Hey, brothers.
If one of you has guns chambered for calibers A, B, and C, and the other has only guns chambered for calibers D, E, and F, then when you go shooting together, each of you will have to bring and shoot your own ammo. If your guns are chambered for the same calibers, then your ammo is interchangeable.
If I had a brother, would I want his guns to shoot the same ammo as mine? Yes, if I thought it likely I could shoot up his ammo for free, or if he ran out of ammo and I could exchange him some of mine for something of greater value. No, if I thought he would end up shooting up my ammo for free.
Parker, brotherless but gotten used to it...