What Goes Wrong, What It Costs, and What to Decide

6.1 What Goes Wrong

A tethered gun fails mostly at its ends. The table lists every failure this dive’s design has an answer to, the answer, and where it is argued.

Table 1 — 6.1 What Goes Wrong

Failure	What the player sees	Guard	Where
Pulse current in the USB ground	Disconnect sound when the coil fires; gun re-enumerates	Separate 24 V return conductor and pin; one join in the gun; floating brick	Vols 3, 5; Power Supplies Vol 5
A second ground join in the box	The same, intermittently	Plastic box; J1’s sleeve joined to core 2 only	Vol 4, 4.2–4.3
GX mated one position out	Dead gun until the key drops in	24 V on the centre pin; VBUS between the pedal pins; brick unplugged during mating	Vol 3, 3.5
Brick plugged in live at the box	Spark at J1; overshoot at J12	Mating order on the lid; LED shows “live”; electrolytic reservoir and SMBJ28A	Vol 4, 4.6
Thin USB cable	Gun browns out or will not enumerate on some ports	28/24 (or 28/22) cable: 63 mV ground drop at the gun’s draw over 4 m	Vol 2, 2.3
Flex fatigue at the gun end	Intermittent USB, a cut-out when the gun is twisted	Constant-flex cable for the power cores; heat-shrink boot sets an 8 × D bend; jackets stopped inside the boot	Vol 2, 2.6–2.8
Untwisted pair at the cups	Nothing, until it is long	Under about 25 mm each side	Vol 5, 5.2
Umbilical crushed by a chair	Kick stops; gun keeps aiming	The brick hiccups on a 24 V short; USB is separate	Vol 4, 4.4
Earthed brick substituted	Disconnects return	Meter check: output 0 V to mains earth must be open	Vol 5, 5.6
Wrong brick	Blown fuse or TVS; or a weak kick	6.2
GX contacts worn or oxidised	Rising drop; then intermittent USB	Contact-resistance check, new and after use	6.3
GX coupling ring backs off	Intermittent everything	Ring checked at every session; see 6.3	6.3
Cable snag	Gun yanked from the hand, PC pulled off the desk	6.4
USB-A plug wear at the PC	Loose fit	Leave it in the PC; the box takes the handling. The USB-A connector is specified for 1,500 insertion cycles (USB 2.0 Table 6-7)	Vol 4

6.2 The Wrong Brick

The junction box’s barrel jack is a 5.5 × 2.1 mm socket, and every brick of every voltage with that plug fits it. Power Supplies Vol 2 makes the point for the gun; with the jack moved to the box, it applies to the box.

Table 2 — 6.2 The Wrong Brick

Brick plugged in	What happens
12 V	The regulated stages run with half the headroom. A coil chosen for 24 V may no longer reach its set current: a weak kick, nothing damaged.
19 to 20 V laptop brick	Inside the 18 to 28 V window (D14); headroom reduced (Vol 2, 2.4 gives the largest coil at 18 V as about 6.7 Ω at 2.4 A). Some laptop bricks have earthed outputs (Vol 5, 5.6), so check.
24 V, the intended class	As designed.
36 V, such as a supply bought for an AimTrak (Ultimarc specifies 24 to 36 V)	Above the SMBJ28A’s stand-off and the window’s 28 V top. The TVS conducts heavily, and either F1 opens or the TVS fails. This is the realistic hazard in a house with more than one light gun.

The guards are the label beside J1 (“24 V ONLY, centre +”), keeping the gun’s brick with the gun, and, if the hazard is judged real, a different jack for the box. A 5.5 × 2.5 mm jack would refuse the 2.1 mm plugs most bricks carry, but the GST60A24-P1J has a 2.1 mm plug, so that would need a re-plugged brick. That trade is not worth making for the first build.

6.3 Connector Wear and a Ring That Backs Off

Contact wear. No GX16 seller publishes a mating life; the one maker figure for an M16 connector of the same size is binder’s “more than 500 mating cycles” (Vol 3, 3.2). A gun unplugged once per session reaches 500 matings in a year of daily play. Wear shows first as rising contact resistance. Because the 24 V contact runs under half its lowest rating, and the regulated stage tolerates a volt of extra drop (Vol 2, 2.4), the first symptom would most likely be on USB: a gun that occasionally fails to enumerate. Check W3 measures it.

The practical policy: leave the gun plugged in at its station. The GX is a connector for packing the gun away, not for every session; the USB-A plug stays in the PC; the brick is unplugged at the wall, not at the gun.

The coupling ring. A threaded ring on a part that takes a solenoid’s kick dozens of times a minute may work loose. A loosened ring lets the plug rock in the socket and the contacts fret. No published figure exists for GX rings under vibration. The check is simple: mark the ring and the panel part with a paint line once tight, and look at it after a session of autofire (W8). If it moves, a thin rubber O-ring behind the ring, or a printed collar that clips over it, holds it.

