|Permatex 81730 sealer flows well when the nozzle is cut small and the sealer is applied to one side of the cavity to allow the air a path to flow out. Over-flow peels off easily.
Elastomer tipped bullets are all the fashion in some circles.
Ballistic engineers figured that one work-around for the standard FBI test involving four layers of denim involves pre-filling the hollowpoint with a soft, rubbery material.
The problem with denim (and cardboard) is that it packs into the hollowpoint on impact. When compressed from the tip, the column of fibrous material does not bulge outward.
We are all familiar with how rubber bands get thinner when they are stretched. Rubber cylinders or plugs also get thicker as they are compressed. For the amount of strain involved, the volume of the rubber shape does not change and the rubber behaves much like hydraulic fluid when trapped.
Consequently, when a hollowpoint bullet has the hollow filled with a rubbery material (but not foam rubber), then when the bullet hits a target the bulging rubber plug exerts outward forces on the sides of the hollow and assist in the initiation of the expansion.
And while I would never encourage anybody to experiment, I will note that a flowable, soft, rubbery sealer might make an acceptable substitute. Further, I might note that the 3mm diameter by 6mm deep hole shown on these Zero 158 grain, genuine pewter medallions would only add about 0.8 grains of mass and a 1.5 ounce tube would be sufficient for over 800 such bullets.