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What the 10mm Auto Cartridge Was, Is, and Could Be Another GREAT article contributed by MakeMineA10mm. The 10mm was a cartridge that evolved from several wildcats, including the 40 G&A and "The Centimeter". Most of these cartridges were based on safely obtaining major power factor while adding magazine capacity, or attempting to achieve the goals of Skeeter Skelton, Bill Jordan, and Elmer Keith for an "ideal" law-enforcement (and therefore personal defense) cartridge, but in an auto-loading pistol format. There were some limits to these cartridges primarily due to the already-existing pistol-formats that they were designed to fit into. Along came Dornaus & Dixon and the Bren Ten. They were designing a brand new pistol and a brand new cartridge to go along with it. It seems apparent that they had more than a passing knowledge of the wildcatting going on, and there is no question they knew of the writings of the famous gun-gurus. With Col. Jeff Cooper on board, they worked with Norma, the only ammo producer who would listen to them, and designed the cartridge and gun, basically, simultaneously. What they came up with, was a marvel of it's time, and this one. They invented a 10-shot modified CZ-75 and the 10mm Auto cartridge, shooting a 200gr FMJ-TC at 1200fps or a 170gr JHP-TC (actual weight 165grs) going 1400fps (actual velocity was 1360fps). What happened to the Bren Ten is equally famous, and it almost took the 10mm cartridge along with it. For a variety of reasons, including defaulting on pre-payments, problems with sub-contractors (especially magazines), and other financial trouble, Dornaus & Dixon and the Bren Ten went under. Luckily, Colt came along and re-chambered and re-engineered their 1911 into the "Delta Elite" and saved the 10mm. The awesome ballistics possible, combined with a proven handgun design made the 10mm an attractive cartridge once again. This had all transpired by the mid-1980s. Then came the infamous "Miami Shoot-Out" in 1986. In the infamous shoot-out with bank-robbers (armed with long-guns), the FBI had several agents killed and more wounded. They commissioned a study to find out what went wrong. The study determined that their marksmanship had been good, but the terminal ballistics of their rounds had been too slow to incapacitate the bad guys. The Chief of the FBI's Firearms unit, knowing of this new cartridge that was more powerful than anything else in an auto-pistol, suggested the FBI adopt it. The 10mm was now blazing hot, with not only the proven 1911 design and the defunct Bren Ten (which is actually a pretty good design), but also the largest and most impressive Law Enforcement Agency in the US as it's first big user! This also promised the possibility of even more pistol designs, as, in true bureaucratic fashion, there would have to be a competitive design contest to pick which pistol to buy and issue, leading more manufacturers to design 10mm handguns. In addition, more high-performance factory ammo was coming into being, including the Winchester Silvertip (175gr JHP @ 1290fps). But, then, a funny thing happened. Along came a man named Martin Fackler, who was a doctor for the Army, studying the effects of wounds simultaneously and as an adjunct to the FBI's Wound Ballistics Workshop. To make a long story short, he said that a 180gr 10mm bullet at 950fps was just as effective in 10% gelatin as the Norma 170gr bullet at 1350fps. In addition, many FBI agents (who are primarily accountants, attorneys or computer people by training) were found to be having a very hard time with the recoil of the full-power round. Since the lighter round was just as effective and was easier to handle, the "FBI-Load" became the new standard of 10mm ammunition. Due to the huge market for law enforcement ammunition, most companies switched over production to the "FBI Load". In the mean time, Smith & Wesson realized there was an opportunity waiting. They realized that they could squeeze the FBI-Load's performance out of a cartridge short enough to fit in a 9mm sized firearm, rather than the 45ACP-sized pistols the 10mm required. They designed the 40 S&W in short order with the help of Winchester Ammunition, and the 10mm began to look very much like an ugly duckling again, as it had after the demise of the Bren Ten and before the Colt Delta Elite came out. This is where the 10mm was, after the huge wave of 40S&W hit in the mid-90s. The 10mm has had an exciting and tumultuous first 18 years of life. Now, for where the 10mm is today. Do not be dismayed, 10mm fans! Rather, take heart, because over 21 manufacturers have made more than 37 models of 10mm firearms. (See the 10mm Firearms Page) In addition, there are over 70 