SADJ has posted an excellent interview with Donald Roebling Award winner Sal Fanelli, a supervising engineer for Infantry Weapons Systems at MCSC. Fanelli commented on the creation of Mk. 318, its modification into the Mk. 318 Mod. 1 “Silver Bullet”, and even the potential for a new caliber for USMC rifles:

Why did the USMC decide they needed a new round of 5.56mm ammunition?

USMC determined that the performance of the M855 was inconsistent. Firing into a block of ballistic gelatin, we determined the M855 had yaw dependency. What we mean by that is from high speed videos of one round from an M4 we observed the path of the projectile doing exactly what we expected. It provided adequate terminal effects, but this didn’t happen every time. Other rounds went straight into the block and straight out of the block – no yaw, no enhanced terminal effects. We considered this inconsistent performance and confirmed that the M855 was impact yaw dependent.

We needed a round that gave consistent terminal effects from round to round, ammo lot to ammo lot and gave the exact terminal effects every time. We needed this performance at ranges from 0 to 400m.

Interestingly, while Fanelli does not give a reason for the inconsistent performance, Major Glenn Dean, US Army, did present one in his paper Small Caliber Lethality co-written with David LaFontaine.

Fanelli also commented on the possibility of the USMC seeking a round intermediate between 5.56mm and 7.62mm:

We did have an ulterior motive. There was a pending requirement for a new intermediate caliber. If we could improve the consistency of the terminal effects of our current 5.56mm round and use it as a temporary solution, it would bridge the gap until we could get the requirement going for an intermediate caliber. The intermediate caliber requirement remains in the discussion phase. Instead, our development resulted in a round that equaled the performance of the current 6.8mm round. Remember the Belgian SS-109 ammunition? It was selected as the NATO standard in the U.S., and was designated the M855? Well, that ammunition outperformed the Vietnam era M193. In a similar way, our improvements have now given us a 5.56mm round that gives us the performance of the current 6.8mm round. Of course, with the same technology approach, we know today that we can get a better performing 6.8mm round and 7.62mm round than the NATO standard 7.62x51mm M80 Ball.

This is one of the few government sources referencing a requirement for an intermediate round between 5.56mm and 7.62mm that I have seen. Even so, as he says the requirement “remains in the discussion phase”.

The Mk. 318 Mod. 1, an improved, lead-free variation of the Mod. 0 round with a higher ballistic coefficient, is competing against the M855A1 round developed by the US Army, according to Fanelli:

The original round was adopted as Mk318 Mod 0. This is a copper jacketed, open tip match, 62 grain projectile. If you don’t know ammunition well enough you might confuse it with the Mk262 which is a 77 grain projectile. The Mk262 is an open tipped match round used by the Special Ops community. Since we were worried about confusion with the Mk262, we had to take another look at the color of the tip. Every color was taken except for solid silver so we decided to nickel plate the entire projectile. As we now jokingly say, “All Marines will have silver bullets.” That round hasn’t been fielded yet, and a potential production contract is in the future. I can just see the Marines out there polishing bullets.

…

There is some additional testing that needs to be done. The USMC has type classified, adopted, and issued the Mk318 Mod 0 as an interim solution until formal testing has been completed on the Army’s M855A1. The decision on which round (Mk 318 Mod 1 or M855A1) will be fielded will be made within a year.

He also provided one of the only pictures of the Mk. 318 Mod. 1 I’ve yet seen:

Nickeled for identification, the Mod. 1 bullet has somewhat predictably been dubbed the “Silver Bullet”.

Fanelli’s interview is one of the first major pieces of information about the USMC’s enigmatic rifle ammunition program. While the story of M855A1 is fairly well characterized, the requirements and research that led to Mk. 318 has not been.