300 Whisper (Fired in an AR-15 using a silencer)
This picture shows a 300 Whisper compared to a .223 Remington.
Notice that the 220 grain bullet is actually longer than the case. The 300 Whisper was designed to be fired in an AR-15 Carbine, through a
silencer (suppressor). This round is usually handloaded to subsonic velocity (below 1070 fps) to avoid breaking the sound barrier, because a supersonic
"crack" makes almost as much noise as a gunshot.
At this velocity you have an extremely quiet round that packs an amazing punch when using the huge Sierra 220 grain HPBT bullet. This
cartridge can provide near MOA accuracy out to 300 yards, and it's about half as loud as your average CO2 powered BB
gun. This round also functions perfectly through an AR-15 magazine.
Handloading the 300 Whisper is much more critical than reloading most other calibers, especially when you're using a silencer. Each powder charge needs to be individually weighed within 1 tenth of a grain. The bullet needs to be as fast as possible (without breaking the sound barrier) and the sound barrier is broken at different speed - depending on elevation. Carefully weighed charges help you avoid serious vertical stringing at long range, and it also avoids an occasional loud supersonic "crack" from a round that slips into supersonic speed. 300 Whisper cases can be made from .223 Remington brass, but they're far better when made from the .221 Remington Fireball brass. Neck thickness is an important consideration. They need to be necked up from .224" to use .308" diameter bullets. This process is a real stretch that results in losing about 5% of your brand new cases. After that . . . . case life is not too bad.
I soon found that it's an expensive project to build one of these custom rifles in 300 Whisper, especially if you're going to use a silencer. Reloading dies for 300 Whisper are proprietary, and they cost three times more than other calibers. The required $200 tax stamp doesn't help either. Don't expect a silencer to be as quiet as they are on TV. However, it's pretty neat to hit a target 300 yards away, and not make much more noise than dropping a large book on the floor.