I personally don't really mind bloom, as it is kinda realistic... There's just too many people complaining about it. Trollish story right below:
Reach happened before 1,2,ODST and 3. When the UNSC released the DMR, the Marines massacred -blam!-tonnes of Covenant. The Marines found the challenge way too easy, and asked Keyes on the PoA to ban all usage of the DMRs, as the DMRs had something called "bloom". Keyes didn't understand why the Marines would want to get rid of such a fine weapon, but he did it anyway. They placed them in airlocks, and released them into space. The DMRs were never seen again.
That, my friends, is the story of the DMR.