DW RZ-10 and Underwood Ammo

[arentol]

I have had a DW RZ-10 for a while now (since 2009). It has handled every bit of ammo I have thrown at a it without even a hint of complaint... Until today.

I recently got my hands on some 135gr, 165gr, and 180gr Underwood Ammo which is rated at 1600, 1400, and 1300fps respectively, and which, unlike Double-Tap which has similar ratings, actually achieves those high numbers.

Anyway, I took the above ammo to the range today and ran a magazine of each through my RZ-10. The 135 and 165's resulted in two FTE's per magazine, but the 180 ran perfectly for 2 magazines straight. One of the FTE's on the 165 even resulted in the round behind it getting shoved into the unejected case so hard the bullet got pushed back about 2mm into its case.

I want to use Underwood as my main carry ammo, and while I am fine with using just the 180, I would prefer to carry the 165 if I could get that to run reliably.

I assume that the main issue is that the springs aren't right for loads this heavy. Does anyone know what springs I should be changing to fix an issue like this, and what kind of change I should be making to them (heavier, lighter)?