+1 on BradH's explanation. "Varmint" bullets are built so that they disintegrate as soon as they hit anything, (prarie dog, etc.) and to do that reliably they need specific parameters for muzzle velocity and spin rate. They use very thin jackets (sometimes internally pre-scored) and soft lead cores, so they're pretty flimsy to begin with. If you exceed the parameters for velocity, spin or both, you can expect the jacket to rupture, the core to spin like clay on a wheel and the bullet to generally disintegrate anywhere outside the barrel. This is BAD because there's no telling what the bullet's parts are going to do-they won't necessarily disappear in a puff, and may fling jacket parts and lead just about anywhere.
Military bullets, like the 55gr spitzer used in M193 are usually made with sturdy jackets that also encase the whole front of the bullet, wrapping around the base to very firmly hold the bullet together, while varmint bullets tend to have open (sometimes WIDE open) tips or very large soft points. The 45gr Winchester bullets I've seen are fairly robust; they're not going to just vaporize on you in most AR-compatible loads. But I wouldn't push them too far beyond normal AR velocities, either.