@Lozmoid:
How about using a very thin coating of superglue to strengthen the rifles, and stop them from cracking? Then prime and paint. I expect the Japanese Infantry would be the most susceptible to this.
Superglue might cause issues, depending on whether it is oil or water based. As a rule of thumb, never put oil-based under a coat of water-based (IE acrylic paint). The oil fluctuates with the humidity and when it expands, even a slight amount, it will cause the paint to be even more brittle.
I’m not sure that it would help much anyway at small amount. If anything, just brush on a few extra coats of varnish, allowing the proper dry time between. Above all, just handle them with care.