5.45x45mm MkVI

This is actually a very old picture, as NSers may attest, I just never uploaded it.

This is the 5.45x45mm Samozniy cartridge, introduced at some point along the AR-3 Rottweiler rifle family line and also used in later variants of the AR-2 Pitbull family.
It mates the 5N7 5.45mm service bullet with the larger case of the 5.56mm cartridge. This was due to large quantities of both the 5.45mm cartridge and 5.56mm cartridge existing in Samozniy Russia at the time.
It was believed that the yawing wounding properties of the 5.45mm 5N7 bullet were superior to the fragmenting properties of the 5.56mm cartridges in reliability and also capability. The yawing of the bullet is also independent of impact velocity and thus range, while the fragmenting 5.56mm bullets require a minimum impact velocity which restricts the effect to ~100-200m range.

Now, it wasn't long after I designed it that I figured that what I'd done was a tad pointless. I'd realised that the 5.45 cartridge as-is is a perfectly good cartridge, tapered case or not (this was probably before I learned how awesome the 7.62x39mm cartridge was, to be honest) and the effects of the bullet could be replicated by simply designing a 5N7-type 5.56mm bullet. I left it in my forces as a kind of curio.
But it seems to have had a bit of a resurgence recently...

www.shooterscalculator.com/bal…
(Scroll for trajectory graph and ballistic data - provided by a friendly NSer)
It's rather impressive, if I say so myself. It has better muzzle velocity from most barrel lengths than either 5.45x39 or 5.56x45 and as the trajectory chart shows, a flatter(er) trajectory than 5.56 US service or idealised loads.