Ok then! I performed the following test, with an M4 shooting approx 380 FPS, hop up OFF, with a witness and my camera taking pictures before each different round.
No Hopup therefore these are unaltered, random shots at the plastic lower.
Please Note that I am an official BB Bastard rep, I am mainly posting conclusions, unaltered in any form for the purpose of this discussion.
First shot is of the TM Plastic lower receiver pre any shots.
.28 Plastic, White, Styrene BB from aprox 10 feet. NO HOP.
Outlined in RED to show the impact marks
.28 Clear tempered glass BB from aprox 10 feet. NO HOP.
Outlined in BLUE to show the impact marks. As you can see, the top right shot, the indent is a little deeper than the plastic. Of course your eyes go right to the bottom where the second shot hit just above the magwell lip, and the pictures show the result..
.40 BB Bastard, Beige, BB from Aprox 10 feet. NO HOP
Outlined in BEIGE to show the impact marks. The first shot hit just to the right I assume of the magwell chip from the second SILICA round, and just took some more out of it. The second hit below the M, and left a beige mark. I found no real impact dents from the .40, they seemed to break appart very easy from impact.
These are my findings and results for the test. As I stated, unaltered, 10 feet, no hopup and just iron sights aimed. So pretty much random shots. The reason I chose 10 feet is that is the standard minimum engagement range for airsoft games, therefore that would be the minimum allowed for most shots to occur vs a plastic receiver.
Draw up your own conclusions! However they are pretty straightforward.