jacketed bullets available, and brass is being (or has been) made by at least seven manufacturers. I think it's safe to say the 10mm isn't exactly a dead or dying cartridge, nor is there going to be any shortage of it's ammunition in our lifetimes. (Especially when one considers some of the cartridges that ammo is still available for today, such as the 45 Schofield, 405 Winchester, 7.63 Mauser, etc.) To my thinking the pinnacle of 10mm design and production came just at the time the FBI loads started predominating the ammo market: the Glock 20. When Gaston Glock directed that Glock make a 10mm, I'm betting he was making a play for the FBI contract. This is supported somewhat by the speed at which he directed a 40 S&W Glock be designed and produced (which happened so fast, Glock actually beat S&W to market with pistols in that caliber!). However, this was a great thing for the 10mm cartridge, because, for the first time, other than the Bren Ten, there was a gun designed specifically for the 10mm, and not a 45ACP gun re-engineered into a 10mm. This makes the Glock 20 an incredibly strong, safe and well-engineered pistol for the 10mm cartridge, which means there is no reason to unduly restrict ourselves to FBI loads!!! With no disrespect to Dr. Fackler, I must say I disagree with him on one point. If the 170gr Norma load made the same wound tract as the 180gr "FBI load," I'd call the Norma load the better one, because, clearly (and admitted to by Dr. Fackler), there is more to wound ballistics than we completely understand. If the wound tract (which is one mechanism we do understand) for both loads is in the ideal range, I'll take the load with more energy over the lower-powered one any day! In addition, the Glock 20 solved a great deal of the recoil problem, through it's wide and flexible polymer frame. I say, ditch the FBI load, and go for the full-power stuff!!! Today, there are numerous sources for full-power 10mm ammo. Below is a chart which shows some of the full-power 10mm ammo available today, and their factory specifications:
So, if you want a pistol that can do it all: 
Capable of flat trajectories (<2" drop at 100 yrds), and
Enough power for hunting big game (such as White Tail and Javelina), and
Terrific terminal ballistics and magazine capacity for personal defense, the 10mm Glock 20 is impossible to beat! And, now, for what the 10mm could be... Although Glock considers their warranty void (as do most other modern firearm's manufacturers), if one uses reloads, done properly and safely, reloading can provide some very high-performance 10mm ammunition. There are numerous places to look for input on this, one of the best being the 10-Ring at GlockTalk and the Reloading Forum at GlockTalk. In addition, look over the reloading page here at the GlockFAQ.com and review my article on Ken Waters' Pressure Ring Expansion Measurement system for determining comparative pressures. The safety and strength engineered into the Glock 20 is definitely the way to go when looking for a high-powered semi-auto pistol. This is also why the Glock 20 is safer than the Glock 21 converted to 45 Super, 400 Cor-Bon, or 40 Super. Why? Because, the Browning locking system uses inertia (the weight of the slide) along with delayed unlocking of the breach to allow pressures to drop to safe levels before the breach opens. You can see in the picture below that the G-20 slide (on the left) is thicker (and heavier) than a G21 slide (on the right). In addition, since the 10mm bore is smaller, but the outside diameter of the barrel is the same, the 10mm barrel is heavier than the 45 ACP barrel, making the 10mm barrel heavier. This added weight increases inertia and slows the unlocking process of the slide, thereby providing more time for the pressures to drop before the breach opens.  In the G-21, the slide is lightened (internal sides of the slide milled away, back to near the ejection port) so that it opens more quickly and easily, so that the residual pressure of the 45ACP is at the appropriate level to guarantee 100% functioning of ejection and reloading every time the slide cycles with those 21,000psi 45ACP loads. Putting a 10mm-pressure-level cartridge like the 400 Cor-Bon or 45 Super in the lightened slide of a G21 is cutting the margin of time between safe and unsafe pressure levels at slide-opening to the absolute minimum. If any minor problem develops, such as a high primer, bullet set-back, or fluctuation in the powder charge, there is no safety margin left to prevent a worse event, such as a case failure or full ka-BOOM. In the 10mm there exists the full safety margin built into the 10mm format pistol by Glock's engineers. We can make that margin even better by installing an after-market barrel with better