I would first look at all SW's on the play field that can score 100 points, expect one to have a small gap.

Hi Toyguy
I do not have a Bally pin - in ipdb.org there is a schema "Mariner" - hard to read. Do you have a schema ?

On my Williams pins I easily can produce "Machine Gun" of "Score-Reel-100-Points-Player-1". With a bit of imagination I can produce "Machine Gun - sometimes" on any "Score-Reel-100-Points-Player-X". With even more imagination I can produce different kinds of "Machine Gun - sometimes" depending on "Player-X".

Look at my PDFs - I once made these PDFs helping on the problem "Aztec - 100-Point-Score-Reel-Player-1 sticks (keeps-on pulling)".

For Your problem - Have a look at "B3" (and "B6") - in my PDF-1 I show "10 Points are given":
Look at Your 100-Point-RELAY -> the "Self-Hold-Switch***" - When the relay is NOT pulling: Is there a nice (big) gap between the two blades / contact-points ? If these contacts are very close together: This can cause the "Machine Gun". Also check: Does the "Anchor-Plate" sits well (on the holding-points) ? Is there a "retaining / returning spring" ? (Nicely pulling ?)
Also check the "E.O.S.-Switch***" on "Player-1-100-Point-Score-Reel" / .... Player-2... At the "End of travel of the plunger": Is this switch widely-opened by the plunger ? Compare ...Player-1... and ...Player-2...: The same setting ?

"Self-Hold-Switch***": From the "Coil of 100-Point-Relay" (Lug on one side) there is a wire running to a solder-Lug on a "Switch attached at this relay" - the "Self-Hold (Circuitry) Switch" - (when the relay is inactive: is there a nice gap between the blades ?).
"E.O.S.-Switch***": In the Score-Reels there are plungers - they travel - at the "end of travel" they hook-on with a teeth (tooth ?) (and when travelling backwards) turn the Score-Reel. (At the End-of-travel: does the switch opens widely ? / On all Score-Reels at "same point of travel" ?)

Toyguy, want to produce (as a test) "Machine Gun on "10-Points-Score-Reel" ? -> Start a game -> Take a Screw-Driver (handle insulated) and carefully hold the tip of the Screw-Driver at the "Self-Hold-Switch on 10-Point-Relay" - at the Bakelite-Holder ! (The relay shall be able to act / move. You ONLY make a connection between the two blades of the Self-Hold-Switch !!!) - As long as You hold-on the Screw-Driver: "Machine Gun" ...

Some theorie: "A-1 to A-8" - Part of the "Reset-Cycle at start of a game": ... Score-Motor (A-4) is turning and with notches it pulses the "No-1-Reset-Relay (A5)" -> therefore "A6" pulses until the turning of the Score-Reel (done by "A-7") opens the "Pos-Zero-Switch A-8".
Actually, "A6" pulses "on / further" until ALL Score-Reels are at Pos-Zero. All Score-Reels have TWO Pos-Zero-Switches -> when the other Zero-Switches (not on the drawing) are ALL open -> "Reset-Cycle" will be stopped.

"B-1 to B-6": 10 points to be given.

"C through K" - The ("C")-100-Point-RELAY is "faulty-always-pulling": Maybe the cause is on "D", "E" ... "K". ((("E" is very theoretical))).

Toyguy, do You have a schema for showing us "interesting parts of schema" ? Greetings Rolf

Hi Toyguy
and the PDFs ... Greetings Rolf
PDF-A-B.PDF
PDF-C-K.PDF

I Have a PDF copy of the schematic if you want it PM me. I have also upload a copy to IPDB, it may be a while before it is available there