The 740/742/7400 series magazine have a bolt release in the magazine, while the 760/7600 series magazines don't have, or need, a bolt release. I think that you could use the magazines in either rifle, but the 7400 magazine would be a PITA in a 7600 because of the need to push the bolt release on the magazine at the same time you push the forearm forward. Conversely, if the 7600 magazines are used in a 7400, there won't be any bolt stop to hold the bolt open when a magazine is inserted or when the last round in the magazine has been fed, fired, and ejected.
Probably something that I'd do in a pinch, but would prefer to have the proper magazine for each particular rifle.
JEff
