I suggest you visit LL Bean. Their gun selection is lacking but it is a nice place.
I thought there was shop near Freeport but I haven't been to Freeport in a few years now that my grandmother moved out of her house there.
And since you will be driving past, I also suggest you hit up Kittery Trading Post. The ammo prices are not that great. Cabelas in Scarborough is also on the way to Freeport.
LL Bean now has a separate building for sporting goods, including hunting. They have very high end firearms. There used to be a small shop in the area, but last time I was up there, which was in 2013, someone told me they were closed.
Ammo selection at Kittery stinks IMO. Cabela's at exit 42 on 95 has a great selection and occasionally it has good prices but it's hit and miss. Lately they've had a large selection of 9mm.
LL Bean now has a separate building for sporting goods, including hunting. They have very high end firearms. There used to be a small shop in the area, but last time I was up there, which was in 2013, someone told me they were closed.
- - - Updated - - -
I notice that list still has Bushmaster listed. Before I went to any of those shops, I'd call and make sure that they are still in business.