My upper receiver is stamped 5.56/223. I haven't seen anything on the lower. Is it safe to shoot 5.56 ammo?

Short answer: yes

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ΜΟΛΩΝ ΛΑΒΕ

That's all I need to know cause. A friend of mine sold his AR and still has 1500 rounds of ammo. He asked me if I wanted to buy it from him for $300. I'd be a fool not to.

Only shooting them outta my AR.

If the gun is marked for 5.56, you can shoot 5.56 in it, bolt gun or otherwise.

In an AR, all the lower does is make the firing pin hit the primer......pretty much. The upper with the barrel really dictates what caliber you can shoot. They are sort of independent of each other.

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Still carrying my S&W 642

Yes, buy the ammo from your friend.

I assume it is factory ammo and not reloads?

Factory loads. I don't shoot reloads in my rifle unless I reload them.

Actually on the AR design the final word is what's stamped on the BARREL, not the receiver.

You can swap a 223/5.56 barrel for 300 ACC/6.8 or a several other calibers and the receiver would still be the same.

Also in general you don't want to shoot 5.56 in a .223 barrel. A know a lot of people do but you don't want to be that 1 in 10,000 guy who has the tolerances stack up against him.

Of the 1500 rounds, 300 are tracer. I shot a couple hundred rounds the other day. No issues. I'm gonna start buying 5.56 probably more than .223.

Yes I did.

What I've seen, the 5.56 is more accurate than the .223. I was shooting PMC bronze, I shot all over the paper. Then I shot 5.56, same position l, I was able to cover groups of 3 with a nickel.