I find this whole idea of using a pistol caliber rifle in a 3 gun match intriguing.
admittedly, I don't do any IPSC or IDPA shooting but both the SWAT Magazine WC 3 gun and the SCTC matches i shoot in semi regularly have rifle targets out as far as 300+ yards. It seems sheer folly to use a pistol caliber gun at ranges like that.
do other 3 gun matches use all three guns on pistol courses or something like that?
as for shooting brens and BARs in the match, I actually conversed with my dad about that, and they're generally considered to be crew served weapons. I personally think it would be suitably manly to compete with a BAR, and if i had one and it was legal, i'd do it myself.
There is no restriction against military bolt action rifles. we've never had someone shoot the match with a bolt gun, but I think it would be neat if someone showed up with their lee enfield number 4 mk1(t) I'm sure it'd pass review as a combat worthy and legal for WC3 gun rifle and scope combination.