10MM Glock 'DUPLEX LOAD' - Two Bullets One Bang

A 'TANDEM TEN' LOAD

What happens if you load two projectiles (bullets) into a semi-auto pistol cartridge? - Does the gun blow-up, - do they just lock together as a single slug – or maybe you might get the result that I was seeking and get two holes in the target for one shot.

When I came across a story by Wiley Clapp in an American shooting magazine (GUNS & AMMO Nov.1990) I was immediately intrigued by the possibilities suggested by that story about loading multiple bullets for use in .38s and .357s. Mr Clap told how to use bullets cast from a Hensley & Gibbs #333 mould to wildcat two or even three-bullet loads in a revolver.

                                                #333 Mould and Cast Bullets

It struck me that my favorite semi-auto pistol cartridge – the 10mm Auto, was a little like the .357” in that they both are longer straight sided cases when compared to a 9mm Luger or the .45ACP. Perhaps - like the .357”Magnum, - there might be enough room inside a 10MM brass case for more than one projectile.

 - Grabbing a hacksaw I cut open a case and then shortened two cast pills to approximate the length that I thought a pair of 100gn x .40”pills would be - if loaded into the 10MM case. Well they looked comfortable and very much 'at home' – and there was room for a normal looking amount of powder to fit behind the twins – so maybe I was onto something useful.

                                         'MOCK-UP' Of 10MM DUPLEX LOAD

If two 100gn 10mm (.40”) pills could be safely fired from one round in a 10MM semi-auto like the GLOCK 20, and give minimum separation on a target at short ranges – seven to ten metres – the surface area on impact would be twice that of a single bullet but still deliver the same momentum, and reduce over-penetration.

 - A problem that I have experienced in Bowling Pin Matches is that often a decent on-target hit with the 10MM will splinter the pin and leave it spinning on the table while the bullet goes straight through without stopping - and carries on unperturbed to the backstop, - whereas a slow soft .45” slug will push the pin gently back and off the table while lodged in the timber pin.

                                                10MM - May be HOT when fired !

I have a theory about the lack of enthusiasm for the 10MM Auto in America. - When Col Jeff Cooper was proposing the development of a powerful cartridge between .45” and 9mm – he should have first gone with something smaller like the .40”S&W (short & weak !) - Then later on, had a public 'eureka moment' and proposed a magnum version for real Americans - called maybe THE .40”AUTO MAGNUM .

It would be the same superior, versatile, flat shooting, powerful, and controllable caliber for modern handguns as it is today – but with an 'All American' name that even  "Bubbers" could vote for – it's a MAGNUM, an American Gun.

The Wiley Clapp .357' double / triple load in GUNS & AMMO was worked-up as a defensive load using very short 'wadcutters' cast from a Hensley & Gibbs #333 Mold. These hard cast bullets weigh only 66.6gn and measure just a quarter of an inch front to back. This short flat shape readily allows three pills to be gang-loaded into a .357" Mag case (and two into a .38”) – and there's no snags to using a flat nosed wadcutter round in a revolver. - Certainly if I could find a  #333 bullet mould in New Zealand I'd snap it up like a cold beer in the desert, and would load up rounds for my six inch barrel RUGER .357".

– But of-course wadcutters wouldn't do in a 10MM semi-auto - which would certainly need some lead-in from a tapered or round nose projectile to help feeding up the ramp into the chamber. What I had in mind was a shallow 10mm pill weighing no more than 100gn with a truncated cone nose profile that would feed in a semi-auto action when poking out of the front of the case, – but the same tapered nose, when reverse-stacked (base-to-base) with the top slug would ease down into the internal tapering case and act like a semi boat-tail back end for the twins and help them slip into place without pushing out a bulge on the outside wall.

First I approached some guys who were swaging projectiles requesting a batch of lightweight 100gn pills,- but they said that they couldn't swage them small enough for me – So next I turned to a fellow Pistol Club member who had told me that he'd made his own mould to cast 9mm Truncated cone pills to feed a GLOCK 19 Compact. - Robert liked the idea – and in only a couple of days he produced samples weighing exactly 100gns that looked great except he couldn't get the lube to sit in the grooves using his homemade sizer / lubricator - so we tumble lubed the samples for testing.

