I wrote up a long winded response to the last few post and when I tried to submit, I lost the whole thing. Perhaps for the good, there must be a windbag filter on this forum
Let me just condense it to this:
My results are not scientific. I put the HDD on a brand new gun (replaced by Crosman on warranty) so I can't compare between the old and the new. But....
1. I did see my ES and SD tighten way up. This is important for me...it might not be for you.
2. I once tried the # 10 spring but only on the old gun with poor results...something was wrong with the gun which is why Crosman replaced it. I got no curve with the #10 and minimal increase in velocity...so no comment on that as to it's HDD qualities.
3. I did get a few more shots with the HDD. If you are really interested in more shots I'd suggest the airtube extension.
4. I would imagine that you might experience a slight drop in velocity with the HDD by design. But I am shooting with it and the stock spring at respectable velocities with Exact Kings. Again, I can't compare. I should have tried the gun stock to get a baseline before installing the HDD but I wasn't aware at first that Crosman had replaced everything (they aren't good at telling you what they did and the chicken scratch on the ticket was tough to read)...the new serial number was the giveaway. BTW I have the hammer tension all the way up but then again I had it all the way up in the old gun and it shot at a lower velocity without the HDD. That's why crosman saw fit to replace it.
So here's the thing: It depends on what you are going to use it for.
1. If you are interested in a flatter smoother curve for target shooting, I think it might be worth consideration to get a HDD.
2. If you are a hunter, I think you could still consider it but not as much. If you are going to hunt, put in a #10 spring and get some good velocity/energy going. Forget shot count, you aren't going to get that many shots off anyway. If you're a serious hunter, take a buddy bottle. To put it in perspective, a Quackenbush or Dragon Claw .50 cal is only going to give you 3 or high power shots max. The idea is knock em down to where they don't get back up with one well placed shot. You need power for that, not a smooth curve.
Check out Ted's Holdover on intalling a HDD: http://www.youtube.com/watch?v=J_rrZWaFqbI
...not for the install itself but for the graphs he took the time to compile. That's a whole lot more scientific than what I'm saying here. He was using an untuned out-of-the-box .25 cal Marauder (he states that in an earlier episode with the Marauder).
Not trying to push the HDD, just putting up what I saw....hope that it helps someone in their decision making.
oops got long-winded again