I've been tooling up and researching ways to design and produce an aftermarket lower receiver for the XCR-L. The thread for AR aftermarket triggers on the XCR and the complete failure of that project has inspired me to go down this route. I see a lot of need for modularity and aftermarket support for this platform, and I'm looking to make this lower be the key stone to all of those bits and accessories that this community has struggled to get over the years.
The plan is to redesign the current XCR lower to support AR aftermarket parts in the FCG, stocks and trigger guard, in that order of importance. The primary goal is to rearrange the FCG to accept AR trigger parts, followed by building in a rear 1913 rail in place of the XCR stock receptacle for potentially infinite stock options as well as allowing for ease of conversion into a pistol without any need for caps, and finally providing accurate holes to allow for trigger guard installs, where possible. The final build will be CNC machined out of 6061 Aluminum and cerekoted.
A 3D printed prototype will be used for fitment testing but a 3D printed lower will not be produced. Such a thing would be a different design with different goals, and I've determined that the XCR is a poor platform for designing a 3D printed lower, due to there being only 2 contact points securing the lower to the upper (only one take down pin and a weak connection using the recoil spring as a tab) versus a typical AR lower. There is not much security there to prevent the bolt carrier and recoil spring from breaking out of a plastic printed lower.
Below is a comparison between the current lower and a mock up of the proposed lower. The dimensions shown are the entirety of the research I have done on the lower so far, more needs to be done.
I am well-versed in CAD using Autodesk Fusion 360 and I am now using it professionally. F360 is versatile enough to design this lower, and my thread on Machining Projects
should demonstrate that I'm able to program and operate a CNC machine. I'm waiting for a desktop CNC machine to arrive that should be big enough to make this lower. I also now work at a production company that has a CNC machine that I have access to, so come hell or high water I do plan to get this done.
My main issue that I really need help with is getting dimensional drawings for the original XCR lower. It's proprietary information and I doubt Robinson Arms is willing to send me a copy of their blue prints. Right now it's just my XCR-L lower, my digital calipers, dimensional drawings for the AR lower on the internet, and my own two eyes (the same ones that I shoot with) that are being used to design this thing. If anyone can help get me accurate dimensions for the lower outside of the FCG, it would make this project a hell of a lot easier. Otherwise it's going to be a lot of measuring and trial and error.
One issue I found out in the making of the mockup is that the XCR BHO/Bolt catch will need to be replaced with a longer one that spans the extra height of the new lower if we want to have a trigger guard. The BHO system is the heart and soul of the XCR lower, and replacing it with an AR BHO seems to be a betrayal. Still, this means that using this lower requires a proprietary part that Robinson Arms doesn't produce.
I will be quite busy with work and two other personal projects that I've put ahead of this one, but this one is the most complex project with the most amount of time invested so far and the one I need the most help with at the moment. I don't want to set expecations too high, and we'll hold off on future plans beyond this. I expect this to take a whole year and that's not including the last quarter of this year where I'm getting other things out the door. Right now, I'm building it for myself and my own use, but once I iron out the kinks I'd like to distribute this info to the community and get manufactured goods to people who want this lower one way or another. I'm really committed to this project and confident that it'll work out, but the bulk of the development comes with getting those dimensions. I don't need any funding, I just need measurements. Feel free to post ideas and thoughts too.