Small Caliber High Velocity Expanding Shroud Armor Piercing Core

The 4mm gauss rifle is a Traveller staple.

It's the standard Imperial Army rifle, advanced enough to be viable all the way to TL15 and simple enough to be handed to TL8 troops.  Easily manufactured at TL12+ (GURPS TL 12, 7, and 10 respectively).

As described in Traveller LBB 04 Mercenary: It fires a 4mm, 4g projectile at 1,500 m/s. (Translated to American that's a .16 caliber, 62gr, bullet going 4,920 fps. 3,315 and change ft/lb of energy at the muzzle.)  Even more, "The round itself consists Of a dense armor piercing core surrounded by a softer metal covering, ending in a hollow-point.  Giving the round both high stopping power and a good armor piercing capability."

In the 3e GURPS: Traveller book this rifle does 8d+1(2) cr, 1/2D 900 and Max 4,500.

Ultra Tech for 4e describes a 4mm gauss rifle with "almost" 5,000 feet per second velicity.  That sounds familiar, doesn't it?

Ultra-Tech gives 6d+2(3) pi 1,200/4,800.  The errata says that it should have pi-, but High-Tech p.163 says,

"4mm to 7.99mm (.16 to .31 caliber): At low velocities (pistol cartridges or black-powder weapons), damage type is small piercing (pi-); this models the behavior of rounds like the .32 ACP (which fires a 7.95mm bullet), .25 ACP, and .22 LR. For bullets of this caliber fired from high-velocity weapons (such as most centerfire rifles), damage type is piercing (pi)."

I defy you to define 4,920 fps as low velocity, so the pi- must be coming from something else.  Gauss guns are assumed to be using Armor Piercing Enhanced Penetrator (APEP) ammunition (it's where the 3 armor divisor comes from) and all the sub-20mm AP ammo gets a step down on the bullet size modifiers in 4e.

A straight conversion of the 3e weapon would simply change the Cr damage to pi-, but should also change the armor divisor to 3.

Compared (averages):

8d+1(3) pi- will punch DR 87 or 1.24" of homogeneous rolled steel armor plate.

6d+2(3) pi- will punch DR 69 or 0.99".

3e GURPS Traveller TL10 Combat Armor is DR 46.

4e GURPS Ultra-Tech TL10 Light Clamshell armor has DR 45.

The average hit to DR 45 will do: ((dice * 3.5) - (DR / 3)) / 2

7 with the GURPS: Traveller converted to 4e version.

4 with the Ultra Tech 4e version.

Not very impressive on the surface of things, but remember this armor completely stops 7.62x51mm NATO.  The average 7.62 NATO AP round will only manage to get 1 point of damage through.

Against someone unarmored, though, the average rolls get: ((dice * 3.5) -1)) / 2

14 with the G:T version.

11 with the UT4e version.

24 with 7.62 NATO ball, 11 with AP.

The given GURPS versions don't sound like "both high stopping power and a good armor piercing capability."  They barely make the victim roll for consciousness!

I have a solution...  Which after crunching the numbers, I am uncertain I like it.

SCHV Expanding Shroud Armor Piercing Core (ESAP): TL10.  A soft metal jacket shrouding the same tungsten-reinforced bulk amorphous metal penetrator of an APEP round.  Damage becomes two linked attacks with the second triggered by the first attack failing to penetrate the target's DR.  The first attack does normal damage with an armor divisor of (0.5); objects without DR get DR 1. Change damage type: pi- becomes pi, pi becomes pi+, and pi+ becomes pi++ (no effect on pi++).  The second, linked, attack does 80% of normal damage and is modified as per APEP ammuntion on Ultra-Tech p.152 but only if the first attack fails to penetrate the target's DR!  Multiply CPS by 15.

This changes the G:T damage to 8d+1(0.5) pi+ trigger linked 6d+2(3) pi- and the UT4e gun to 6d+2(0.5) pi+ linked 5d+1(3) pi-.

Against the DR 45 wearer: ((dice * 3.5) - (DR * 2)) * 1.5 trigger linked ((dice * 3.5) - (DR / 3)) / 2

The GT gun firing ESAP does 4.

The UT gun gets 1.

But against unarmored targets...  ((dice * 3.5) - 1) * 1.5 linked ((dice * 3.5) - (DR / 3)) / 2

GT gun: 42 points of damage and the linked attack does not occur because the first attack penetrated the DR.

UT gun: 33 points of damage.