New AR upper if different caliber.

[ExtremeX]

[chicoredneck]

The question to ask is, what do you plan on using it for? All these calibers have a specific nich. If you define your intended use it will help point you to a cartridge more suitable to that task.

[r8dr rider]

308 and don't look back

[ExtremeX]

If I went with something more of a longer range cartridge it would be to extend the range of the 5.56 for target use and possibly even hunting.

Honestly I’m just looking for something else to shoot out of my AR; I haven’t giving much thought to the application side. I would be nicer to have a bigger bullet for CBQ ranges that has a little knock down power.

The only rifle caliber I own is the 5.56… just looking for something new, but would like to build on the existing platform I have now.

[chicoredneck]

I would look at was most affordable then. I'm going to guess the 6.8 would be if you don't want a 7.62x39, but I'm not positive.

[Jarhead]

6.8MM

[Steelheader]

6.8 SPC for distance outside of CQB. My personal "next" build. I am going to use it for pigs and possibly deer hunting here in the REAL NorCal.

[CHS]

Here are the unique advantages of each of those rounds mentioned:

.458 Socom = Replicates .45-70's massive power in a small semi-auto package
7.62x39 = more power than 5.56 utilizing a very common cartridge
6.8 = more power than 5.56 utilizing a cartridge more suitable for an AR's magwell
6.5 Grendel = More power than 5.56 in a very long-range capable package also more suitable for an AR's magwell

Here's the thing, all of those cartridges besides the 7.62x39 were specifically designed for function in the AR platform. The 7.62x39 was basically just sorta "made to work" in the AR.

[berg]

If you want a .30 caliber you can just get a 7.62x40. All you need is a new barrel and you can use all your existing parts.

http://www.ar15.com/forums/t_3_121/547534_My_first_Ar_build_7_62x40_wt.html

http://www.ar15.com/forums/t_3_121/382761_7_62x40mm_Information_Thread.html

[vta]

You might consider 450 bushmaster. It uses your existing 556 magazines with no modifications. Comparable to the 458 socom.

[GettoPhilosopher]

.300AAC? I don't have personal experience with it, but a .30 cal cartridge from an AR using standard bolt and magazines with full capacity sure sounds attractive to me.

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[Bartin]

308 and don't look back

That doesn't work for what he's asking

I want to do .458 socom but I'm waiting until I get a reloading setup because commercial .458 is extremely expensive, as you mentioned

[DannyInSoCal]

The 6.8 SPCII (.270) has been greatly improved due to higher quality ammo.

40% more knock down power for hunting within 200 yards compared to .223/5.56 -

I prefer the 110gr ammo...

Plus it splits the difference between .223 and .308 without having to build a new MUCH more expensive rifle -

But it all depends on your definition of "long range"...

If you are going out past 400 yards -

You actually are better off with the .308 -

Unless you're just killing paper.

In which case just stick with .223/5.56 and go with a 1/7 twist 24" barrel and 77gr ammo...

[zfields]

.300AAC? I don't have personal experience with it, but a .30 cal cartridge from an AR using standard bolt and magazines with full capacity sure sounds attractive to me.

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Ballistics similar to 762x39, but 3x the cost.

300 AAC in a free state with a suppressor maybe.

[Don29palms]

If you want to use 7.62x39 get an AK.