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Basically there is one difference.... The carry handle. The A3 designation denotes a flat top upper with the inclusion of a removable carry handle that is included(should be if they call it an A3) in the purchase. An A4 is just a flat top upper without a carry handle.
Some folks think it has to do with M4 cuts on the receiver, this is not true. This is where the M4 designation from Colt comes into play.
If you'd like to have a carry handle, get the A3 package. If you are going to run an optic and BUIS, just get an A4 and save yourself some money!
The above may be true but many manufacturers still sell A3 uppers as flat tops w/o feed ramps and A4/M4 uppers w/ feed ramps. Something to bear in mind. A3/A4 are mean different things between mil/civ world and between civ. companies.
Basic rule of thumb: If your barrel HAS ramps, you can use a ramped or standard reciever. if your barrel has NO RAMPS then you must use a standard reciever.
If you're buying a complete upper it doesn't matter.
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