Proof

Doing up Twilight 2000 in GURPS with equipment upgrades has caused some interesting things.

Things like how much DR does an Outer Tactical Vest have?

How much DR should the vest itself have?  SAPI plate?  ESAPI plate?

The OTV plus SAPI is supposed to be proof against 7.62x51mm ball at essentially muzzle velocity and it must take three of these hits.

7.62x51mm at that velocity is 7d pi.  Which is 7-42 points of damage, so a DR of 42 stops it dead every time.  A DR of 24.5 stops it on average.

The vest is DR 12/5*.  The split DR uses the higher value agains pi or cut attacks, lower against all other types like cr or imp.  The asterisk means it's flexible armor and subject to the blunt trauma rules.

12 is not enough to stop most 7d pi attacks, but it's not supposed to.

The SAPI plate is DR 23.  This combines with the 12 to give a DR of 35 against rifle fire.  The SAPI plate is also ablative armor, and loses 1 point of DR for every 3 hits it receives, regardless of if the damage penetrates.

DR 35 is going to stop nearly all 7d attacks!  But it won't last forever.

Next, the ESAPI plate is supposed to be proof against .30-06 Armor Piercing, 7d+1(2) pi-.  The same penetration criteria stands for acceptance and most places say its more effective against normal penetration too.

So I gave it a DR of 35 but made it hardened level 1 armor.  This gives a total of DR 47 against pi or cut attacks!  It's still ablative as the SAPI plate.

The 8-43 range of the .30-06 AP normally gets an armor divisor of 2.  That would halve our DR 47 to just 23.  What hardened armor does is eliminate the same number divisor steps as levels (the steps are 100, 10, 5, 3, 2).  In this case, the DR 35 of the ESAPI stops 35 and the underlying vest 6, so 41.  So, nearly any .30-06 AP shot is blocked by this combination.  Again, not forever.

If it stopped there, it'd be fine.

.50 BMG does 7dx2 pi+.  That's 14-84 points of damage running, on average, to 49.  On average, a .50 ball round barely penetrates an OTV and ESAPI combination.  Notice that it readily penetrates the plate though!  This appears to be accurate believable based on a couple of YouTube videos shooting Level IV armor plates with a Barrett!

.50 BMG AP does 7dx2(2) pi.  This drops the OTV+SAPI DR to 17 and nearly any hit breezes right on through.  The OTV+ESAPI still gets 41.  On average, 8 will still get through.  Again, this kind of result is borne out by a couple of YouTube videos.  It's scary, sometimes, how plausible GURPS numbers are compared to a reality check.