I am not an expert, but at least that is specific enough that those who are can give you an answer. I believe I am correct about the following:
- The M16A4 and C7A1 use the same barrel, lower receiver and recoiling parts.
- The M16A4 fire control is safe-semi-3-round burst, while the C7A1 is safe-semi-full.
- I think the C7A1 is still using a proprietary rail on the upper, while the M16A4 uses the now-standard Picatinny 1918 rail.
- Canadian furniture is moulded from a plastic that has better low-temperature impact properties than that used in the US varieties.
Edit to add: The M16A4 I describe is the US Rifle M16A4, which was manufactured variously by Colt and FN Manufacturing, Inc. It is possible that Colt makes a gun they call the M16A4 for commercial sale which is different than the M16A4 described. They certainly have done this in the past with the M16, M16A1, M16A2 and M4 Carbine. It is important to remember that Colt owns the trademark to these names (except M4) and can apply them to single-shot .22s with foreends moulded out of coleslaw, if they want to. They are under no obligation to follow any pattern when making commercial rifles.