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I was just looking around at the "photos of your AR's" thread and I saw one lower receiver that said it was multi-caliber. Is a rifle capable of firing different caliber rounds, from the same barrel? If you can, what is the range of rounds and how could you figure this out. Im used to shotgun shells where, you can't or really wouldn't even want to try to shoot any shell other than the appropriate size. Less experienced with rifles, and I guess this question could apply to pistols as well, just looking to beef up my fundamentals, thanks for the information.
 

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If you swap the upper and the magazine you can fire whatever caliber you want that's available in the AR platform.
 

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It can fire whatever round that is stamped on the barrel (with limited interchangeability for certain calibers like 5.56/.223). ARs are like legos. Pretty easy to change parts around. "Multi" caliber is stamped on some lower receivers recognizing that different uppers with different calibers can be attached to the same lower receiver. This is the case even if the lower receiver was stamped with a specific caliber. What matters as far as calibers are concerned is what barrel and sometimes bolt is attached to the upper receiver. You can't shoot a 6.8 SPC from a 5.56 upper with a lower stamped "Multi" just the same as you can't shoot a 20 ga from a 12 ga shottie. The barrel (and sometimes bolt and mag) are what matters, not what's stamped on the lower receiver.
 

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From the same barrel? Well, kinda/sorta, but generally no, not in an AR platform.

There are plenty of rounds of the same caliber that can be fired in the same barrel, like .22 short and long in a .22lr barrel, and .38 spl in a .357 mag barrel, but in those cases the caliber is the same, but the brass is shorter. A bit more obscure, but you can generally fire original rounds in an Ackley improved barrel of the same caliber as well.

I suppose you could fire a smaller caliber round of the same parent case out of a barrel, but I would imagine it would leave quite the mess in the barrel. 7mm-08 in a .308 barrel is probably not too bad, but I bet a .25-06 in a 30-06 would leave a bunch of copper and lead in the barrel.

With a gas operated AR, I think that only the correct case would cycle the action even if another round could be fired safely.

One exception - if an AR lower has a blowback upper in .22lr attached, you may be able to run .22 long or short rounds through it with a bit of recoil spring tweaking.
 

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I was just looking around at the "photos of your AR's" thread and I saw one lower receiver that said it was multi-caliber. Is a rifle capable of firing different caliber rounds, from the same barrel? If you can, what is the range of rounds and how could you figure this out. Im used to shotgun shells where, you can't or really wouldn't even want to try to shoot any shell other than the appropriate size. Less experienced with rifles, and I guess this question could apply to pistols as well, just looking to beef up my fundamentals, thanks for the information.
The reason they are marked multi is because you can build it into whatever caliber you want. As spydermonkey stated the caliber is determined by upper.
 
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