Firing prone question

[Colt Ward]

Rules questions being answered at the rate they are . . .

So its come up and seems off to me . . . when a mech is on the ground, standard rules say it has to prop up with a arm to fire weapons- be they in the torso or other arm.  Being on the ground propping up, its a +2 to the THN for being prone.  But BMM says you add a +1 for propping on a arm, which means +2 for being prone & +1 for propping on a arm to make it a total of +3 TH?  I do not have BMM, just TW and the charts from the box which just gives the +2.

[RoundTop]

BMM p.30

"Add a +2 Target Number modifier to weapon attacks made while prone, in addition to any other standard modifiers."

That is the same as TW.

What you are missing is the next line:

"One-Armed Prone Fire (Optional): Under this optional rule, a prone ’Mech with one arm destroyed may still prop itself up using its intact arm and fire any non-limb mounted weapons. Apply a +1 Target Number modifier (in addition to the standard prone modifier) to all shots made in this fashion."

That is TacOps one-armed prone firing, where you are missing an arm, and is an optional rule (otherwise you cannot shoot from prone when missing an arm.)

[Colt Ward]

Okay . . . so, does that also apply when a prone mech is propping up and firing weapons in one arm?

Since it sounds like TW is 2 arm prop vs TacOps 1 arm prop optional rule

[nckestrel]

Okay . . . so, does that also apply when a prone mech is propping up and firing weapons in one arm?

You can only fire weapons from an arm, while prone, if you have two arms.  You only get the +2.  This is the TW and BMM rule without the optional rule.

Since it sounds like TW is 2 arm prop vs TacOps 1 arm prop optional rule

That's exactly what it is.  Some TO rules made it to BMM.  The additional option in BMM is the Tacops 1 arm prop.
In TW, you need 2 arms.  You can fire the torso and one arm.  +2.
TacOps presents an optional rule, that BMM lists as an option, you can use when you have only one arm.  You can only fire torso, and there's both the +2 and the +1.
The +3 is only when you have a single arm and you can fire any arm weapons then.

[RoundTop]

And I had a raging debate with an old player a month ago who insisted that you could always use one arm to prop and fire with the standard +2 penalty.  I showed him rulebooks going back to 2nd ed. that you always needed both arms to fire from prone.

[Paul]

And I had a raging debate with an old player a month ago who insisted that you could always use one arm to prop and fire with the standard +2 penalty.  I showed him rulebooks going back to 2nd ed. that you always needed both arms to fire from prone.

Yeah, one of the more typical misconceptions.
In their defense, it is a little bit hard to follow when you quickly read the rules. Usually I say "Do a pushup with 1 arm", which makes it easier for some to be at peace with.