6.4 Snags

A cable on the floor will be stepped on, caught by a chair, and walked through. The question is what gives way.

• In this design, the GX and the base plate take the pull, then the gland at the box, then the USB-A plug in the PC. If the umbilical is caught near the box, the box (screwed down by its flanges) holds and the gun is pulled from the player’s hand. If it is caught near the gun, the base plate takes it.
• The PC end is the expensive end. A floor box screwed to the desk leg or floor stops a yank reaching the PC’s USB port. This is the strongest argument for screwing the box down.
• A breakaway connector would let go before anything else. No stocked magnetic or breakaway connector was found with eight contacts and a rating for the 24 V pulse (Vol 3, 3.9). The GX should therefore be treated as a non-breakaway joint, and the cable routed accordingly: from the box straight to the floor in front of the player, never across a walkway.
• Tripping. A 3 m cable lying from the screen to the player is a trip hazard for anyone else in the room. The fully wireless architecture’s main advantage over this one is exactly this (Fully Wireless).

6.5 The Bench Checks

Every one uses the lab’s own instruments (the Analog Discovery Pro ADP3450, the programmable bench supplies, the NanoVNA, calipers, a luggage scale). They are numbered so that other volumes can cite them.

Table 3 — 6.5 The Bench Checks

#	Check	How	Settles
W1	Fit	Calipers: the GX16 plug’s clamp bore; the panel part’s hole and nut against the printed base plate	Whether the boot of Vol 2, 2.8 fits; GX16 or GX12
W2	Can a slip happen?	Calipers: key projection ahead of the pin tips; pin recess in the shell; then try to touch a pin to a wrong socket with the key out of its slot	Whether the slip of Vol 3, 3.4 is physically possible at all
W3	Contact resistance	Four-wire: 1 A from a bench supply through each mated contact, ADP3450 across it; new, after 100 and after 500 matings	The 20 mΩ seller figure; wear
W4	Reservoir discharge	Scope the GX centre pin to position 7 after unplugging with the brick live a moment before	How long the gun’s pins stay live (Vol 3, 3.6)
W5	Hot-plug overshoot	Single-shot capture of V_SOL at J12 as the barrel goes into a live brick	Whether the SMBJ28A works hard; whether C4 needs damping (Vol 4, 4.6)
W6	Ground offset	Gun ground minus the PC’s USB shell, two channels, idle and in autofire	The 63 mV estimate; that firing does not move it (Vol 5, 5.4)
W7	USB under fire	USB decode or host disconnect log through several minutes of autofire; then with a 0.5 m lead for comparison	Packet integrity; no disconnects (Vol 5, 5.5)
W8	Mechanical	40 N pull at the gun and at the box gland; ring witness mark after autofire	Strain relief; ring backing off
W9	24 V drop	Brick plug and J12 voltages during a regulated pulse	The real loop resistance against Vol 2’s 0.36 Ω
W10	Cable delay and impedance	NanoVNA, time-domain, on the open-ended 4 m USB pair	The cable meets 90 Ω and 26 ns (Vol 5)

6.6 What the Whole Wired Setup Costs

Prices as read on 2026-10-19 (2026-10-18 for the brick, from Power Supplies), from the pages cited in the volumes. Several sellers refused automated reading; those rows give no price rather than an unverified one.

Table 4 — 6.6 What the Whole Wired Setup Costs

Item	Pick	Price	Source
24 V brick	Mean Well GST60A24-P1J	$19.40 (DigiKey) to $24.86 (TRC), 2026-10-18	Power Supplies Vol 2
USB cable, 28/24, 15 ft	Monoprice 5440 or equivalent	not read	Vol 2
4 × 20 AWG flex cable, 3 m	Alpha Wire 85004 (or an unrated 4-core)	not read	Vol 2
Expandable PET sleeve, 3 m, 1/2 in	any	not read	Vol 2
GX16-8 connector pair	Tayda A-8801 and its mate, or equivalent	$1.09 for the A-8801 part	Vol 3
Heat shrink, adhesive-lined, for the boot	stock	stock
Micro-USB pigtail	a short micro-USB cable, cut	stock	Vol 3
JST VH plug for J12		stock	Reference Electronics
Junction box: FB27, gland, bushing, PJ-392, LED, resistor	Tayda	about $4.00	Vol 4
Barrel jack J1	Same Sky PJ-005A (or Tayda DC-099, $0.06)	not read	Vol 4
Known total		about $25 to $30, plus the cables and the PJ-005A

The cables and sleeve are the unknowns, and they are the bulk of whatever is missing: a few dollars for the USB cable and the sleeve, more for flex-rated control cable bought by the foot. A builder using a cheap unrated four-core cable for the first bench umbilical (Vol 2, 2.10’s fallback) will land under $40 for the whole wired setup including the brick.

