Chuck
New guy here on the forum, but I shoot a 27 in IDPA/GSSF.
I use 22 mags for reloads, but I start w/the 27 mag (ie 9 rd) w/a peirce +1 mag extension (ie gives me 10 rds).
If the stage has multiple strings, I will have to start w/a 22 or 19 mag, in the gun, on the later strings.
I've gotten some funny looks @ a couple State IDPA matches, from ROs, and had 1 individual make a point that I should be shooting ESP, instead of SSP, w/that set-up.
Not alot of us "little gun" shooters out there, so sometimes you have to talk to the RO, if there is a question. I usually carry my rule book around w/me. So far it's worked.
Good shooting,
Jay