Air arms s510 hammer spring?

Go use a coin or maybe even 2 that fits or go to your local hardware store buy some washers maybe 2 that you can use to shim your hammer spring try one first angle CHRONY and please don't blame me if you end up going to jail.

On my S400 MPR it went from 6ftlbs to 12 by opening the transfer port restriction screw, not sure if the 510 has one, and adding one 90 durometer oring on the spring guide. The instructions below show to cut a piece of copper but this was to large and my rifle wouldn’t engage the trigger.

http://www.airguns.net/general_aa_mpr_power_upgrade.php

Oh for sure he's gonna go to jail now...

For your slight bump up in power you want, that is the best I can offer. Shim that hammer spring.

 I would check to see where the transfer port is set first. It's likely fairly restricted. You may could just open it and get the increase you want, without any change to your starting fill pressure or power curve. Since this is basically the only simple adjustment on the rifle, its most likely how the factory sets the power when they build it. Hopefully yours is easily adjustable and not anti-tampered somehow. 

Here's a link to a Huma regulator instruction guide, it show how the gun comes apart and good pics of how the transfer port was adjusted. Mostly closed in this rifle's case: 

http://foto.huma-air.com/foto/installation%20guide%20Air%20Arm%204xx-5xx%20made%20by%20Coverleaf%20from%20Airgunforum.pdf

IMO the hammer spring is NOT what you need to adjust. Take for an example the FAC guns with the power wheel. All it is is a variable diameter transfer port and it will take the rifle from very low power to 30fpe with the turn of the dial. Most importantly, it does this without altering starting fill pressures or power curve. They also have a LOT of hammer strike, too much really, and I would imagine the 12fpe guns are setup the same way. I think the transfer port adjustment would give as much power as you needed easily. Especially if it is mostly closed like in the Huma guide.