1919 links like in the pic above are a different matter than a M60, M240, or M249. The last three aren't that hard to link by hand if you need to. I've had to make SAW belts in the field from boxed ammo before. It's a pain, but if you don't want to deal with th jams that ALWAYS happen with mags in a SAW you can do it.
I've linked up leftover 7.62 by hand at the end of a range day or to remove tracers when shooting the M240 too.