I'm gonna ask the GPC question: will tracer bullets fit?

Why wouldn't they?  

stancrist said:

Why wouldn't they?  

That is Tony's issue with Grendel. It is too short to fit ballistically equivalent tracers.

I see no reason why 6.5 Grendel could not be loaded with the 120gr tracer bullet from 6.5 Swede.

The Swedish Military Ammunition Site (amkat.se)

Likewise, it seems to me that the 6mm XM734 tracer projectile should fit just fine in the 6mm ARC.

needs to be ballistically equivalent though. if the site was up, I'd dig it out from the GPC article.

I don't know what you mean by "ballistically equivalent" tracers.  Tracer bullets typically don't have the same BC -- or the same weight -- as Ball projectiles.

The Swedish 120gr 6.5mm Tracer would probably be a quite usable companion for 6.5 Grendel 120-123gr Ball.  Ditto for XM734 Tracer and 6mm ARC Ball.

As I said, there was a case against it in Tony's article (@autogun) - since the webpage is down I cannot quote it. Tony, can you help?

P.S.  In any case, those are just examples of existing tracer bullets that would fit those two cartridges. 

If required for future military use, new "lead free" tracers of appropriate weight would be developed.

The issue is that the reason for creation of the 6mm ARC was effectiveness at long ranges (700m+).  This required extremely high BC bullets which was only achieved with an optimized shape and density requiring a lead core.  According to Hornady, the goal was to equal or better the terminal performance of the 7.62x51 at distances beyond the effective range of the 5.56x45.  As far as I know, there are no tracers that approach the BC of the ones needed to achieve the goals of the 6mm ARC.  The whole point of tracers is to give the gunners a visual indication of where the rounds are going, and to my knowledge this is impossible with the current design of the 6mm ARC.  It is doubtful that even an effective OWL design would work without an optic specifically designed to track a 6mm diameter glowing object at ranges beyond human perception.

A while back during a discussion of the 6mm SAW round, several people noted that the BC of the ball ammunition was surprising poor.  Nathan's TFB article states "The initial design parameters were for a round effective to 1,000 meters, and which accepted a tracer that would give a visible daylight burn out to 800 meters,"  Given these requirements, both ball and tracer round needed the same effective BC to at least 800m.  Given this "chicken and egg" scenario, it appears that the design of the ball round was dependent upon what was achievable with a tracer.   

He's talking about ballistic trajectory matching which is important for your tracer to do in order for it to have as close as possible poi as your ball rounds at a given range.

For grendel, you're correct that one could be done but it would so handicap your ball projectile due to having to trajectory match the tracer that grendel essentially loses it's appeal because of the limitations on performance a trajectory matched tracer would place on the ball round ballistics