I have got to agree with everyone above, that the 223 is far from ideal,
but that being understood you might want to hand-load your ammo.
23.5 grains of varget powder to start, 24 grains in most rifles is accurate ,
and this hornady bullet in hand loads ,
have been used rather frequently on small florida deer successfully
stick to heart/lung shots at under 200 yards
https://www.hornady.com/bullets/rifle/22-cal-224-70-gr-gmx#!/
http://www.handloads.org/loaddata/default.asp?Caliber=223%20Remington&Weight=75&type =Rifle&Order=Powder&Source=
and yeah, if its the only rifle you have it works,if your rifles got a 1-in- 7 or 1- in- 8 twist rate.
check your state game laws it may not be legal, and thats important, to know, before use.
its legal here in FLA and theres a 5 cartridge mag capacity limit
http://myfwc.com/hunting/regulations/taking-game/
but seriously a more practical answer is get a real deer rifle like a 308 win, 25/06 7mm08, 257 roberts , or 6.5mm swedish mauser etc.
I think many of the guys Ive hunted with have tried using the AR15 rifles on hog and deer hunts,
theres no question they work in the hands of a decent shot with those hand-loads,
but as stated, get a real deer rifle.
Ive had excellent results with my 257 roberts and 100 grain speer bullets 38 grains of varget
https://www.speer-ammo.com/bullets/r...sptz-sp-bullet
the 100% copper expanding bullets in 70 grain weights will function, but theres obviously better options