This is one of:
 http://www.berklix.com/~jhs/src/bsd/fixes/FreeBSD/src/gen/sys/\
  netinet/ip_fw2.c.REL=5.5-RELEASE.diff
  netinet/ip_fw2.c.REL=7.4-RELEASE.diff
  netinet/ipfw/ip_fw2.c.REL=8.4-RELEASE.diff
  netpfil/ipfw/ip_fw2.c.REL=9.2-RELEASE.diff
  netpfil/ipfw/ip_fw2.c.REL=12.1-RELEASE.diff
  netpfil/ipfw/ip_fw2.c.REL=CURRENT.diff

 5.5 netinet/ip_fw2.c		autoinc_step = 100
 6.4 netinet/ip_fw2.c:		autoinc_step = 100
 7.4 netinet/ip_fw2.c:		autoinc_step = 100
 8.0 netinet/ipfw/ip_fw2.c:
 8.4 netinet/ipfw/ip_fw2.c:	V_autoinc_step = 100
 9.2 netpfil/ipfw/ip_fw2.c:	V_autoinc_step = 100
12.2 netpfil/ipfw/ip_fw2.c:	V_autoinc_step = 100

*** 7.3-RELEASE/src/sys/netinet/ip_fw2.c	Fri Sep  3 17:08:02 2010
--- new-generic/src/sys/netinet/ip_fw2.c	Fri Sep  3 17:08:44 2010
***************
*** 151,157 ****
  	u_int32_t		value;
  };
  
! static int autoinc_step = 100; /* bounded to 1..1000 in add_rule() */
  
  extern int ipfw_chg_hook(SYSCTL_HANDLER_ARGS);
  
--- 151,157 ----
  	u_int32_t		value;
  };
  
! static int autoinc_step = 20; /* bounded to 1..1000 in add_rule() */
  
  extern int ipfw_chg_hook(SYSCTL_HANDLER_ARGS);