Dillon and Hornady case feeders are great, except they often have trouble with the odd length of 300 Blackout. Dillon ships a shim washer for use with 30 carbine, but it's way too thick for Blackout and raises the plate so much that blackout brass can get under it.
This shim is 0.030" compared to Dillon's washer which is 0.130". This raises the case feeder plate just enough to prevent blackout from jamming in the gate in most feeders, using the small rifle shell plate.
This shim allows you to use the small rifle plate with 300 Blackout, when the gate is properly positioned. The metal gate does need to be pulled out about 5/32 of an inch to prevent upside-down cases. On some feeders this causes "bridged" jams of brass that does not fall down fast enough. This shim helps solve that problem. Every case feeder is different, however, and some may work fine without this shim.
The gate can be positioned with a 5/32 hex wrench as the following picture demonstrates:
(5/32 hex wrench not included) Shim is precision hardened steel and made in the USA.