My solid education on bullet performance, in particular expansion and penetration, comes from working for Barnes Bullets for almost five years. Not only was there thorough R&D testing, but during every production run bullets were pulled from the machines and sent to the lab where they were loaded and test fired for bullet performance. If anything was out of spec, the machine was adjusted before completing the run. Barnes tested thousands of bullets a week.

Pistol bullets have relative narrow velocity ranges of desired performance. They are slow moving to begin with. The velocity at which a bullet is designed to begin expansion is critically important. The velocity at which full expansion is achieved with the desired penetration and maximum energy dump is also important for self defense.

To test the expansion of bullets pulled off the production lines quickly and cheaply at various velocities, Barnes would load a range of powder weights for test cartridges and shoot them into a vertical water tank. After decades of recording the results, Barnes had a very good understanding of the difference 50 fps. lower velocity could mean to a bullet's expansion and penetration. Barnes also performed thousands of gel block tests on their own and competitors' bullets.

When you start with a barrel that is 3.1" in length, and almost all ammunition testing is done from 5" barrels (full size guns) for velocity and expansion norms, you will get lower than advertised velocities. How much? Various tests have indicated in pistols that the variance is about 50 fps. per 1/2". Porting releases gasses before they are finished pushing on the back on the bullet, thus decreasing velocity up to another 50 fps.

So, if a 9mm bullet is designed to start expanding at 1000 fps and get full, expanded penetration at 1175-1250 fps., as is common, but instead leaves a ported Shield barrel at less than 900 fps., the likelihood that the bullet will perform as desired is reduced.

There are myriads of tests and photographs available on the internet that confirm the above.

There are new loadings available today that account for short barreled self defense weapons and have bullets that expand adequately at lower velocities--like 900 fps.--but they will generally not achieve the desired 12-14" of penetration. A slow bullet that expands bleeds energy quickly and runs out of steam. Heavier bullets, even when expanded, retain velocity better, and therefore will usually penetrate better than a lighter slow bullet. Weight retention is a key factor for penetration, more so even than velocity.

Barnes had a .38 spl. loading designed for 2" snubbies that deliver velocities substantially below the typical 5" barrel service revolvers from which .38 spl. ammo is usually tested. Because the bullet expanded and retained 100% of the weight of the all-copper bullet, penetration was adequate. This is not an increased velocity loading; it is an increased performance loading.

If you choose to believe that porting has little to no effect on bullet performance or velocity loss, you are entitled to that opinion. Those who have actually done the testing see different results.