Can you swap a 6.5 upper with a 5.56 and interchange them anytime you please?

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With few exceptions, any 5.56 upper and 6.5 Grendel upper will work on the same lower. Just need different magazines for the lower.

If the primary purpose of the 5.56 upper is to shoot at a lower cost, a 5.56 upper that resembles your Grendel upper (style, barrel length, weight, etc) should serve as a nice practice weapon. When the switch is made to the larger round, everything would feel familiar.

Ditto everything Drifter said. The only difference between a 5.56 and a Grendel is the bolt, barrel, and magazine. The lowers are the same, so you can pretty much switch with impunity...