Forgotten Weapons Plays With XM19 SPIW Magazine

Around the same time that Small Caliber High Velocity was starting to percolate through the US Military, so was another set of programs designed to increase the hit probability of the individual soldier. Considering that 5.56 was ultimately adopted as the standard loading does not take away from the ingenuity of the designers at the time, who developed many remarkable solutions.


Various items proposed to increase hit probability included using multiple projectiles including rapid fire (which exists today in the AN-94) and shooting multiple projectiles from the same detonation in the same vein as a shotgun, just with the projectiles stacked on top of one another.

To me, one of the most interesting concepts was the use of flachettes, which are basically ballistic darts. In the case of the XM-19, the dart was accelerated to over 4800 fps. This high velocity caused its own issues, but the idea continues to be of real interest to weapons designers, especially those looking at underwater weapons.

The magazines were an interesting design. Rather than use the standard shaped wire spring common even to today, the magazine used roller springs, which provide constant tension. Coiled wire springs provide more force when compressed as most shooters know when trying to put the last rounds in a magazine.


The design was ahead of its time and even has recently been reintroduced to the MSR through the new TorkMags.

For even more gratuitous photos and even more history, check out Forgotten Weapons. 

Nathan S

One of TFB’s resident Jarheads, Nathan now works within the firearms industry. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

The above post is my opinion and does not reflect the views of any company or organization.


  • Constant force springs like those shown have been adapted for firearm magazines by many inventors, including Gene Stoner. They are typically seen in higher capacity designs, like the 50rd magazines that Colt and NWL-Dahlgren developed for the SEAL teams during Vietnam. More recent applications have been the Eagle and Ram-Line pistol magazines of the 1980s/1990s, and the current TorkMags for the AR-15.

    Besides the reduction in the height of the magazine, the constant force spring allows just that, a relatively constant force in lifting the the stack of cartridges from beginning to end. With conventional coil springs, you risk either lacking enough force to lift the last cartridges in the magazine, or too much force when the magazine is full. The latter causes drag against the bolt and makes the magazine too difficult to seat when the bolt is closed. The downside to constant force springs in magazines has always been reliably keeping the springs attached to the magazine tube. The rivets pop, or the holes in the spring or magazine tube egg out or crack.

    Here is Stoner’s magazine patent from the early 1960s.

    • Bierstadt54

      Do you know if someone has tried using a constant force spring for detachable shotgun magazines? It seems like it would address the problem of plastic deformation on the front of the shotgun shell.

    • Here is an even earlier constant force magazine spring design from the 1950s.

  • Beardedrambler

    Some 10-22 mags use a coiled spring. Is this the same type of thing?

  • LazyReader

    Looks neat, like Buck Rogers rayguns

  • kreatin

    I believe it was eagle industries they made a coil spring mag for the mini 14. I still have a couple. Will upload pics when I get home

  • tsh77769

    This is an example of a good design that is very workable but may have a slightly tarnished rep due to imperfect execution by Ramline and Eagle.

  • Tony O

    Huh. Pretty sure Ian was supposed to credit us with providing him the magazine. Considering this specific magazine is one of like only 2 or 3 out in the wild, and my shop just happened to have one, it would have been nice to be mentioned. Did him a solid, after all. Or maybe I’m just bitter.

  • noob

    hmm did the p90 originally use constant force springs? or did they always use box coil springs?