I just got an AR-15 and some ammo (5.56, 55 grain). I haven't shot it yet but while messing around with it this morning I noticed how the hole out of the barrel is smaller than I expected. I grabbed a 5.56 round and stuck it in the tip of the barrel bullet point first (opposite direction of how it would be shot out of the rifle) and I noticed that the entire bullet part of the round didn't fit. I'm thinking it is designed that way to allow the rifling in the barrel to score the bullet as it travels through before it exits. Do I have it right and it is supposed to be this way? I just want to be sure before I take it to the range. TIA.

You got it right. In fact you inadvertently did a shot out muzzle test typically done on older rifles. If the bullet head completely burrows into the bore it's been worn away enough to affect trajectory/accuracy.

2 minutes ago, siderman said:

You got it right. In fact you inadvertently did a shot out muzzle test typically done on older rifles. If the bullet head completely burrows into the bore it's been worn away enough to affect trajectory/accuracy.

This is just want I wanted to hear. Thanks. Stay tuned for more newbie questions. LOL.

I would add to make sure the rifle shoots 556 and is not a 223 rifle.

18 minutes ago, Tony13 said:

I would add to make sure the rifle shoots 556 and is not a 223 rifle.

It is in fact a 5.56 rifle.

As mentioned above. 

The barrel is .223, the bullets are .224. 

The bullets will deform to seal the grooves, maintaining pressure.