Is it Time for the 6x45mm Cartridge?
Patrick Sweeney takes a new look at this 1960’s wildcat for the AR platform in his 2010 book The Gun Digest Book of The AR-15 Volume 3
Below is an excerpt.
By Patrick Sweeney
The original caliber for the AR-15 wasn’t the .223/5.56, it was a slightly smaller cartridge. The .222 Special delivered the kind of performance that the designers wanted, which was basically a 50-grain bullet at under 3,000 feet per second.
The Army, trying to keep the AR away and keep the M14 in the running, kept moving the goalposts. Finally, they insisted that the bullet used had to penetrate a steel helmet at a distance farther than their own research had indicated soldiers fired on opponents. The special got stretched and boosted, until the 55-grain FMJ was at 3,100 fps.
And there it stood, until the mid-1980s, when the SS109 came about. That was intended for use against swarms of Soviet infantry in Western Europe. What, there never were swarms of Soviet infantry in Western Europe? Musta worked.
Seriously, the expectation was that the Soviets would roll West, and the NATO allies would be faced with Russian, East German, Polish and who knows who else mechanized infantry piling out of their BMDs, BMPs, and BTRs, lining up and assaulting the NATO positions. They expected to face lots of targets, and not only armed ones, but armored infantry. So, the push was for armor-piercing performance, leading to the SS109 and the later M-855, with a 10-grain steel penetrator tip inside.
The new rifle also received a new barrel twist, one turn in seven inches, to fully stabilize the SS109 and the tracer as well. Only the tracer really needed the new twist, but the military approach was/is a “one size fits all” approach, so there it was. I had a talk with Mark Westrom, CEO of Armalite about that, and he remarked that it would cost more to prove to the Army that a 1:9 twist was better than it would cost to re-barrel every rifle in inventory to 1:9.
Fast-forward to Somalia, the infamous “Blackhawk down” incident. There, good shooters (Rangers, Special Forces and Delta operators) spent a long time, and a lot of ammo, shooting at people who in many instances didn’t fall down when hit. To everyone’s surprise, small bore ammo designed to penetrate to a fare-thee-well failed to do more than create simple perforation wounds on unarmored opponents.
So the system stayed quiet until we were in another shooting war, and reports came back. This time, they came back too often, and too frequently, and over a long period of time, to be ignored. As a result, the now well-known Mk 262 load was developed. What it does is simple: it takes advantage of the too-fast twist of the M16A2 and M4, the 1:7 twist, and loads a 75- or 77-grain bullet in the case. The longer bullet is less stable than the shorter, 62-grain M855, and thus overturns on impact or soon after.
The next step was a refinement, the Mk 262 Mod 1, which included a cannelure in the bullet. The cannelure is a place to crimp the case neck into, but it also strategically weakens the bullet. When it begins to overturn on impact, it then breaks apart at the cannelure.
Stop wringing your hands. Lots of bullets overturn, tumble in the parlance, and lots of bullets have cannelures. And some have both, such as the old loading, the M-193, the 55-grain load from the Vietnam era.
The Mk 262 carries its speed better and offers longer-range performance. In fact, it offers too much long range performance, at least as far as hitting is concerned. You see, it puts the Army on the horns of a dilemma. The load is so accurate in some rifles that a skilled shooter can hit his target far beyond the effective ballistic “thump” of the bullet. Yes, a 77-grain bullet is gonna hurt, but when it has dropped to the performance of a .22 rimfire magnum, it gets tough to justify it.
What’s worse, not all (in fact, very few) of the soldiers who might get their hands on it can actually make use of its range. Yes, I’d rather poke a .224 hole through a bad guy at 700 yards, than let him walk off unscratched. The awful truth is, the Army doesn’t teach enough about marksmanship to let soldiers do that. The qualification course goes out to 300 meters.
There is no feedback, so if you nick the edge of the target you get scored the same as if you center-punched every one. Beyond 300 meters is a mystery, and many soldiers will be told to not shoot at the 300-meter targets, to save the rounds. That way, they can use the extras to make sure they get this close in. After all, with 20 targets coming up, and 20 rounds, you need only a dozen hits to pass. So, if the far targets are chancy, save your shots for the sure thing.
Which is a less than reassuring skill set to have, wedged behind a boulder in Afghanistan whilst being thrashed by a tripod-mounted PKM from 800 meters out.
No, the Army spends time teaching marksmanship skills to only a very few. They haven’t time, being too busy with a whole raft of mandated courses they have to teach first. But that doesn’t keep shooters from dreaming. The first dream was to stretch the existing round more. The longest-lasting and most-desired is to go back to the very beginning.
The 6×45 is a way to get heavy hunting bullets (or light varmint grenades) into a .223 case. If your state doesn’t allow .22s for hunting, this is your caliber.
6×45
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 downrange. 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 6×45 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 6×45 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 6×45 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” soft point, 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 soft points, all-copper hollow points, 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 58 grains 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 6×45 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 6×45 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 6×45 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 6×45 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 6×45 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-2 in 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 6×45 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 6×45, you need only a new barrel. The bolt and magazines of your 5.56 will work just fine, thank you very much.
Read more on this topic at TacticalGearMag.com