I have an RRA and Spikes "muti caliber" complete lower. I'm tossing the idea of getting a complete upper of another caliber. What options do I have or not have?

Common: .22LR, 9mm, .223 Rem., 5.56x45, .300 Blackout, 6.5 Grendel, 7.62x39, 5.45x39, 6.8 SPC/SPC II, .450 Bushmaster, .458 Socom, .22 Nosler.
Olympic used to make .45 ACP.
AR Five Seven makes an upper in 5.7x28mm that uses P90 magazines.

There are about 1,000 options OP, but the ones listed above are the most common it all depends on what you want it for?

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Pretty much, this.

Not sure what your lower markings have to do with it, though.  

Lower markings have nothing to do with what caliber upper you can or cannot attach to it.

macon armory makes .45 ACP Uppers

What modifications to the mag well would I need to do for these?

Any recommendations on companies that sell these?

Depends, some are semi-permanent/pinned in parts, some are drop-in and easily removable parts, some just require a special magazine, and something like .300 blackout require no changes to the lower and uses a standard magazine.

It would be easier to help you out here if you could narrow down more to what caliber you're looking at or what it is you want to do.

Narrowed it down to 300 blackout or 6.8 SPC/SPCII or .308 if that is possible.

.308 is obviously not possible, the round is longer than the magwell.  You need an "AR-10" pattern receiver set for that.

.300 Blackout has just a unique barrel.  All other parts are interchangeable with a 5.56mm AR-15 setup.

6.8 SPC will require a barrel, bolt, and different magazines.

Fuck. So absolutely NO other caliber can be used with the same lowers I have without doing any modifications?

That's only true for .308.  .300 BO and 6.8 SPC work fine on a standard AR15 lower, you just need special magazines for 6.8 SPC.  The barrel/bolt that he mentioned will all be in the upper.

not sure how you got the above out of what other people are saying. Nothing about the lower is modified to run 300blk, 7.62x39, 5.45x39, 458 SOCOM etc. It's only pistol calibers, generally, that require anything to be done to the lower, and that's just magwell blocks and buffers etc usually. It's the upper where almost all the differences are.

There are a zillion calibers you can shoot from an AR15 magwell, lots of cartridges have been designed specifically to fit into the AR15 magwell.

The "multi" marking is mostly just so people don't get butthurt over being assembled for some other cartridge. There is no functional difference in the lower.

Honest question, how familiar are you with firearms in general? Firearms roughly are segregated by how long of a cartridge that their action can accomodate... AR15s shoot, at maximum, 5.56/223 length cartridges.  308 Winchester / 7.62x51 is a much longer cartridge that will not fit into an AR15 length action, which is the reason for the AR10 / 308 size AR variants.

What is it that you are trying to accomplish?  Your choices are all over the map. 300 Blackout is essentially a fast magnum pistol & suppressor specialty cartridge. 6.8 SPC is a higher performance hunting and defensive cartridge, specifically designed to fit in the AR15. 308 Win is a much larger, full power rifle cartridge for larger game hunting, long range target shooting, and in the military context for sniping and belt fed MG use.

I'm familiar with firearms. Not an expert whatsoever. Not a hunter or .mil. I'd like to take it to shoot at my local range. Already have 2 ARs 5.56/.223 so I'd like to get a higher caliber upper that I can use on my 2 lowers.  From what I understand .300 blackout is an option that I have.

.300 Blackout is a decent option, its performance is very similar to that of 7.62x39mm.  If you want something with a bit more power, 6.5 Grendel or 6.8 SPC are great options.  They're probably two of the most powerful, available, and useful rounds available for the AR-15 platform right now.

If I wanted another AR-15 upper this second, I'd go 6.5 Grendel.

I love 300blk more than most. In supersonic loads it's almost as good as 7.62x39, but not quite. The real beauty of the caliber is when loaded with heavy subs and used with a silencer. To me, I probably wouldn't own 300blk without a silencer. So, for me, that's the question...will you silence it?

Being hunters, the .300 black is a great round, we have two rifles that were specifically built around that particular round and we use them for hunting deer up to about 125 yards, they don't get used for targets, just hunting and they are easy to reload which is one of the major reason I built them.

If all you are doing is punching paper, there is really no other reason to have a different caliber, it will punch paper with the best of them.

I'd like to. Hell I'd love to. Hopefully IF the HPA is passed I for sure will.

Any brands I should completely stay away from? Any brands I should consider?

brands of uppers? All depends on the budget. BCM makes damn good stuff, but if you're on a budget PSA stuff works too. I have 2 Radical Firearms 300 blk uppers and they've both been great. They had an issue with bbl extensions coming loose (torqued prior to heat treat, or something like that), but I've never had an issue with them personally. I do always go with the upgraded BCG though. For budget builds they work just fine for me.

I have a BCM 5.56 upper already. Love it. I'm debating which would be less expensive. Build or buy. Barrel, upper receiver, parts kit, rail, gas block and tube, flash hider/comp? Opinions on costs?

Generally for low/mid price range stuff, buying an upper put together for you is cheaper. It's not until you get to the high end where noveske charges you $200 to install different furniture. On the other hand building gets you EXACTLY what you want. I also happen to think assembling ARs is fun, but to each their own.

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