so im not new to the handling of m16,ar 15's. ive been around them since 04, even slept with one. so a question i keep finding myself asking is about these black rifles is...i know they can be chambered in many calibers, .223, 6.5, .308, .458. how does this work. does the lower mag well have to be larger as well to accomodate the larger rounds? or does it turn into a one single shot rifle? so in theory could i use any lower and any upper chambered in different calibers? or does the lower need to be the correct caliber match?
The differences you cite are the reason for AR10 versus AR15 type uppers and lowers. Case length of the .308 for example requires the longer magazIne well and bolt stroke found on the AR10 platform. Not 100% sure what the specific length is but of the calibers you mentioned I believe the .308 is the only AR10 round.
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so in theory could i use any lower and any upper chambered in different calibers? or does the lower need to be the correct caliber match?
As long as you have a mil spec AR 15 lower, you can attach a mil-spec AR 15 upper of any caliber to it. Some calibers require a change in the bolt due to larger case diameter. For example a 5.56 bolt will not work with 6.8 SPC ammo.
... does the lower mag well have to be larger as well to accomodate the larger rounds? or does it turn into a one single shot rifle? ...
Some loads for the 5.56/.223 have bullets too long to feed reliably stacked up in the magazine. These rounds, which tend to be 80gr and above, would have to be fired single shot. The case is the same length as normal but the bullet is longer, making the overall cartridge length too long for the mag even though the single round will chamber properly.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
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