The primary aim with the inserts was always to prevent clogging. A clogged HP will act like and FMJ in terms of performance. This will lead to over penetration. In most contract bids, a penetration over 20-22” is an automatic disqualification as the fedbois and major police depts (NY and LA) are risk adverse. They would rather an under penetration not stop a suspect then have to pay out on a pass through hitting a bystander. It also slows the filling of the hollowpoint cavity with fluid which slows expansion. This is an engineering issue that had to be overcome but making a HP open faster is much easier then stopping clogging.
I did forget to mention a couple of other rounds that do something similar to the insert. Browning X-Point which forms an X of lead in the cavity. It’s an elegant solution to get around the patent on the other idea, Federal Hydroshock with a thin cylinder formed out of lead in the center of the cavity. Both suffer from being fragile and hard to manufacture.
Something to remember is almost all major developments in self defense ammo are driven by the FBI get test. It’s only in the past 10 years that the industry started waking up to developing ammo specifically for CCW and not just rebranding LEO ammo.
I know what the stated purpose of the inserts are. I am saying that in reality, they do not perform that function. Feel free to pull the plugs out yourself and try the rounds through four layers of denim or a heavy clothing layer if you do not believe me. Without the plugs, the rounds penetrate a little less and expand more rapidly, as you said, which I think is effectively just compensating for poor hollow point design; here is the one Youtube test I have seen that tried it.
That is why I view them as gimmicks, not actually useful additions, and is why I think many manufacturers do not bother. Velocity and hollow point design will make cloth a nonissue, generally speaking. HST is effectively the JHP king, and it does not need any plug to function reliably or penetrate adequately.
Browning X-Point isn't an elegant solution since it does not work generally (Hydrashocks are solid based off what I have seen though). It's nice branding, but not actually nice function. Pretty much all of these are marketing, and every single instance I see of the x point functioning as designed is not fired into a clothing barrier, which IMO renders the test utterly worthless. Just because many carry options are marketed initially to LEO and feds doesn't change the fact that they are using marketing gimmicks to try to make their product seem better than it is. They are still selling a product to people, they are just people with a badge.
40 S&W seems a bit better but also overpenetrates based off of this.
I am well aware that gel tests are the standard. That is why I am referring to them for information rather than taking my redneck tests as gospel. Barring access to whatever LEO or fed tests that are supposedly being conducted, this is what I have to go off of. The tests I see from manufacturers are typically worthless since they use bare gel, and I do not put any stock in their results whatsoever.