Yes, but if you look young (under 18), they may ask for an ID to prove you are old enough.
I have been asked for my Mass LTC when buying pistol ammo in southern NH. There is no law that I know of that requires this, it was just the store's policy - per the sales person. Move further into NH and they could care less as long as you are over 18
Similiar situation: my primary is MA, and we also own in ME. Setting aside any previous discussion of what BATF considers residency, I've never had an issue purchasing ammo in ME as a MA resident. The only questions I've ever received are "Is this for a pistol or a rifle" (Wally wanting to distinguish between being 18 vs 21) and "Ya sure you gotta enough???" (everywhere else in ME)