Which Carbine Run Bartertown?

In order of introduction...

In GURPS terms, the unreliability of the M1 doesn't occur often enough to show.  Same for the notably finicky XM177E2.

Also note even though the XM177E2 and M4 are shown as LC2, it's because of the short barrels not RoF.  If we were to allow machine guns on this comparison, the XM177E2 should have RoF 15 and cost $32,000; the M4 should be RoF 9/- because of 3-round burst and should be unobtainably expensive.

The 5.56 guns sling lead much farther.  5.56 magazines are by far the cheapest.  5.56 magazines are also the lightest both in total and per round.

The SUB-2000 is lighter and cheaper.  9mm ammo is cheapest.

The M1 Carbine doesn't really win in any category...

The M1 Carbine's 4d+1 pi does 5-25 points of damage with an average of 15 per shot.
The XM177E2's 4d pi does 4-24 points of damage with an average of 14 per shot.
The M4 Carbine's 4d+2 pi does 6-26 points of damage with an average of 16 per shot.
The SUB-2000's 3d-1 pi does 2-17 points of damage with an average of 9 per shot.

The M1 Carbine sends 15-75 points of damage down range (45 average) per turn.
The XM177E2 sends 12-72 points of damage down range (42 average) per turn.
The M4 Carbine sends 18-78 points of damage down range (48 average) per turn.
The SUB-2000 sends 6-51 points of damage down range (28 average) per turn.

The M1 Carbine sends 150-750 points of damage down range (450 average) per magazine.
The XM177E2 sends 120-720 points of damage down range (420 average) per magazine.
The M4 Carbine sends 180-780 points of damage down range (480 average) per magazine.
The SUB-2000 sends 64-544 points of damage down range (304 average) per magazine.

The M4, the heaviest gun, is 13.4 lb. with a basic load of 210 rounds.

Going up to, but not over, 13.4 lb. with whole magazines only...

The M1 Carbine will have 210 rounds (12.9 lb.).
The XM177E2 will have 210 rounds (13.3 lb.).
The SUB-2000 will have 224 rounds (12.7 lb.).

They all get six spare magazines for this weight range.

The M1 Carbine sends 1,050-5,250 points of damage down range (3,150 average) per basic load.
The XM177E2 sends 840-5,040 points of damage down range (2,940 average) per basic load.
The M4 Carbine sends 1,260-5,460 points of damage down range (3,360 average) per basic load.
The SUB-2000 sends 448-3,808 points of damage down range (2,128 average) per basic load.

This is already getting kind of silly.  But should I bother to calculate carrying a 9mm pistol and two spare magazines?  It should end up being about the same because you're not shooting the pistol until your carbine is jammed or out of ammo.  You'd get a few points more damage from using the pistol mags in your SUB-2000 but not enough to shift the preponderance.

I did comparisons of various pistols earlier.