Well - ok I suppose there is no advantage to registering multiple length uppers but you can if you want. Not sure about different calibers, but maybe you are not interested in that anyways.
You don’t register uppers. The only thing registered on an SBR is the lower.
You can SBR one lower and have 20 different uppers of different lengths and/or calibers for it as long as you can return it to its original configuration.
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