As Undergiver points out, it bites both ways. Sometimes in our favour, sometimes against us.
Its all in the way each brand of mag is marketed.
And if you built a semi auto rifle that fired 9x19mm and used HK P7 mags, then it seems you could use P7 40S&W mags and get 13 shots out of your semi rifle. Actually, 13 plus one in the chamber.
http://www.rcmp-grc.gc.ca/cfp-pcaf/bulletins/bus-ent/20110323-72-eng.htm
Its all in the way each brand of mag is marketed.
same reasoning why we can have 10rd LAR Pistol mags for our AR-15's, and I also believe you can 12rounds of 9mm in a 40S&W Mag.
And if you built a semi auto rifle that fired 9x19mm and used HK P7 mags, then it seems you could use P7 40S&W mags and get 13 shots out of your semi rifle. Actually, 13 plus one in the chamber.
http://www.rcmp-grc.gc.ca/cfp-pcaf/bulletins/bus-ent/20110323-72-eng.htm


















































