They learned about firepower, but also primarily that the old LE standard of taking cover and returning fire against a highly offensive, military style, mobile opponent was a good way to get dead.
The FBI knew this but just got fat and lazy and forgot the lessons the mobile heavily armed gangsters of the 1920's & 1930's taught them through hard lessons. Experienced hard nosed lawmen with gunfighting experience were called in to combat the equally hard nosed gangsters. The Federal Agents were also heavily armed with the likes of Thompsons, BAR's, Gov't Model .45's and .38 Supers, S&W 38/44 Heavy Duty revolvers and at some point the new S&W .357 Magnum. Complacency will get you killed.
