300 AAC / Blackout Brass Trimming jig for Cutoff Saw

This is a jig to work with the Harbor Freight #42307 Mini Cut-Off saw to trim .223 / 5.56x45 cases down to make 300 AAC cases.

The jig uses two of the saw mounting holes to position the case for trimming just behind the shoulder of the case. The jig is designed to position the case based on the shoulder rather than the case mouth to eliminate variations due to case neck length. The pocket also has an additional 0.050" wall around it to better hold and position the case than a simple half-round pocket.

The saw vise must be removed from the saw base to use this and a 0.050 shim placed under the jig due to a slight raised lip where the vise normally mounts. I tried several times to print an undercut using support but did not have very satisfactory results. Some tweaking of the position is required to get just behind the shoulder. There is enough slop in the mounting holes to do this. I use 1/8" x 2" toggle bolts to clamp the jig and mount it to the workbench.

This is a work in progress. I am looking at ways to make the cut off neck easier to remove, eject or drop away. I will post that version when I perfect it. In the mean time I am including both the STL file as well as the Solidworks file if you care to modify the design.