Still falling...block

Just an update on my 3D printed falling block rifle.  I had a question on how strong the 3D printed barrel threads will be, or how much force it would take to blow the barrel forward out of the receiver.  Someone much smarter than me did the math on the thread shear strength using the rough numbers for PLA, and came up with a minimum of 6,815 lbf, with "ideal" threads giving me 13,583 lbf.  That's well over the amount of force that the DAG training ammo can generate.

I think I've got my design far enough along that I can make a test print.  I'm not sure what I want to do with the top of the handguard.  I don't really want a top handguard, but it looks like it's missing something without it.  I can't make the front of the receiver smaller or longer because of the barrel nut.  I like the Space Cowboy look, so I might end up with some sort of optic mount(my Savage Axis barrel doesn't have a front sight on it anyway).  

Help! I'm falling... a 3D printed falling block rifle

New project time, because apparently I don't have enough projects going on...

Ever since I discovered the very low power plastic DAG 7.62x51/308 training ammo(10g plastic bullet with a total range of only 300 meters), I've been looking for projects to use it with.  I also decided that I wanted a lever gun of some sort, for no reason I can explain.  And, obviously if you're here, you know I do a lot of 3D printing.  So I decided to combine all three into a new project.  I'm going to 3D print a lever operated rifle specifically for the DAG training ammo.

I want to use as many 3D printed parts as I can, mostly because I don't feel like machining anything.  "Locked breech" and "3D print" don't generally go together, so I decided to go with a single shot falling block action because of it's high strength in a smaller size.  With a falling block, it's easy to get a whole lot of strength without having a massive receiver.  It's also a very simple action with very few parts(and if the print proves to be too weak, easy to replicate in metal).  I'm going to use an AR15 hammer/trigger, and a floating 1911 firing pin(mostly because I have all these parts on hand already).  The DAG training ammo has a smaller case head than standard cartridges, and the action is designed around that so standard 308 WILL NOT FIT.  For the barrel, I'm going to use one from a Savage because of the way it threads into the receiver with a locknut to adjust the headspace.  I'll also be using the 3D printed stock I designed for my SCR style project, mostly because I have it and it fits me well.

Still very much a work in progress, but here's how the action looks so far.  The "H" shape of the breech block gives me four locking lugs and a whole lot of surface area and shear area.  Napkin math using PLA numbers and the Lilja equation says that that the locking block should be able to handle 2100lb of bolt thrust with a safety factor of 2.  I have no idea how much power this training ammo has, but it's pretty wimpy, and for reference a 22LR makes about 900lbs of bolt thrust.  My biggest concern is that the case head might start crushing the plastic on the breechface, and if it does, I'll make a metal insert for it.

Here's how it looks with the breech closed and and ready to fire:

When the lever is opened, it pulls the block down, the nose of the lever then hits the extractor lever pulling the case out, and the block cams the hammer back to half cock(I'll cut a half cock notch into the AR hammer).  Once closed again, the hammer can be cocked the rest of the way with the new lever attached to it and sticking out the side of the receiver(which isn't ideal, but I can't come up with anything else that will work with the AR FCG).