I currently have an RRA Tactical Entry 16" carbine. I also want a 20" AR. Instead of buying another complete rifle I was debating on just buying a 20" upper and swapping it out with my 16" upper as needed.

If I zero both uppers on the same lower, is it reasonable to assume that the zeros would be retained while swapping out uppers. I basically assume this to be true since GI rifles retain their zeroes even after field stripping for operator maintenance. I don't mean to ask a stupid question, I'm just asking to make sure someone out there doesn't know something that I'm not aware of before I throw down $500.00 for a new upper.

Thanks for your time,

Brent

Quoted: I currently have an RRA Tactical Entry 16" carbine. I also want a 20" AR. Instead of buying another complete rifle I was debating on just buying a 20" upper and swapping it out with my 16" upper as needed. If I zero both uppers on the same lower, is it reasonable to assume that the zeros would be retained while swapping out uppers. I basically assume this to be true since GI rifles retain their zeroes even after field stripping for operator maintenance. I don't mean to ask a stupid question, I'm just asking to make sure someone out there doesn't know something that I'm not aware of before I throw down $500.00 for a new upper. Thanks for your time, Brent

Think of it as swapping out lowers, as the sights and barrel are part of the upper receiver assembly.  The zero will stay put.  

what the other's said.

What the others said.

You'll have ZERO ZERO change.
Ha! I'm so witty sometimes.

I do it all the time....

DCM upper, varmint upper, 3 gun upper....   same lower (well, I DO pull the lead out of the buttstock that's in there for HP matches) when using it for the other uppers.

Increased utility, lower total cost....  benefit of modularity