Projectile speed drives the initial weapon choice

Should a weapon be shortened while retaining seven-percent-light-speed projectiles, or keep its length and reach ten percent?

Keep its length and use the faster projectile because of the increase in impact energy.

The fictional design choice is between shrinking a weapon while retaining a projectile velocity of seven percent of light speed and keeping its existing length to reach ten percent. The initial preference is firmly for the faster weapon. Moving from seven to ten percent is described as almost a fifty-percent increase in speed. The reason given is the greater energy delivered at impact.