I have a Colt CAR-15 that has the reciever marked '.223'. This is an older gun that was purchased in 1990.
I have one too - manufactured around 1981 and I purchased it around 1992 - No forward assist, solid aluminum sliding retractable buttstock, thinner chrome-lined barrel, no sear block and older style bolt carrier that registered lightning-link owners need to make their gun go full auto. That's what I think you are describing.
It says 'COLT CAR-15 Gov't Carbine .223'
- This may be a later model - does it have the heavier barrel?
Do you know if it is ok to fire 5.56 through it? I am pretty sure I have with no issues. I know that guns marked 5.56 fire both 5.56 and .223
No problem - you can fire 5.56 through it all day long.
What do you think an original CAR-15 in excellent condition is worth?
The Colt CAR-15 (which is actually an SP1 variant) in 98% condition would probably sell for between $1,700 - $2,100 in the current market.