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The 5.6×57mm Triple-Bore Tround was an experimental military tround cartridge developed by David Dardick for use in the H&R SPIW.


The tround has a triangular casing not unlike most other tround variations. However, this variation has a rounded angle on its profile, allowing for a faster rate of fire. As the name suggests, it has three bores in it. Each bore has an AAI-standard, 5.6mm saboted flechette in it, the same type as used in the XM144 cartridge. When fired, the sabots were discarded early in the bores while the flechettes continued down the main barrel. These rounds have proven during testing to be lightweight.

One of the requirements for the SPIW program is that the weapon has a 60-round ammo capacity. The H&R SPIW has a 20-tround drum magazine, and with each tround having three projectiles in the same case, it achieves the effect of a 60-round capacity.


There is a flaw with the ammunition. Because the trounds are taped together in the drum magazine, a variation of only a few thousandths of an inch in the plastic tape would cause the Trounds to bulge excessively and split during initial function firings.

Another immediate and fundamental problem concerned the three-shots-at-once theory that the SPIW is based on. The three bores were in fact one common space: every time the H&R prototype was fired, gas leakage began as soon as the flechettes left their tround. The first flechette exiting the muzzle triggered a further dramatic drop in pressure. At best, this reduced the muzzle velocity and consequently the range and accuracy of the other two flechettes. At worst, the pressure drop just might leave one or both of the remaining flechettes stuck in their respective bores, waiting to act as a serious obstruction when the next shot was fired.

Both of these drawbacks made the SPIW dangerous to operate.


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