i have a G3 parts kit i bought over a year ago, and want to build it into something different.  i was wondering if the roller locks would work for a lighter round ie. 243 win, or a heavier one  ie 376 steyr.  does anyone know the max COL that a G3 mag will feed.  the 243 should be no prob, but the 376 is about .250" longer.  the 243 would have shaved sights and be used for varmit hunting.  i would really like to build a semi 376, though.  it would have a 16" barrel with the standard sights.  i think 300 grn slugs out of a G3 would be awesome.  

Yes you could make it work, but lets go into some detail on the delayed blow back action.

The basics are that the bolt, rollers, carrier and recoil spring work as a unit, and are spec to work with Nato 7.62 ammo.  If you change the ammo, you will need to rework the perceived mass of the bolt on lock up.

Think of a basic blow back action like that of a 22lr.  If you step up the ammo into the mag class, then a stronger spring or heavier bolt is required to keep the action closed long enough to allow the bullet to exit the barrel before the spent case is ejected. In regards to going with a lighter round, such as a 22 short, then the spring and bolt mass needs to be lighter or the action will not cycle.

On the HK delayed blow back action, the rollers cam into the trunnion.  Since the bolt must press back against the rollers to drive/unlock them from the trunnion, it in effect makes the bolt and carrier act as if they had more mass than just the weight of the two parts.

If you went with ammo like the 243, then chances are that you would need to lose perceived mass from the carrier, or relationship of the bolt to rollers.  This would allow the rifle to unlock at the correct time, and cycle the spent case out of the action.  
On something like a 300 short mag, you would need the increase the perceived mass of the carrier/bolt, or the ammo energy would unlock the bolt too soon and you would have the carrier trying to blow back out of the stock.
Also, here is the biggest hurdle once you did figure out the new formula for the b/c perceived mass, the chamber needs to be fluted to allow the case to be gas glided out the chamber. Finding someone to flute the chamber in the off caliber will be really hard pressed.

Bottom line is that is easier to just use another platform/rifle, which incorporates a gas system.  This allows the bolt unlock/timing to be easily tuned by barrel gas port size, and not having to break out the slide ruler to achieve the correct perceived mass weight of the B/C to allow the delayed blow back system to function correctly.