According to the Sporting News Record Book, the NL record for SLG by a rookie in 100+ games is .621 by George Watkins in 1930. The AL record is .618 by McGwire in 1987. As of last night, Braun's SLG is .643. If he plays every game the rest of the season, he needs to slug .575 the rest of the way to finish at .622 and he would then have the record. In the games since August 14, when his SLG was at .684, he has slugged just .214. But he slugged .716 in June and .673 in July. Even with his currect slump, his August SLG is still .600. So I think that he will come out of it and slug well enough the rest of the way to break the record.
The SABR Baseball List and Record Book lists the best slugging percentages by rookies who qualified for the league lead. Here they are. The number after the year is the player's rank if he was in the top 5.
- Mark McGwire-.618 (1987,1)
- Wally Berger-.614 (1930)
- Albert Pujols-.610 (2001)
- Ted Williams-.609 (1939,4)
- Hal Trosky-.588 (1934,4)
- Walt Dropo-.583 (1950,2)
- Dale Alexander-.580 (1929,5)
- Johnny Mize-.577 (1936,3)
- Joe DiMaggio-.576 (1936)
- Mike Greenwell-.570 (1987)
- Fred Lynn-.566 (1975,1)
- Mike Piazza-.561 (1993,4)
- Frank Robinson-.558 (1956,4)
- Dick Allen-.557 (1964,3)
- Tony Oliva-.557 (1964,3)
- Rico Carty-.554 (1964,4)
- Jason Bay-.550 (2004)
If Braun had enough plate appearances right now, he would be leading the league. If he plays every game the rest of the way, he might get 502 PAs and could finish in first. But whatever happens, he will likely crack the above list and that is usually a good sign.