Diana Diana underlevers, grindy cocking stroke

My experience with airguns that have an applied rail on the receiver is that the interior can have burrs and rough edges. When the piston is ran up against this it scars the piston. Then the metal to metal grinding starts and sounds exactly like what you are describing. With a simple tear down, it would reveal this. Sand/polish both receiver and piston. Grease the surface and the grinding is gone.
 
I have the same issue with a D65. It's very accurate, but sounds as if there is sand in the gun! Is this normal? What pictures would show this?
I was told by a top tuner that with the 60-65s due to the three springs, two inside one, that thats normal as you cock it the springs will rub against each other. Was told you could tar them up to help that but then risk slowing it down by a good amount.
 
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With the Diana underlevers, I would first check that the front stock screws haven't gone in too deep, due to stock wood crushing under them from overtightening. This will cause interference with the cocking lever and extra friction and sound, and it is a progressive condition.

IME, every springer needs to have either the stock screw pockets hardened with superglue, or flat washers added under the star washers, if not already by the factory. I do both. Only then is the proper function and durability of the stock / receiver interface guaranteed.

A few drops of thin, quality gun oil (I use V-Mach) into every accessible joint of the cocking system of my sidelever Dianas (I've had three) worked wonders with regards to quiet, smooth and low-resistance cocking action. No need to dismantle, even though it is a good idea if you have the tools and the skills.