MRod 10# spring strength differ?

[zaphod621]

Have owned my MRod .25 for a couple months now and have been a good little lurker around here trying to learn as much as I can before asking any questions. I have been noticing some very different tuning settings on some of your rifles that i cannot duplicate with mine. My rifle was used so I don't know any adjustments the PO did but here is what I started with:
2800 fill, JSB Kings, 16 shots.
844
854
853
846
847
844
841
836
832
827
827
821
815
810
803
796
791 (2000psi)
I was intrigued with the results some were getting with the 10# spring so I got one.
Results: only change was the spring: only 12 shots to 2000psi
880
874
868
862
853
847
840
829
819
812
803
791
Clearly needs less Hammer spring. I opened the transfer port to 4.25 ccw and backed off the spring all the way ccw and left the stroke alone( stroke screw is very hard to turn but that's for another day). Played around with the spring on the initial shot looking for about 830fps and a climbing shot count from there and found that 3/4 cw was about right.  Boost my fill to 3000 and dailed in 3/4 turn cw on the hammer spring.
Results:
839
851
852
853
855
858
855
855
850
848
848
845
848
842
834
828(2300) 16 shots within 4% of peak and just over 40fpe. Very accurate, just over 3/4" five shot group at 100 yards. I can live with that but was just noticing that others have different shot strings than mine with what looks like the same setup.
This seems like a pretty good string to me but some are realizing morelike 45fpe and longer strings than I am with their spring dialed in at 6 or 7 turns cw. If it turn mine in 6 turns I get a very high initial shot and fall off a cliff after that.
Could the difference be the springs give a varying amount of tension?
Also is it common for the stroke screw to be difficult to adjust? Mine is very stiff to turn so I left it alone.

[Bullfrog]

I suspect much of the difference is from slight variations in the factory valve between guns. I've had 3 factory valves in my .25 Mrod and each one gave me different velocities and air usage efficiencies on the same settings, sometimes drastically so. The various transfer ports could be ever so slightly different from valve to valve and that could make the difference. For instance, my first 2 valves couldn't get me shooting about 870fps with 31 grain barracudas on a 3200psi fill, my third factory valve would let me get up to 900fps on the same fill, but at the cost of not being able to depower my gun (it used the same amount of air no matter whether I shot 40fpe or 60fpe).

It is ideal if the stroke screw is hard to adjust. Crosman used to make them with set screws in the hammer that kept pressure on the stroke screw so it wouldn't slip every time the hammer hit. Crosman stopped doing that on later Mrods and therefore the hammer stroke has the tendency to slip a little every time the hammer pounds the valve stem. Some people fix that by making their own set screws and tapping out the little hole on the hammer for the screw to screw in to. I personally use blue loctite inside my hammer (a LOT of it cured for 3 days) and that seems to do the trick. If your set screw is hard to adjust that's a good thing because it means it will have a harder time slipping than if it was easy to adjust.

[rsterne]

I suspect the springs may be from different sources.... everyone refers to them as a 10# spring, but with no dimensions or wire size / # coils.... They may (or may not be) a 10 lb./in spring rate, and even if they are, you can get that through different dimensions.... That will require different preloads to arrive at the same SHAPE of shot curve....

Differences in the port sizes, alignment, and even how tight the bore is, will affect the velocity at the PEAK of that curve.... The bottom line is that you have to tune for (example) 2 clips (16 shots) with the first shot about 3-4% below whatever your peak is.... I would do that with the stroke and velocity screws set at maximum.... What that peak velocity is, is less a factor of your spring, and more a matter of the ports and barrel.... There could be a difference in the Chrony's as well....

Bob

[zaphod621]

The bottom line is that you have to tune for (example) 2 clips (16 shots) with the first shot about 3-4% below whatever your peak is.... I would do that with the stroke and velocity screws set at maximum.... Bob

Yeah thats pretty much my approach to it all. My problem is suffering from "shot-string envy". In reality the difference between 40fpe and 45fpe down range at say 50 or 100 yards is only 3fpe and with the game I will likely be taking with this 20 to 30fpe is way more than needed. It's all too easy to fall into the trap of thinking it's a race with everyone else, when it's actually a contest with myself. This shot string is getting pretty darn close to optimum and I am pleased with that. Always appreciate the advice from all.
Zap

[FuzzyGrub]

The main advantage of a heavier spring is going to the higher fill pressure.  You do get some more velocity, but that is mainly the job of the transfer port adjustment.   What you have looks to have a good curve.  If you want more velocity, open of the transfer port, some (CCW).  You will get higher velocity, but the curve will get deeper, and ES will go up.