This isn't any kind of big deal, so don't condemn a nice rifle over it.

Yes, the problem you describe is common and normal.

These rifles were set up to fire the ammunition of the day - Kynoch ammo loaded with Cordite lit by a much larger diameter Berdan cap that used a copper (corrosive) or brass (non-corrosive) primer cup. What was ideal for the protrusion and shape of strikers for these primers isn't for the smaller modern Boxer primers. Some of the strikers in these rifles weren't even round. The firing pins probably need to be re-shaped and re-radiused by a smith who works regularly with DRs.

Be wary of Federal factory .470 ammo. There are two common problems with it. It is usually overloaded. A dealer friend recently chronographed a batch of it from a new Heym with 24" barrels and got 2,220 fps, which is way too fast. Your result of under 2,000 fps with 26" barrels is the other problem - lot to lot consistency is often poor.