I've been testing different things to see what is possible for improving the standard valve trigger pull. I've been using a modified RT on/off.

but the reactivity created, kicks the trigger back only about half way. the reduced shaft of the bottom of the on/off pin does not consistently return the trigger when it has to work against the wieght of your finger. this will end up causing accelerated bolt and sear wear.

I do have a better understanding of how the shims in the ULT work. normally, the on/off is approx. .555" long and the hole in the valve body where it goes is approx. .590" deep with the o-rings in it. when the the gun is pressurized, the o-rings and the on/off are pushed down by the air pressure until the on/off is stopped by the body.
this means that the pin can go up into the o-rings way past the point of sealing.

with the ULT, shims lengthen the on/off so that you can make the o-rings stay in place. this holds them up higher. so that the sear does not push the pin way past the point of sealing. instead it can be made so that the pin just seals. thus increasing the recharge rate to it's maximum point.

with the homemade e-mag, http://www.automags.org/forums/showt...hreadid=125891

I've been using the modified RT on/off along with a quad o-ring for the on/off top o-ring and a soft buna o-ring for th e small o-ring inside the on/off. by using lvl10 shims with it, I've been able to increase the recharge rate to the point where the standard valve can do 18 bps, without drop-off. the quad o-ring definitely helps. it makes it possible for the pin to open to a larger area of flow, faster, because the o-ring seal is closer to the edge of the o-ring.

the modified RT on/off works well with the e-mag mod because the pin does not have to push back and return the trigger. it does not feel the wieght of you fingers. it also makes it possible to use much less force to actuate the sear. by fine tunning the on/off a high rate of fire is attainable even when using a lvl 10 that slows the bolt down and uses more air.

on a side note, after trying quad o-rings on the the lvl 10, I'd say they don't realy help any more than using plain buna-n o-rings.