Nope. Shorter barrels don't develop less velocity because of less pressure. By that I mean the kind of pressure that causes hard bolt lift, sticky extraction and a safety issue in general.
It is because they truncate the length of the cylinder in the piston-cylinder system that is the bullet-barrel arrangement. The work done by expanding gas moving the bullet (which manifests as velocity) is described by an integral. If P is pressure and v is volume,then the work W is W = Integral of P(v)dv. By shortening the barrel you shorten the interval of integration. Since the integrand is nonnegative this results in a smaller value of W. Less work done means lower resulting velocity.
Peak pressure, the kind that causes problems when in excess, occurs relatively early in the bullet's travel down the bore. If you develop a load resulting in hard bolt lift and expanded primer pockets using a 26" barrel and cut the barrel to 20" you'll still get hard bolt lift and expanded primer pockets along with less velocity.
