I saw the shield in CUP couple of days ago, and immediately had to try it out. I really like the idea, and how it was implemented, so thanks to the author for this cool feature :)
Now regarding the problem at hand, there is a certain spot that shield doesn't protect you from bullets: https://www.youtube.com/watch?v=FMCqXlIT_v0
Also, it would appear it only protects you from some pistols, almost any other rifle goes through, even the 9 mm MP5 (from CUP)
I'll have a look at that certain spot that you can shoot through and get it fixed.
Regarding penetration, personally with this model of shield anything 556 and up should be able to penetrate it but I'll sort out "small arms" penetration values so we don't have any issues like you pointed out. I'm unfamiliar with most vanilla a3 rounds however any gun you use for testing try make sure it isn't using AP type rounds (not sure if that is even in vanilla or possible though) but could be a thing to look out for.
I'll do some more testing myself and see what results I can find.
Ah, I really hoped it could stop 5.56 and 7.62x39 rounds (not 7.62x51), as infantry never carry anything smaller than 5.56, be it vanilla or modded, so you don't get much chance to use it.
Nevertheless, it's a very nice addition to the game, thank you for looking into this!
After some testing and new understanding of how the Fire Geo works, I've managed to fix this issue so the shield should no longer have certain glitched spots to fire through.
The shield will now withstand about 15 rounds of 5.56 before it breaks and can be penetrated, obviously this will be different for each Round so a smaller one like 9mm should take longer/more to penetrate.
You can see from this video the effects of this, the shield is not invincible so I guess anyone that uses it in the future will need to use it in a tactical way and not over expose the shield to too much fire.
Apart from that, thanks for the report.