im not 100% sure how the timing in a V6 works, but it should be similar to the timing in a V2/3
where the tappetplate meets the gears, the gear should have a little nub on it that the tappet plate hits with the gear turnes
in a V2/3 you can see above how the gear has a little nub and how the tappet plate is next to it. the pic above shows proper timing with the nub at 2 o-clock
i cannot seem to find any pics of what the gears in the M1a1 look like, but im sure its close. The tappet plate should have a small spring that connests it to the mechbox shell to pull it forward(and the nub on the gears pulls it back). if your timing is wrong, that would explain why your not firing even with compression.