                         SAMPLE 100grain CAST TRUNCATED CONE BULLETS

Thinking 'all care'.. I decided to use brand new Winchester 10MM cases and hand seated the Winchester large primers using a LEE Ram-Prime. I ran all the new cases through the sizer die before priming and bell-mouthing them. The 100gn bullets were covered with a slightly tacky coating of lubricant so I wiped clean all the bases using a rag on a flat surface to ensure the sticky brown lube didn't glue them together as one slug.

The first five cases were loaded with one only 100gn projectile over a light charge of Winchester Superlight (WSL3) – really just to ensure that the cartridges would feed in & out of the GLOCK 20 chamber and confirm that the bullets 'truncated cone' shape would work. - Robert had sized the castings to .401” and I seated them down to give an overall cartridge length of 1.160” which looked right and they barrel-dropped and fed up from the magazine when hand-cycled through the action.

                                        Glock 20 10MM - Functional & So Good-looking !

Not wanting any complications caused by a compressed powder load, I had measured the powder space below the duplex load which indicated that a maximum powder charge of WSL3 might be 5.0gn. I also needed to ensure that more than half the capacity was filled behind the bullets to eliminate the chance of the propellant flame 'flashing' across the powder surface and resulting in an instant detonation with high pressure – so I chose a bottom limit powder charge of 3.0gn WSL3.

Hand weighing each charge on scales I loaded five duplex loads each of 3.5gn, 4.0gn, 4.5gn. & 5.0gn powder charge and the paired 100gn pills balanced and fed easily into place for the crimp to be applied to each case just like bought ones before sealing them into clearly marked plastic bags.

                        LOADED DUPLEX 10MM AUTO CARTRIDGE & 'MOCK-UP'

When I got to the outdoor range it was a dull day with just enough of a breeze to blow my note paper away every time I forgot to weigh it down with a rock. - I set-up two IPSC buff targets and stuck a large black patch in the A-Zone as an aiming point and fired the five 100gn “singles” - making a fair group but had to hand eject as they were too light to work the GLOCK 20 action.

- The first twin-load chambered - and POP as two 'snake eyes' appeared near enough to the black patch. - Yeahh!, quickly in with the magazine holding the remaining four 3.5gn loads – and a nice set of paired holes built up on the cardboard target – this was getting to be fun . - The big 10MM GLOCK 20 was running like an Austrian watch on these twins and the paired holes were appearing right where they should.

 - It wasn't until I got to the 4.5gn loads that the “plastic phantastic” began to feel like a 10MM – and the 5.0gn loads did get the GLOCK to move a bit - ejecting the spent cases three or four metres to the side. A bit far to retrieve them conveniently - but no excess pressure signs. The 10mm brass is a lot easier to find than lesser cases – try picking 9MM cases from the gravel when they're mixed in with all those broken bits of Desert Eagle in the dirt!

-Having used all my test loads at seven metres I had to go home and load more to try on bowling pins – and I also needed to check the spread at longer ranges and pick a standard powder charge to run with. - Close examination of the targets showed that some of the hottest 5.0gn loads had tumbled and gone through the target sideways – and the lighter charges had given the widest separation on the target, suggesting that a powder charge somewhere between 4.0gn & 4.5gn as thrown by my LEE Powder Thrower should give the optimum result.

I loaded fifty more of the 'tandem tens' again using new Winchester cases with a thrown powder charge of 4.3gns of the WSL3 propellant. I was working the next couple of weekends but in due course I got to the range and set-up another clean IPSC Target at 25 metres and test fired five rounds - with the paired holes ringed by felt pen between each shot. - the widest spread at this 25metre range was 135mm (5.5”) and the friendliest pair were only 20mm (0.5”) apart.

- So far, so good – but the big test was to see how bowling pins reacted to my home-made pin- pushers. - setting-up a couple of slightly used pins and carefully aiming ..'whack' - the twin loads certainly knocked the plastic coated pins back and off their table – with all the 100gn pills staying under the plastic coatings rather than blasting splintered exit holes. - I'd have to shoot pin matches to prove the load, but certainly the duplex rounds demonstrate the versatility of Col. Coopers ideal calibre for semi-auto pistols. It would be hard to develop a similar load for shorter auto calibres such as the 9mm, .40”, or .45”.

Marty K
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