Gun Stats: M252 81mm Mortar

Now that GURPS Gun Stats is out, I can finally stat the gun that, while absent from GURPS High Tech, will forever live in my head rent free: the M252 81 mm mortar.

Steel Rain, Steel Pain

The M252 is the medium mortar deployed by US Army and Marine infantry battalions, falling between the company level M224 LWCMS in both branches and the M120 120mm mortar used by the US Army (the Marines discontinued the M327 Expeditionary Fire Support System in 2019 and currently doesn't have a heavy mortar). It is a smoothbore, muzzle-loading, high-angle indirect fire support weapon.

Manufactured by Watervliet Arsenal in the US, it is a licensed adaptation of the Royal Ordinance L16A2 (see below). It was adopted by the US (replacing the M29)  in 1987 and remains in service today.  

Name: M252 81mm Medium Extended Range Mortar System

Date: ~1987 for the M252 (the L16A1 was designed in 1956)

Configuration: Bipod and baseplate or vehicle mount

Caliber: 81 mm (3.2 inches)

Barrel Type and Length: 50 inch smoothbore (this does not included the Blast Attenuator Device (BAD) as that portion of the barrel doesn't contribute to barrel pressure).

Overall Length: 56 inches (including BAD)

Action: Muzzleloader (8-16 rpm sustained, up to 30 rpm maximum)

Empty Weight: 35 lbs (bipod 27 lbs; baseplate 29 lbs; sight 2.5 lbs; total system weight 91 lbs) 

Cost: $24,717

Propellent: Smokeless powder

Ammunition Class: Semi-fixed

Reference Load:  While there about a dozen rounds for this weapon the most common one is the M821A1 HE round (or the effectively identical for game purposes M821A2, differing only in the specific model of multi-option fuze).

Projectile Class: High-Explosive

Projectile Weight: This is actually very hard to determine from sources. The total cartridge weight is 9.42 lbs (or 9.1 lbs or "about 10 lbs" depending on the source), but this includes the primer charge inside the tail assembly and the four horseshoe shaped charge increments. The charge increments weigh basically nothing, about an ounce each if my memory is accurate. So that might explain the difference between the 9.1 and the 9.42 lbs weights in different sources. In which

 case each charge is 0.08 lbs, the primer should be about same, so we'll call projectile weight 9 lbs. This includes 2 lbs of Composition B (or 1.7 lbs of RDX according to some sources).

Muzzle Velocity: 216/489/682/850/1001fps for 0-4 charges. We'll use the median of 682 fps, for most calculations, since we are going to use the real range data for Max.

Length-to-diameter ratio: 19.6 in/3.2 in =6.125, rounded to 6

Sectional Density:  0.88

Charge Weight: As noted above, 0.08 lbs each, 0.32 lbs for all four.

Cartridge Weight: 9.42 lbs with four charges

Ammunition Cost: $1000
Additional Notes: The BAD somewhat mitigates the hearing loss hazard, giving +1 to HT rolls to retain hearing (but mortar crews still wear earplugs, even in combat). The M64A1 sight is an illuminated 1.5x magnification telescope, but since this a mortar sight that will nearly always be aiming at a stake, not at a target this probably doesn't matter in GURPS.

GURPS Stats:

TL:8

Damage: HE has KE damage (at least when on a impact or delay fuze setting): 6dx2 (0.5) pi++. Although I'll note GURPS High-Tech doesn't give this for mortars, (presumably because these are indirect weapons that you fire at a 25 hex square) so I'll leave it off the weapons table. 2 lbs of Comp B is 6dx3 cr ex. It has a "high fragmentation" body so that's [5d].

Acc: 3

Range: 

1/2 D: 1220, but again not listed for mortars in GURPS High-Tech because nobody gets hit with a mortar shell at 3,000 yards. 
Max: 6,407 yards

Weight: 35/9.42

RoF: 1

Shots: 1 (2)

Bulk: NA (it's a mortar)

Rcl: 78, which is ridiculous, but also doesn't matter (except for calculating ST) and is another thing not on the table for mortars.

ST: 42M this seems high compared to other mortars, but I can't see that I made any errors.

Cost: $24,717

LC: 1

Skill: Artillery (Cannon)

Notes: Minimum Range is 77 yards at charge 0 (note this is both the range for maximum elevation at minimum charge and the minimum arming distance for the fuze, to prevent short rounds and misfires from blowing up the gunline). As noted it has the BAD. Typical crew is five: squad leader, gunner, assistant gunner and two ammunition men, but you can operate it with only two. The complete system includes besides the mount noted above, a canvas bag with the cleaning brushes, aiming stakes, night aiming devices (NADS, essentially tritium globes that attach to the stakes), etc. this weighs another 30 lbs, but is typically paired down to essentials when using the gun away from vehicle support. 

Artillery (Cannon) (IQ-5)

TL	Weapon	Damage	Acc	Range	Ewt.	Rof.	Shots	ST	Cost	LC	Notes
TL8	M252, 81 mm Mortar	6d×3 [5d] cr ex	3	77/6500	35/9.42	1	1(2)	42M	$24,717	1	[1] [2 [3]

[1] First Range figure is minimum range, not 1/2D. [2] HE-AB round with multioption fuse with proximity (AB), near surface burst (also AB), time delay (to penetrate light structures) and impact settings. [3] Blast Attenuator Device gives +1 to HT rolls to retain hearing.

Additional Ammunition Options:

Cargo Projectiles: 108 yard radius

IR Illumination or Illumination: $800

White Phosphorus: $500

Smoke (Red Phosphorus): $350

Development Rounds: These rounds are currently in development and therefore will be (according to GURPS Gun Stats, anyway) TL9 when and if they are deployed.

Advanced Capacity Extended Range Mortar (ACERM): HE with glide wings, control surfaces, laser guidance and GPS. Max Range is 21,000 yards. $10,000
Non-Lethal: Similar to DPICM, this munition spreads submunitions over the radius of effect. However these munitions are flash-bang grenades. HT-5 aff (108 yard). $5,000

It's not clear to me what GPS actually does for indirect fire weapons. GURPS Gun Stats p. 40 says that it adds to Acc and can switch the SSRT range penalty to the Long Distance Modifiers (B241). However the Using Artillery box, GURPS High-Tech p. 139, says that neither Acc nor Range penalties apply to observed indirect fire. The specifications for the ACERM is accuracy to within one meter.

Variants:

L16A1 The original British system, still in service. It can fire any of the rounds above, these stats are with an older TL7 HE round with an impact fuze ($600) TL7,  6d×3 [4d] cr ex, 24 lbs, ST 41M, $100,000. Does not have the BAD.

M252A2 Lightweight version available in 2020, 31 lbs, ST 41M. Bipod weight is 18 lbs and the baseplate is 21 lbs. The 2.9 lbs sight has x4 magnification.