That doesn't make sense to me. Even if guns use the same ammunition, they still use different magazines (with some exceptions and those are represented in the game). Having ammo and magazines as seperate items would be a hassle to play and a hassle to code I imagine.
And what do you mean by:
This would add some life to older weapons which can accept large ammo types.
?
Do you mean put larger caliber ammunition into a gun, than it usually holds? That is physically impossible.
Besides
You get stronger ammunition for many old weapons later on, made out of alien alloys.