[captain_dave]

So how strong does an AR15 style lower receiver have to be. Couldn’t it be made of plastic or even wood, so long as the holes all line up?

[fireman15]

People have printed lowers out of plastic with 3D printers but they are for demonstration purposes and not considered strong or durable enough for actual use with standard cartridges. You can use an inexpensive cnc milling machine to save a lot of effort when machining. https://ghostgunner.net

[Charles Martel]

So how strong does an AR15 style lower receiver have to be. Couldn’t it be made of plastic or even wood, so long as the holes all line up?

There have been a number of polymer lower receivers sold. The early ones had an integral buttstock, because the weak point is the rear, where the buffer tube threads in. Newer ones have metal inserts in the polymer to strengthen that area.

I haven't seen one made of wood, but it might just work.

[1_Inch_Group]

They do make polymer lowers

[WhirlwindAttack]

I've seen 80% polymer lowers for sale. My buddy has a carbon 15. They even make 80% Glock style lowers.
http://www.polymer80.com/