chamber support than the factory Glock barrel. Such barrels are made by Jarvis, KKM, and Bar-Sto, to name but three. Be sure to discuss the amount of chamber support with whomever you look at for an aftermarket barrel supplier. Even with the factory Glock barrel, with judicious selection of powders, careful application of Ken Waters' expansion ring measurement system, and use of top-quality components with a commitment to quality in assembling them, we can exceed original Norma full-power load ballistics!! With the fully-supported, extra-length aftermarket barrels, one can reach far into 41 Magnum ballistics territory!!!! This is not a reloading article, but I will give you some guidance at powders to which to look for top performance in the 10mm. One warning: Many of these powders are not the easiest to work with as their shape and size does not meter uniformly enough to not weigh each charge. To get top performance you will have to sacrifice by doing things by hand and carefully, such as hand-weighing powder charges on a scale individually! These powders have been used and tested by numerous persons and have been found to be in the correct burning rate range for the 10mm:
Blue Dot
Accurate Arms #9
Alliant STEEL
IMR 800XOther powders that have been worked with and appear promising are:
Hodgdon LongShot
Hodgdon HS-7
Vectan SP-2
VihtaVouri N-105Some obsolete powders that I have a suspicion would work very well are:
Win571
AL-8Norma lists some loads with it's R-123 powder as "factory load equivalents," and they display good -but not absolute top- performance, but I have no idea if it is the powder the original 10mm loads were loaded with or not. According to it's location on the burning rate charts, and comparing it to the other top-performing powders, it appears to me that R-123 is not the best powder for 10mm, but only testing will tell The following companies (at least) have made 10mm brass:
El Dorado (PMC-USA)
Federal
Hornady
PMC
Remington-Peters
Star-Line
WinchesterI make no claim as to any particular brand being heavier or more suitable than others. I have had good luck with all of them, and bad luck with a couple such as early-production brass R-P cases (nickel-plated ones seem fine), and some PMC that were soft. Just be sure to restrict the heaviest loads to new, once- or twice-fired brass at most. Beyond two firings with heavy loads is not recommended. That brass should be set aside for light, practice loads. Lastly, although it is not technically necessary, as long as you develop your loads with the factory spring, it does help to acquire an "extra-power" recoil spring, such as those from Wolfe or ISMI. A recoil spring of about 22 to 24 lbs seems ideal. Below, I've listed many of the 10mm jacketed bullets that are, or have been available to reloaders over the years ("*" means no longer available):
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Can I convert a .40 to .357 sig with just a new barrel?
Can I shoot .400 Cor-Bon, .45 Super, or .40 Super? The .45 Super is derived from a stronger case (less capacity) and modern reloading components. It can theoretically be fired from a stock .45 Glock. Doing so is not recommended without the addition of at least a 24# recoil spring. This load works in all the standard Glock magazines. .400 Cor-Bon is a .45 necked down to .40 cal. Case neck tension is a major concern as the bearing area of the case neck on the slug is minimal. This load requires an aftermarket barrel and at least a 24# spring (26# preferred) and is problematic feeding from hi-cap magazines. It appears to feed satisfactorily for PC 10 rounders. .40 Super is the .45 Super necked down to .40 cal. It is inherently stronger than the .400 Cor-Bon and has a longer bearing surface in the case neck reducing case neck tension problems. It uses a small rifle primer by the way. This load requires an aftermarket barrel and at least a 24# spring (26# preferred) and is problematic feeding from hi-cap magazines. It appears to feed satisfactorily for PC 10 rounders. [MarkCO]
According to Garey Hindman, son of the late Ace Hindman (final developer of the
.45 Super), you cannot get enough recoil spring in the G21 to handle the .45
Super. Yes, people are doing the conversion with only a #24 lb. spring, but it
must be beating the hell out of their pistols -- not to mention potential
feeding problems. Garey puts in a 25 lb. recoil spring and stainless guide,
Mag-Na-Ports the slide and Jarvis replacement barrel to slow down slide
velocity. This prevents frame battering and slows down the slide enough for
the expended case to eject reliably. I would follow this approach with the .40
Super as well. http://www.acecustom45.com
[JT]
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