While I understand that your race does not employ anything but large metal projectiles at targets, we have a problem with how you're using the DR system. Which pains me, because I don't enjoy it — however, I'm the reviewer, so here I am.
I'll start first with the problem outside DR.
1.
Right now, we have several weapons in the setting that fire a round within a tenth of the speed of light.
One actually is a series of weapons fielded by the Gartagens. Quite frankly, I don't know how they were approved. Regardless, we can't use them as a basis to approve of what you're doing, and I don't even feel good about using them as a measuring stick.
Another is one of my design — the Chiaki destroyer's signature weapon, the Mass Driver Gun. Leigh already explained problems I had with the weapon, and I am fixing them in another tab. Its flaws are long overdue for a nerfing.
Origin has a starship weapon that fires nearly at the speed of light. That's too much, but that weapon also doesn't appear to be attached to anything other than an Abwehran/Origin product and the Atuan. Atuan somehow got that through, but I can't do much about it now.
Where am I going with this? Roughly speaking, you are talking about firing huge objects (1 ton!) through space at a speed that is absolutely mad. Nevermind I don't have a weapon against which to judge it — all I have are three rounds that go at a certain speed with defined weights.
That's not enough for me to review. I can't approve these rounds without more context.
I said three rounds because the 75 mm round confuses me. Is it used in handheld weapons as well as starship weapons? Because firing a 75 mm round at weighing 8+ pounds at 5,000 m/s generates 33,885,085 foot-pounds of energy. That is recoil energy. For comparison's sake, a round fired from a M1A1 Abrams tank generates 9,297,865 foot-pounds of energy. I'm not approving the 75 mm round out of a handheld weapon. You have your ADR4 rifle.
2.
Without (continuing to get) too technical, the amount of mass you add to each larger caliber isn't enough to justify the DR, especially when fired at the same speed.
When it comes to kinetic energy, speed > mass. You could justify the largest round being SDR5 if you significantly increased its speed instead of its mass. However, that's not going to happen, as I'm not sure 0.15 c makes sense anyway.
LATE EDIT: Also, nevermind that most gauss cannon rounds that do crazy damage are doing it through means other than kinetic force.