What is the reason this stock mod is so prevelent?

[BradBrownBess]

I have heard several opinions but never thought much about it. Why do so many stocks have the half moon carved out above the trigger on the lock side at the seer?

[Taylorz1]

Is that where a sear bar that comes below the bottom of the lock blows the wood out if the hammer is let down without a nipple or with the barrel off ?

[HIB]

Brad,      Stop by and we will see if any in the collection have this condition.   I know of one spring blow out that might answer your question.    HIB

[rich pierce]

Is that where a sear bar that comes below the bottom of the lock blows the wood out if the hammer is let down without a nipple or with the barrel off ?

Bingo! Also with wear a sear bar often drops below the lock plate. If the lock is removed and reinstalled for cleaning this is bound to happen.

[Bob Roller]

Is that where a sear bar that comes below the bottom of the lock blows the wood out if the hammer is let down without a nipple or with the barrel off ?

Bingo! Also with wear a sear bar often drops below the lock plate. If the lock is removed and reinstalled for cleaning this is bound to happen.

That can happen with the sear bar and it will only occur at the REAR of the lock inlet.Forward of that will probably be a mainspring link.
The whole thing looks like a low end job to me.A powerful mainspring can.if it breaks can knock the bottom off of a lock mortise and
in the past I got a number of requests for forged springs to replace too stiff cast springs that had created catastrophic failures in nice rifle
stocks.I told them to return the lock and the bill to fix the stock to the lock maker IF it could be fixed.None of these people would buy a lock from me because I insisted until the day I made the last one in 2019 for the time and skill involved in this type of work.Fine wood,good barrels and small trim and cheap locks were the thing then.
Bob Roller 

[T*O*F]

Also with wear a sear bar often drops below the lock plate. If the lock is removed and reinstalled for cleaning this is bound to happen

This seems to be most prevalent with locks that have stirrups, especially back action locks.  It is just good practice to always move the lock to half-cock when removing it.

[BradBrownBess]

Thanks everyone - yep that is on a military musket right above the trigger at the lock plate. That pic is a Charleville - and I tend to see it more on military muskets (I at least notice it more) - which might make sense as they would be constantly cleaned and the lock taken out, etc. I'm sure early mainsprings (1774 model is the mortise pic) and being mass produced might be prone to weaker metal on the mainspring.
Now of course this VERY well may have been carved out long after the musket was sold out of service - or in the field I would think a soldier would have to make do with what he had and hollowing out that half moon may have been "making do". Many of these were brought here for Rev War and so many of these military muskets were used until just slap worn out.
Here is a Charleville Lock back - similar to a Bess or most military muskets of the time.

[Avlrc]

This is interesting. I have had a couple of rifles that had this, I never knew what caused it, but now I know.  The funny thing is I showed one of my inflicted rifles to another collector & pointed out this hole.  Jokingly he said it was an "oil hole" to lubricate the lock workings, LOL.

[Bob Roller]

That hole guarantees an escape for the oil if overfilled and TOF is right about half cocking a lock if it has a two position tumbler or go
to full cock if it doesn't.
Bob Roller