For scale: Ultimarc charges $25 more for the AimTrak with recoil ($120.00) than without ($95.00), on 2026-10-19, and ships the recoil gun without its supply (Vol 1, 1.6). The wired architecture is the cheap part of a recoil gun; the solenoid, its driver and the shell are where the money goes.

6.7 Decision Rows Proposed

For DECISIONS.md, all Proposed:

Table 5 — For DECISIONS.md, all Proposed

#	Part / choice	Candidates	Pick	Why, and why not the others	Vol
WD-1	Umbilical	USB 28/24 + 4 × 20 AWG flex in one sleeve; + 2 × 18 AWG and a pedal pair; unrated 4-core; Cat 6A S/FTP; multicore with untwisted data; chainflex; 28/28 USB; coiled cord	USB 2.0 28/24 (or 28/22) shielded + 4 × 20 AWG constant-flex (Alpha Wire 85004 class), PET sleeve, 3 m	Real 90 Ω pair; separate 24 V return; pedal on spare cores; flex-rated. Under regulated drive the gauge moves the regulable coil only 8 % from 26 to 18 AWG, so 20 AWG is chosen for robustness and diameter. Cat 6A: 24–26 AWG power and no spare; untwisted multicore: no defined impedance; chainflex: too stiff; 28/28: fails 125 mV	2
WD-2	GX16 pin map	24 V on the circle; VBUS beside ground; six or seven pins	GX16-8: centre +24 V; 1 Pedal, 2 VBUS, 3 Alt Pedal, 4 D+, 5 D−, 6 USB GND, 7 24 V RTN (clockwise from the key, gun’s mating face)	Meets D31 by geometry: no rotation moves the centre. VBUS slips into a pedal input behind 1 kΩ (about 4 mA), never onto a data pin or a reversed supply. Settles CI-7’s pin question	3
WD-3	Far-end junction box	plastic passive box; diecast 1590A/B; moulded Y-split; USB-B socket on the box; box with fuse/switch/hub	Passive plastic box (FB27 class): PJ-005A, PJ-392, LED + 10 kΩ, M20 gland, bushing; USB passes through; pedal sleeve to USB GND by a window tap	A metal box joins the jack sleeves (a second ground join); every electronic addition duplicates the brick’s or the carrier’s protection or adds a pulse-loop contact	4
WD-4	USB shield termination	both ends; PC end only; gun end only	PC end only (USB-A shell); folded back at the GX	No spare GX pin; no chassis in a printed gun; a gun-end bond is a parallel ground path	5
WD-5	Mating order	as D31; plus LED; plus switch	GX first with the brick unplugged; barrel in with the brick unpowered; brick into the wall last; “live” LED in the box; order on the lid	Keeps the only live source during GX mating the current-limited VBUS; avoids hot-plug overshoot; a switch is a pulse-loop contact and easily left on	1, 4
WD-6	Pedal ground	USB GND conductor; 24 V return core	USB GND, tapped in the box	The pedal input is referenced to logic ground; the 24 V return carries up to 0.5 V of pulse drop	4

Correction to a sibling, minor. Power Supplies Vol 2 and D31 give the GX16’s current rating as “4 to 7 A” from sellers. On 2026-10-19 Olimex listed its four-pin GX16 at “20 A / 220 V”, so the seller range is wider still (5 to 20 A). The conclusion is unchanged: no maker datasheet was found, and the design runs every contact at under half the lowest figure.

6.8 Questions for the Builders

1. Adopt the pin map (WD-2)? It also confirms CI-7: the pedal jack goes in the box.
2. Base-plate width (W1): does a 16 mm GX16 panel part and its 18.8 mm nut fit the printed magazine’s floor plate, or does it fall back to GX12?
3. LED or switch in the junction box (WD-5)?
4. The pedal TVS part for the box, chosen with the carrier’s input protection.
5. First umbilical: buy Alpha Wire 85004 by the foot, or start with an unrated 4-core and upgrade if it fails at the boot?
6. Brick discipline: is there a 36 V AimTrak-class brick, or any 12 V brick with a 5.5 × 2.1 mm plug, in the same room? If so, consider a keyed connector at the box (6.2).

6.9 What Could Not Be Verified, Across the Dive

• GX16 maker data: none exists that was found; contact resistance at eight pins, mating life and clamp bore are seller figures or unknown (W1, W3).
• Prices of the USB cable, the 85004, PET sleeve and the PJ-005A (seller pages refused automated reading on 2026-10-19).
• Alpha Wire’s flex-test method behind “up to 14 million” cycles.
• The gun’s 5 V draw (150 mA is Power Supplies’ estimate).
• The slip itself (W2): whether a GX16-8 can be mated one position out at all.
• Hot-plug overshoot at J12 (W5) and reservoir discharge after unplug (W4).
• Coupling and packet integrity under autofire (W6, W7): expected clean, unmeasured.
• Linear Technology AN88 was identified but not read; its content is cited only for the general mechanism.