Safe-ish

I am 100% unsatisfied with the safety on my SCR rendition.  It works reliably, but I don't like how it's layed out.  When I flipped the selector lever to the other side of the receiver, I had to move the the detent and spring too.  To be honest, I just kind of made it up as I went along, and it shows.  I'm still using the original detent just with the base ground off. That bit is OK, the real problem is the spring.  I made the detent pocket as shallow as I thought I could get away with so that I had more solid plastic in the receiver, but that left me with a spring that could only be 3-4 coils long.  Additionally, to remove the selector, you have to use a punch to override the detent and knock it out.  I don't see this as a good long term solution.  I also had to notch the back side of the safety lever for the head of the screw for the Rise Armament anti-walk pins I'm using, but that's no big deal.  Here's what I've got now:

For Receiver Mk3 I've decided to add a channel for a longer detent spring.  It goes from behind the detent into the hole for the trigger pin.  This way I can use a longer spring, and can take some tension off the spring by pulling the trigger pin when I want to take it apart.  It's will be a little fiddly to assemble, but a lot of the springs on ARs are like that.  While I was revising things, I also thickened up the trigger guard a bit to make it a little tougher.

Flexin' like a ninja

Like most things on this gun, the buttpad is 3D printed.  I could have printed it in regular plastic, but where's the fun in that?  While 7.62x39 isn't exactly a stout cartridge, I decided I didn't want a hard plastic buttpad.  Instead, I used Ninjaflex TPU filament.  If you've never used Ninjaflex, the filament coming off the spool is slightly stiffer than a rubber band.  It's hard to print with, and has to be printed very slowly.  It's printed fairly course with .2mm layers, I've had mixed results printing with thinner layers.  I printed it with 4 walls and 15% infill, making it just a little bit squishier than the Limbsaver on my Mosin.  Nice and flexible:

I, of course, only have red Ninjaflex, and I didn't want it red, so I had to paint it.  Remember my love of SEM from my previous Impala posts?  Well, SEM to the rescue again.  If you're not familiar with SEM Colorcoat, it's made primarily for automotive interiors.  It's also one of the few things that works well on printed TPU.  It sticks great, and because it's made for plastic, fabric, and vinyl, it flexes without cracking.

Speeding things up a bit

When I test fired my SCR, I had a few light primer strikes.  Not completely unexpected, it's a common problem with AR platform 7.62x39 steel cased ammo like I use.  Instead of swapping to a heavier hammer spring which would increase the trigger pull, I learned that bobbing the hammer can be an effective solution too.  The lighter hammer moves faster, which imparts more energy to the firing pin, which hits the primer with more energy.  There are pages and pages of internet arguments about it, but the overall consensus is that it works.  As a bonus, it also reduces lock time, the time between when you pull the trigger and when the primer fires(although admittedly my senses aren't fine enough to ever notice).   Anyway, I got out my grinder and whacked some mass off the hammer.  I didn't go as far as some people have because it's easier to take more off later than put it back on.  If it doesn't work, I'm not out anything but a few minutes time.

The other thing I did was make a knob for the charging handle.  IMO, the handle is too short for a bolt action.  It puts my hand too close to the pic rail on top of the upper receiver and is just uncomfortable to use.  It would be fine in a standard semi-auto where you don't use it as much, but having to use it for every round just wasn't going to work for me.  I printed a fairly basic ball knob that just friction fits inside the existing handle.  It's only 5/8" longer, but it makes a world of difference in use.  It's printed solid, so I'm not worried about it breaking off.

Stocks and bonds

Since the printed grip section ended up so ridiculously strong, and the back end won't see the kind of loading that the grip section will(and since I was getting low on filament), I dropped the rear section down to 4 shells thick when I printed it.  All the other settings were the same.  I'm not the biggest guy and even with the lower shell count, it's still tough enough that I can stand on it no problem.

Since I had to make my stock in two pieces, I've got to glue it together.  As mentioned earlier, from my experience with printed prop guns and swords my preferred method for this kind of thing is to run a hardwood dowel through the parts and glue the whole thing together.  It's quick, and makes a stronger parts than it would be if it was all printed.  For this stock, I'm using one long dowel all the way through, and a short one to aid alignment.

For this kind of thing, ordinary superglue is my adhesive of choice.  My preference is Gorilla brand superglue because it tends to be a little bit thicker than other brands.  I dribble some down the dowel holes, then slide it all together.  No need to be stingy here, the more glue, the better.  Note though that if you're going to leave it as a raw print with no sanding or painting, spilled or squeezed out superglue tends to leave a white haze on the glue joint.  Since superglue sets so quickly, you'll likely only get one shot at putting things together, so be prepared.  Once glued together, I put the stock in a clamp to let it sit overnight.  Superglue is a moisture cure adhesive, so the more humid it is, and the more moisture is in the wood dowel, the faster it will set(also why it bonds skin instantly).

In an unrelated matter, I cut a piece of Harbor Freight floor mat to put in the back of the upper receiver.  Since there is nothing to stop the bolt carrier's rearward travel until it hits the lower receiver, this acts as a buffer for the bolt carrier so it doesn't smack the plastic of the receiver when you rack the bolt.  If it doesn't hold up, I'll try a different material.

In another unrelated side note, for those who are curious, my trigger breaks around 5.5-6lbs(best I can measure with my shoddy equipment), and the break is fairly clean.  Not the greatest trigger ever, but acceptable given how wonky it is.

Testing, 1, 2

I started printing my SCR stock last night, and the print failed part way through.  Looks like a nozzle clog or something, nothing major.  But it did leave me with a half printed part that I didn't have a use for, and since I'd been curious about how strong the stock would be, I decided to do some destructive testing.

This part is Ziro Marble PLA(not PLA+), printed at .16mm layers, .45mm line width, 7 shells, 15% infill, and 75% fan speed.  I clamped it in my vice, put a prybar in one of the holes, and started pulling.  While in no way was this a scientific test, my calibrated arms(from years of working on cars) tells me I was putting about 75lbs of force on it before it broke.  Most importantly, it didn't shear across the layers, it broke out jaggedly meaning I had very good layer adhesion.  Interestingly, it doesn't look like I had great bonding between the lines of print in each layer.  Regardless, I'm not going to worry about the strength of this stock anymore, it should be more than strong enough for how it will be used.

Investing in stocks

Through much trial, error, and cursing, I got a stock modeled that is reasonably comfortable without having to reach too far for the trigger.  3rd time's a charm.  I lowered the grip portion even more, and made the whole thing narrower.  It's closely patterned after a Marlin 60, which is the most comfortable rifle I have in the house at the moment.  I printed it out in Vase mode, so it's only one line thick, but it used a minimum of plastic and I'm just trying to get a feel for how it fits.

Since my print volume is limited, I can't print this all in one piece.  One of the tricks I use while 3D printing prop guns and swords is to add a tube through the middle of the parts, then once it's printed, glue in a 1/2" wood dowel rod.  This both helps align the parts, and adds a lot more strength than you'd expect.  I also added a 2nd, shorter pin hole for alignment purposes.  I haven't finished modeling the butt plate yet, but here's how the pieces look inside: