https://thecastlearms.com/product-details?id=3507346

German 8mm S.m.K. Patronen Steel Core Lacquered Steel Case - 15rd (Armor Piercing)

Description Warning: SOLD AS-IS, MAY or MAY NOT FIRE RELIABLY, CONDITION MAY BE BETTER OR WORSE THAN PICTURED. This is antique German Patronen S.M.K. ammo from the 1942-1943, mid WWII era, but it may be slightly newer or older. 

DISCLAIMER--We have about 15 unique variants of this 8mm ammo. There's a chance you may get something slightly different than what's pictured. What you can count on though is that you will receive German WWII Era steel 8mm. There is a chance you may receive S.S. ammo instead of S.M.K. We are extremely reasonable and can work with whatever concerns you have before you order if it is important to you to receive a specific variant/head stamp, or whatever else you might want. If you do no specify beforehand, however, there's a possibility what you will receive is slightly different than what is pictured. In any event, what you will be receiving will be a remarkable value and of historical import. 

BOX OF 15 ROUNDS

Original WWII German ammunition, perhaps the best ever produced! This is armor piercing ammunition with a hardened steel core and lacquered steel case. All we have seen in this lot have the headstamp "hlb" and date to 1943.

The blue line through the white label denotes that this is steel ammunition. 

The German troops first experimented the "Reversed bullet" as an early anti-tank method against the British heavy tanks of World War I . This technique was crude, but proved effective against Mark I tank models, although it was not safe to use for the infantry and soon became obsolete.

As a replacement, the Germans conceived the "K bullet", which was purposely developed as Armor-piercing ammunition. The K bullet was in use by Battle of Messines Ridge battle of Messines Ridge in June 1917 and had one out of three chance to penetrate 12–13 mm thick armor at a range of up to 100 meters.

With the British deployment of Mark IV tank, which had more armor thickness, the K bullet soon proved ineffective, leading the Germans to design highly specialized anti-tank solutions, with the creation of the powerful 13.2mm TuF cartridge and the first anti-tank rifle, the Mauser 1918 T-Gewehr. (from external source)