Flat straight lumber can be made from crooked, or twisted trees, or parts of trees that are from the side that transitions from the tension side to the compression side. So sometimes, due to how it grew, wood is under tension inside, even though it was dried and milled straight, and then as you remove wood during a project, that is, wood that was helping to stabilize it, it then is permitted to wander. Sometimes it can be corrected with heat.
Wood is made maleable with heat. Being new to gun making, I haven't done it to a gun stock, yet, but have done it literally hundreds of times to various species wood used for bows and arrows where precise alignment is critical. Since I'm quite familiar with it, I wouldn't hesitate if that's what I thought a gun needed and I calculated it could be pulled off. Various sources of heat can be used depending on the condition of the wood and the desired effects. I made a steamer for a few dollars that I've used a lot over the years, usually on unseasoned/green wood... mainly because dry heat can open check/cracks in wet wood. When the wood needing corrected was isolated to a small area, I have also steamed it over a kettle of boiling water on the stove. BUT for dry seasoned wood like a gun stock should be, I usually use a heat gun. It can get dry wood very hot and bendable without checking... or scorching as a torch would be prone to do.
Basically you want to keep the heat gun several inches away, keep it moving over and around the area to be bent until it's hot enough you can't hold it in your hand, maybe go a minute longer, then make the correction, overcorrecting about 5% because it will spring back a little, and hold it there until thoroughly cooled throughout. It should move relatively easy if it's soft/hot enough. Don't force it or its likely to break or crack. And hey, I don't want to be responsible if it does
