack
 

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The 6.5 and 6.8 mags have different followers and they're tweaked a bit to make them wider on the inside so the rounds stack better.

As for the lowers, an AR lower is an AR lower. The only difference between a 5.56, 6.5 and 6.8 AR are the barrel, the bolt and the magazine. Everything else is identical.

I picked the 6.5 Grendel myself. I've built two now, and there might be a third in my future.
 

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So, are the 6.5 and 6.8 followers different from each other, or are they interchangeable?
 

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