# General > General Guns & Ammo >  Calibers that fit the Standard AR 15 Magazine

## Rick

We've had a lot of discussion about what uppers are available for the AR15 platform but I don't recall seeing a thread on what calibers will fit a standard AR magazine. Here's a list to start with. And or correct as needed and I'll keep the list updated. 

Double Stack:

.17 Fireball
.17 Remington
.19 Calhoun 
.223 Remington 
.20 Tactical 
.204 Ruger 
.25-223
.300-221 Fireball 
.300 AAC Blackout
.375 Reaper
5.56 NATO
7mm Whisper
7.62 X 40mm 
6 X 45mm 
6.5 X 45mm
.257X45mm 

Single Stack: 

.440 Corbon (Requires mag spacer)
.458 SOCOM 
.458 RMW
.50 AE (Requires mag spacer)
.50 GI (Requires mag spacer)
.50 Beowulf (Requires mag spacer)

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## finallyME

I know some of those can use the same bolt mostly because the case is the same, just necked up or down.  Is that the same for the rest of them?

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## Wildthang

Just to be safe, it is not recommended to fire a 5.56 Nato in a .223 chamber! But supposedly you can fire a .223 in a 5.56 Nato chamber!

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## kyratshooter

OMG the .223vs5.56 post!!!

Truth is, that is hardly relevant with an AR build and has been mostly reserved for the few old Mini14 shooters still clinging to the units.  On top of that all it takes is about 100 rounds fired as fast as one can pull the trigger to erode the throat of the .223 out to 5.56 specs.  the difference is only a couple of thousandths of freebore in the chamber throat. 

I do not think I have seen a .223 AR chamber offered in all my scanning of offerings.

As for the "list", some of them use the BCG and magazine unaltered.  Almost anything you see there with the designation X45 falls in that list.  6x45/257x45/6.5x45/7x45 as well as the .204 Ruger/.300bo.

Several others use specifically altered bolts or the BCG from the 7.62x39 chambered AR.  You have to be careful which you choose and what you buy to match it.

Yes, there are many calibers that one can choose from if they hand load.  Most will never see their name on a label at the LGS.

Most of them are still viewed as radical wildcats but several are good enough to hang on to a long useful career and following, which is what it takes for a wildcat to become standardized like the .300BO and .204Ruger.

I am looking real closely at the 6x45 for my next build.  It only requires a change in barrel.  Brass can be formed easily from the parent .223/5.56 case by simply running through a resizing die.   There are a host of good bullets on the market and they can be held to good velocity (70gn hollow-point @ 2800fps--80gn soft point @ 2400fps).  The 6x45 has enough following that RCBS offers reloading dies as a standard caliber at the normal price.

In my personal opinion it is what the AR should have been chambered for back in the 1950s when it was introduced.

But none of them give you the ability to shoot as cheaply as the 5.56.  Surplus ammo is ridiculously cheap and even the reloading components are cheap compared to other calibers.  55gn fmj bullets are often less than $0.09 cents each, allowing reloading of the round for a max of $0.15 cents per round!  

I know people that have a Dillion progressive set up exclusively for the 5.56 and they are reloading for a thousand rounds at a time.

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## Rick

> Almost anything you see there with the designation X45 falls in that list. 6x45/257x45/6.5x45/7x45 as well as the .204 Ruger/.300bo.




They won't load from a standard AR 5.56 magazine? I thought the only difference in the 300 blackout, for example, would be the upper. The magazine would work just fine. Am I wrong?

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## kyratshooter

You are correct.  They should feed with no problem.

Most of those calibers, and the 5.56x45 parent case, are used because they will work with minimal complications, such as seating the bullets short enough to fit the AR mags.  Most of them only require a barrel change.  That is usually a 30 minute operation on an AR.  I am sure there are places where you can purchase a fit and head-spaced complete upper.  I have not looked into that. 

In fact, the calibers from .243-.30 have gained a reputation for being more reliable than the 5.56x45 because the larger bullets do not provide the steep angle on the feed ramp to hang up and jam.

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