Each rail should have its own screws. As said above the ones in the receiver are grub screws and not designed to hold anything to it, just as protector (like the thread cap for the muzzle when the suppressor is not attached).

Have you ordered/received the rail? There should be four (4) screws minimum with it.

There is also a technique to screwing these down/in. Don't just start at one end and go along the rail. You want to do them in an order and only slightly tighten each screw so the rail is both squarely, and firmly attached without too much pressure/tightness on any one screw. If possible try use the two screw method. IOW screw two screws at the same time. Like you would when installing a set of rings.

Not really needed especially on the 223. The four screws will do the job and recoil is fairly low.