I have a DPMS "Kit" AR-15 in 20" heavy barrel with a 1:9 twist. I can tell you what I shoot. I have two loads.

One is range fodder or urban problem resolution ammo. 55gr with the maximum grains of BLC-(2). It will shoot just over MOA out to 100 yards.
The other load is a 69gr HPBT that has a maximum charge of Varget and it will shoot MOA at that same distance.

The rifle does have a flat top and is scoped. It also has an "A2" stock because I am a dinosaur.

I am amazed at how well the 223 shoots. We shot out to 500 yards with open sights in the Corps. In fact, I shot better with that than I did with the M14 that I went in with! 230/250 if I remember right.

Bullets aren't that expensive, buy some and take an extra crack at the range.

I will say this though, the 223 in my opinion is a tad light for deer sized game. Then I saw where you live and since Florida deer aren't too much bigger than big dogs, you may be OK!