The bi rail stl in no longer needed for this part. I left it in on case someone wants it. .Consider this part for educational purposes only until you do your own testing.
Could also possibly be used as a barrel cooler by stuffing a wet rag in the end to keep the pla from over heating!
update: 9/3 added version 2 which includes longer body and 3 screw mount holes. 2 of which go into the upper which should help reinforce the barrel retainer.
9/7: added version 3 that eliminates the barrel retainer. This slides over the upper and takes the place of the barrel retainer. I discovered after receiving my barrel that I made the rail too long. This should make up the difference and should be a stronger overall part. The dimensions and thickness is the same as the barrel retainer so the same screws can be used. 9/9: v4 minor improvements for fitment. Should be able to print this without scaling now depending on your printer and its calibration settings. Mine printed almost right on the money. Minimal trimming and tight fit. included step files and fusion360 files for v4. Last pic is V4.
9/14: Noticed and fixed an issue where I performed the boolean operation with the barrel retainer instead of the upper, I changed mounting locations during this process and forgot the upper had no fillets. My apologies for anyone that downloaded it. It wouldve still worked by rounding off the forward edges on the upper tho. I started doing this myself but decided to print another.