I have three different dates of Yugo M33 ball ammo: 1988 660gr, 1997 & 1998 680gr. In all cases, I have noticed that with some of the cartridges, the bullet rattles. You don't notice any rattle by shaking them, but by tapping them on the bullet. That is, flicking your finger at them. Also, you can tell by gently tapping the tips horizontally into the end of a table. Some are quite solid where others has a slight rattle. This leads me to believe that the lead core is loose in the jacket. This can only diminish accuracy.

The next time out, I intend to check and shoot all of the solid ones in one group and the rattlers in another to see how it affects accuracy.

Has anyone else noticed this?

This came up before somewhere before, I believe over at biggerhammer. The core is loose but it isn't a real problem. As the projo goes down the bore it is squeezed and tightens up on the core. Yugo isn't known for being an accurate ammo to start with, more of a plinking round, so your 3" groups might open up to 4".

Shoot it and let us know what comes of it. That's how we all learn.

Listen to Jeep, the Yugo stuff is machine gun ammo and came linked in some cases.  I read some stuff about the jacket separation but its not really going to matter. Its not match stuff so you shouldnt expect that kind of accuracy.  There is a lot of better stuff around for a few cents more.  Not to mention that the Yugo stuff is mildly corrosive.  Get your ammonia out when you are done shooting.

some of the TNW stuff I got a long time ago did that as well but none of the IK 98 I bought did.