Firearms restrictions differ depending on the state of residence (and sometimes city). In general, restrictions are based on magazine capacity. Bullet caliber is not usually considered. Some states have additional limits based on bayonet lug and muzzle break. There was a Federal limit on some firearms classified as Assault Rifles, but that law expired and has not been renewed.

The Enfield being a bolt rifle with low capacity (10 rounds or less) is legal in most states. You can get them at many gunshops. I have a Mk4 No1 and a Mk1 No3 in my collection as well as a M1 Garand and a Springfield 1903.

You must purchase a rifle or pistol from a licensed dealer/gunshop or if unlicensed person, face-to-face. I beleive you must be 21 or older for a pistol and 18 or older for a rifle. It is illegal to ship a firearm to an unlicensed person. For collectors like myself, There is a collectors license that allows buying/receiving firearms specified as collectable via mail.

PS - Enfields are C&R and as such collectable. The rule is 50 years for C&R and 100 years for antique. Antiques are typically not regulated at all.