.510 Whisper

The .510 Whisper is a subsonic rifle cartridge developed by SSK Industries for use in suppressed rifles. It fires a .50-caliber bullet weighing 750 gr at roughly 1050 ft/s.

Overview
Despite the large bullet, this round is very quiet, similar to the noise from a .22 Short when used with a suppressor. When used with a very high ballistic coefficient bullet, the round has only lost about 100 ft/s out to over 500 yards. This gives it more downrange energy at that distance than traditional .30 caliber rounds such as the .308 Winchester and .300 Winchester Magnum. SAKO TRG-S test rifles have achieved MOA and better accuracy with this cartridge at 600 yards.

Powders such as H110, 4227, or surplus WC820 are well suited for subsonic loads and give a low standard deviation. Necessary components such as barrels, reloading dies, etc. are available from SSK. Sako TRG-S actions are most often converted to this caliber. The Thompson Center Encore is also suitable, but is rather lightweight for shooting this heavy recoiling cartridge, especially when using 950 gr bullets.

Design
The case of the .510 Whisper is based on the .338 Lapua Magnum that has been trimmed down to a length of 1.875 in and necked-up to accept a .510 in diameter bullet. Neck wall thickness is reduced to 0.013 in and a small shoulder is cut into the body for headspacing. Overall length of the loaded round is about 3.34 in.