62gr M855 or 55gr M193: Must choose ONE or THE OTHER for 16" bbl 1:7 twist

[lysander]

That was my understanding. The slower twist rate barrels would not stabilize the tracer rounds?

Did shooting in arctic cold conditions have something do to with the choice in 1:7 barrel twist rate as well?

In the mid 1970s NATO began to look into the standardization of a second caliber round. It was quickly settled that the round would be based on the cartridge case of the M193, the the projectile was still up for debate, the US preferred the XM777, a 53 grain bullet with a steel penetrator, as this could be used in the M16A1 with the 1-12 twist barrel. The only other real contender was FN's SS109/SS110 which had been designed for the FN Minimi for the US Army's SAW program. This combination (SS109/Minimi) were designed around 1-7 twist rifling.

The range and penetration requirements for the SAW were carried over as a baseline for the new NATO cartridge. Eventually, the SS109/SS110 combination was adopted as NATO standard in 1980. That same year the Minimi was adopted as the M249. These two factors cemented 1-7 twist as the only twist worth considering when the M16A1E1 came about.

So, why was the SS110 so long? The XM287, a 68 gr ball round designed for the XM207E2 (Stoner 63), was the exact same length as the SS109. The tracer version of the XM287, the XM288, wasn't any longer than the XM287 and both would be stable out of a 1-9 twist barrel. So, why was the Belgian tracer so much longer that it required a faster twist? It all goes back to the NATO/SAW program tracer burn-out range requirement. That was set at 800 meters, and that required a lot of tracer composition.