One of the first wildcats for the AR, and other rifles, it is a simple one to effect: basically take a .223 case with a neck not work-hardened too much and pop a 6mm neck expander stem through it. The result is a .223/5.56 case with a neck that will hold a .243-inch bullet instead of a .224-inch bullet.
As a deer-hunting round, this offers some prospects. In a bolt-action rifle, unless it is one scaled for the .223 and the .223 alone, you can gain useful case capacity by loading the bullet longer. The longer-loaded bullet doesn’t protrude into the case, and you end up with as much capacity as the .223 had.
However, we have not that luxury in the AR-15. The magazine dictates just how much length we have to work with, and no more. That, combined with the fixed location of the case mouth, means we cannot use a longer, more aerodynamic bullet to keep the speed up down range. It also limits the weight we can use, as a heavier bullet decreases case capacity (the room for powder) and thus gives us a double whammy in velocity loss: More weight and less powder.
However, improved powders have changed that somewhat since the 1960s, the last time anyone looked at the 6X45 in rifles.
The modern look is interesting, as it combines with the sudden increase in the AR, with a mild deer-capable cartridge. There are states that do not allow .22 rifles for deer hunting. However, a 6mm such as the 6X45 is allowed. So, a 6mm loaded with soft point bullets, say an 80-grain bullet at 2,800, is plenty good enough to drop a whitetail.
Now, since we can’t always depend on the velocity printed on the box, and a lot of ARs for hunting would be handier, in a 16-inch-barreled carbine, we’d be talking more like 2,650, but that is still good enough to drop any whitetail who ever walked the American continent, given a well-placed shot.
The 6X45 has been around since at least 1965 as a recognized wildcat. In all that time, it didn’t get much traction. Why does it now? Two things: new powders and new bullets. In 1965, if you could push an 85-grain bullet much past 2600 fps, you were doing great. And the bullet so-pushed was a plain old “cup and core” softpoint, with not much ability to retain weight or shape and penetration.
Now, we have powders that can push the same weight at 2800 fps, a more useful velocity. And the bullets being pushed, bonded-core softpoints, all-copper hollowpoints, will retain weight, penetrate and work like they are much bigger bullets than they are. At the other extreme, varmint bullets are much better than they were in 1965. They are more accurate, fragile, and able to be pushed to higher velocities. If you want warp speed, a Hornady VMax of 58grains loaded to 2950 fps is your choice, and if you want a bit more range even if it means giving up 75 fps, then their 65-grain VMax at 2875 fps will vaporize varmints at distance.
All of which makes the 6X45 a much more attractive hunting/varmint cartridge than it used to be. However, there are some touting it as a replacement for the 5.56 as a defensive load. There, I have to part company with them. The 6X45 as a deer cartridge works well because of the new generation of expanding bullets. In a military context, expanding bullets aren’t allowed. Yes, police and non-sworn taxpayers can use expanding bullets, but the fewer offerings in the 6X45 make it less useful. I know, I know, it’s like the getting-your-first-job conundrum: You have to have experience to get a job, but if you haven’t had a job, how are you going to get experience?
If people don’t buy the 6X45 for defense, how can they expect the ammo makers to load defensive ammo for it? Not my problem.
The 5.56 gets around the “no expanding bullets” problem by using long-for-their-weight bullets that tumble and break. The 6X45 is boxed in in that regard. Any bullet you can push fast enough to break up is too short to be broken. And any bullet long enough to be breakable is too heavy to push to a speed where it breaks.
The pilots among us will talk of the “performance envelope” which is a graph of speed and altitude, turning radius, range, etc. Operating “in the corner” or “on the edge” means going right up to the limit. The U-2 worked that way. It traveled so high, where the air was so thin, and so close to the limits of its ability, that pilots could not make turns that were too tight. To do so would mean the wingtip of the inside wing (the wings were very long for its size) would slow down, and fall below the stall speed of the aircraft. The sudden drag of the stalling wingtip would put the U-2in a flat spin, which was usually not something the pilot could recover from.
The 5.56 is operating in a corner of its performance envelope: there is just enough room to push a 75- or 77-grain bullet fast enough to make it break up when it tumbles. The 6X45 does not have that room.
So for the military it isn’t a viable option. But for hunters and the non-military defensive user, it offers many advantages. And the biggest of those is that to convert a rifle to 6X45, you need only a new barrel. The bolt and magazines of your 5.56 will work just fine, thank you very much.