Hi guys, and hopefully girls! I'm new to the whole AR thing. Never built one never owned on. My question is how much needs to be changed out , to switch from a 5.56 to a .308? Barrel is obvious but there must be more. Any help would be appreciated. Just trying to get a clear picture of things before I get started.

Everything.  Cartridge is longer. Gun parts are longer.  Buy another rifle. 

or, look at alternate calibers that are designed to run in the AR15 platform:

1.  6.8 SPC (new barrel/bolt/mags vs. 5.56)
2.  6.5 Grendel (new barrel/bolt/mags vs. 5.56)
3.  300 Blackout (new barrel vs. 5.56)

Each one does something a bit different, but all generally were designed for the AR15 platform and would be considered "a larger caliber" vs. 5.56.  There are certainly some others out there, but I think those cover the main/common cartridges.

Or, get a .308 rifle.  they are fun

Hope that helps!

With a Colt 901 you can go from 308 to 5.56mm, but yeah you can't go the other way around... 

I like the 5.56/.223for ammo availability, just wondering if it's feasible to switch between calibers. Kind of thinking one setup for cyotoes and another for bigger game. I have standard rifles that cover most North American game, but I'm thinking one gun for way back woods. Kind of 1 1/2 rifles instead of 2. I know 308 will kill gophers, I'm just wondering how hard it is to go back and forth. Thanks.

What jondelong said. You can build a 556 rifle and then have another upper in one of those calibers available. There are also some big bore options that would work which I don't think he listed. But if you want a 308 then you are gonna have to go 308 which is a larger gun. There is the option of different calibers in 308 also....308, 243, 260 Remington, 6.5 creedmore and 338 federal.....off the top of my head...


I suggest either a 6.5 Grendel to cover all your needs or get a 556 rifle and a 308 rifle...

Thanks, easy Anser is just stick with the 5.56 I guess. I've already got a .22 a .243 wssm, .30 Remington and a .30:06. Just thought it would be cool to have one for travel that could cover a greater range of use, without taking as much space and weight.

On a side note which opperating system and why. I'm not going to be doing 3 gun or anything. Mostly cyotoes and an occasional tin can. And I'm in no way going for tacticool. Just a good solid mid range gun( few hundred yards). Thanks again, learning as much as I can before I get to far into one.

Since you haven't set a budget, we'll assume you want to bargain shop during these salad days.
Start with Palmetto State Armory and "pin together" a rifle and an extra upper.

PSA complete lower with MagPul furniture $149 shipped to your FFL

+

Complete 18" SS barreled upper w/ 15" MLOK rail and BCG $330 shipped

Needs iron sights or optic

Thanks, budget is a concern for sure. I'll look at those.

The easiest second caliber to use is 300 blackout, which uses a .308 bullet in a 5.56 casing.  It fits in your standard 5.56 magazines, and the ONLY part you technically need to change out is the barrel, though it's nice to have a separate upper to make swapping easy.  I just picked up a complete 300 blk upper for only $200, and no need to register a second gun!

Although FMJ 300 blackout is a terrible choice for hunting (it lacks punch), if you  the Lehigh defense hollow points then it is actually a great hunting choice:

https://www.lehighdefense.com/products/300-aac-blackout-whisper-194gr-subsonic-maximum-expansion-ammo?variant=1066138648

https://www.youtube.com/watch?v=Zuu5OMNy2gs

The Lehigh defense has good bone penetration capability, but then expands quite reliably in soft tissue, creating a large 4 pronged wound pattern similar to a broadhead.  Although it lacks the velocity and energy of a full sized .308, it makes up for this with its reliable expansion and larger permanent wound cavity.  More and more people are finding that this round is great for deer, boar, and even a lot of people are reporting it works great for elk.  The main difficulty in using this round for hunting is that the bullet drop is more significant, which may limit its effective range to around 100-200 yards (although it retains its energy quite well, so it would be lethal at longer ranges if you're able to compensate for drop).

You can't

AR15 is 5.56mm, .223, 6.5 Grendel, 6.8SPC, 300blk, 450BM, 458 SOCOM etc.

AR10 IS 308 size.

But once you go up to an AR-10 platform, you get .243, .260 Rem, 6.5 Creedmore, 7mm-08, .308 Win and .338 Federal.

However, if you're not dependent on the magazine well, you could get a .338 Lapua Magnum or .50 BMG bolt action upper that fits on an AR-15 lower.

If you want to move between calibers but use the same lower (say, to use the same trigger and whatnot instead of buying two of everything) rather than having two guns, this is pretty much the answer. Get a 901 with your choice of 5.56 upper.

Much easier to move to 5.56 on a rifle beefed up for .308 than to go the other direction.

Please check out and get subscribed to my YouTube channel! I make my own gun parts! [spam deleted and account locked - Paul]

Gonna jump into this thread as I'm curious about this as well.

I just bought a S&W M&P. If I wanted to, can I use a complete upper in 7.62x39 or 51? Or is the mag well dimensions too small? What am I limited too regarding my lower?

Thanks

A 7.62x39 upper would fit the standard AR lower.  
A 7.62x51 upper would require a large frame lower, as stated in a couple of responses above.
e.g.
Palmetto State Armory PA-10 (.308) Parts & Rifles

Got it,

Thanks

If you want to shoot 7.62 X ? in a S&W MP15 with a minimum of modification, get a .300 BO (Blackout is 7.62 X 35) Upper Assembly (Barreled Upper Receiver).

7.62 X 39 uses a .310 diameter bullet, not the same as .308/7.62 X 51 AND .300 BO (7.62 X 35) which use the same .308 diameter bullets.

.300 BO uses all of the same parts as a 5.56/.223 AR15 Type, with the exception on the barrel, including the magazine. and the Bolt Carrier Group.

To convert from 5.56 to 7.62 X 35, Remove the magazine, Check the Chamber for empty, separate the Top or Bottom Receivers, attach the .300 BO (Blackout) Upper assembly, and swap the charging handle and Bolt Carrier Group from the 5.56 Upper Assembly to the .300 BO Upper Assembly.

In Today's AR-15 Market, you can buy a .300 BO Upper Assembly (without BCG) for $200.  A separate BCG will cost you $70-$100.