BRASS 12 gauge dummy rounds / snap-caps / fake bullets. Looks like the ones used by the US Army during WWI and World War Two in the Model 1897 Trenchguns.
Dry Fire Snap Caps - Dummy 12 & 20 Gauge Training Rounds
To Dummy, or not to Dummy -The Firearm Blog
B's Dry Fire Snap Caps - .40 S&W-TRNGAID-DRYFIRE-DUMMY-.40 S
12 GAUGE 2 3/4 SNAP CAPS DUMMY TRAINING ROUNDS SET OF 8 SAFETY
A-ZOOM Action Proving Dummy StrikerCap Snap Cap 45 ACP 2PK
B's Dry Fire Snap Caps ® - Dummy 12 Gauge Training Rounds
DELUXE SNAP DUMMY ROUNDS TRIPLE-K .357 MAG, 6 CAPS PER PAK
B's Dry Fire Snap Caps - 12 Gauge
CARLSON'S Snap Cap [ 20 Gauge
shotgun dummy rounds, dummy rounds, dummyround, dummyrounds, inert
12 GAUGE 2 3/4 1oz. SNAP CAPS DUMMY TRAINING ROUNDS SET OF 8