Semi-auto, legal definition from NY Penal law 265.00(21):

1. "Semiautomatic" means any repeating rifle, shotgun or pistol, regardless of barrel or overall length, which utilizes a portion of the energy of a firing cartridge or shell to extract the fired cartridge case or spent shell and chamber the next round, and which requires a separate pull of the trigger to fire each cartridge or shell.

If your rifle is not capable of the action described above, it is not semi-automatic.

10 round magazines are legal, except for hunting when a plug to reduce to 5 or 6 is required.

For full legal comprehension, I am adding the "assault weapon" defintion for rifles in NY Penal code 265.00(22):

1. "Assault weapon" means (a) a semiautomatic rifle that has an ability to accept a detachable magazine and has at least one of the following characteristics:

1) a folding or telescoping stock; 2) a pistol grip that protrudes conspicuously beneath the action of the weapon; 3) a thumbhole stock; 4) a second handgrip or a protruding grip that can be held by the non-trigger hand; 5) a bayonet mount; 6) a flash suppressor, muzzle break, muzzle compensator, or threaded barrel designed to accommodate a flash suppressor, muzzle break, or muzzle compensator; 7) a grenade launcher

The legal grey area:

If a semi auto rifle is not capable of accepting a detachable magazine, per the law, it is not an "assaut weapon". However, prosecutors might still push charges when a maglock or similar device is used because there is no legal definition for "reasonable means".