I have reloaded brass for my HK-91 (bought in 1982, still have it) and offer the following caveats.
1. The chambers are large. As Kevin noted, HK was all about reliability and the large, fluted chamber aided extraction. The guns still shoot pretty well. You will need a .308 small base die to size the cases. Use a stout press, the effort is high.
2. The cases are COVERED with combustion residue. This is what the flutes do. They convey high pressure gas over the case to "float" it for extraction. Then, I had a bitch of a time cleaning the cases before sizing. Now, tumbling with stainless steel pins would get them very clean, if not pretty.
3. The extractor, because of the violent action, puts a big ding on the rim. They sometimes do not fit the shell holder. I kept a little file next to the press. Also, as Kevin noted, the cases will probably be lost before they get real bad.
4. The case mouths sometimes get a BIG, v-shaped ding. They can be reloaded by using an expander and then sizing.
The good news is that rounds loaded to less than military pressures have reduced effects of all the above and make the rifle more pleasant to shoot. That is, if an 18" barreled .308 can be called pleasant.
