The story of todays post is.... NEVER BE AFRAID TO LEARN SOMETHING NEW.... or at least open your eyes to existing problems....

I am always careful to make sure that the area between the front of the hammer and the back of the valve can't get pressurized, either from the hammer movement itself, or from an air leak through the valve stem during firing.... If that happens, you can get (in the worst case instance of a leaky valve stem) a machine-gun action where the pressure partially recocks the hammer and fires the gun again.... until the reservoir is empty (yes, I've had that happen).... Travis suggested that I flute my hammers to prevent this, but I double checked, and the sear slot in the bottom of the tube vents into the trigger, and out through the bottom.... However, I haven't been paying enough attention to the BACK of the hammer, where a vacuum can build up and slow the hammer strike.... The only gun I ever vented there, on a suggestion from Scott (Motorhead), was my B-51.... WHY I haven't been looking for this problem in other guns I don't know.... complacency, I guess.... Well, that just bit me.... in spades....
While I was triple checking the space in front of the hammer this morning, I looked at the area behind it.... Yes, there is a rather large cocking slot, with a matching one in the receiver.... but guess what.... There is nowhere for the receiver to vent, other than past the bolt at the ends, in an MRod receiver, with the rear bolt.... I had noticed some large ES values yesterday when I had the SSG backed off with a large gap, and the lightbulb finally went on.... With an SSG (or SSS or TSS), the spring quits pushing the hammer before it gets to the valve stem, and it coasts the last part.... What if there is a vacuum behind the hammer?.... Yup, that will start slowing the hammer down before it gets to the valve.... In addition, that vacuum is unlikely to be 100% consistent, shot to shot, which could increase the ES.... and the more gap you have between the spring and hammer, the worse both might be.... Time to find out....
I set the SSG for 1.5 turns of gap, and shot an 8-shot string.... The average velocity was 811 fps, but the ES was huge, with shots going from 790-834 fps.... definitely not good.... I stripped the back of the gun apart, and machined two slots in the bottom of the rear plug (SSG housing), at about 4:30 and 7:30, in the section that inserts into the tube.... The slots were 3/16" wide and 1/8" deep, and continued back 3/16" behind the rear of the tube.... That is the equivalent area of a 1/4" hole through the rear plug to prevent any vacuum forming behind the hammer on firing.... Here is what you see with the action out of the stock....

There happens to be a slot in my stock at the location of the vents.... If that isn't the case with a different stock, a different vent design may be necessary.... Anyway, I assembled the back of the gun again, adjusted the SSG gap to the same, 1.5 turns out, and fired a 34 gr. JSB through the Chrony.... Instead of 811 fps, it was 956 fps.... WOW !!!.... Obviously the vacuum behind the hammer was a reality, I had moved up onto the velocity plateau.... I went through the process of documenting the velocity vs gap with the vented rear cap.... On the graph below, the dotted red line is the new data, with the vent.... while the solid red line is yesterday's data, without the vent....

There are two important things to notice.... First, it takes a LOT more gap to get to the knee of the curve with the vents.... which means that without them, the vacuum behind the hammer was slowing it dramatically.... Secondly, the slope of the downslope part of the curve is a lot shallower.... It takes more gap to lose the same amount of velocity.... To drop from 944 fps down to 760 fps now takes a 3 turn increase in gap instead of two.... That makes sense, because the bigger the gap, in the unvented setup, the more the vacuum would reduce the hammer velocity.... There is absolutely NO doubt this gun needs the area behind the hammer vented....
Since I had dramatically increased the hammer strike at the same settings, I could now do two things.... I could reduce the preload on the spring, making the gun easier to cock.... I reduced the preload to my original 0.40" (which was not enough yesterday, with the vacuum), and shot the data shown by the purple line.... As you can see, the "knee" of the curve still requires 3 turns more gap, even though the preload on the spring is reduced from nearly 8 lbs. to just over 5.... Since the larger gap means the spring is not compressed as far (and starts at less preload as well), the maximum cocking force, when the gun is set just below the plateau, at 944 fps, has dropped from 19 lbs. down to just 15 lbs.... In fact, I could further reduce the preload if I wish, and that would allow me to decrease the gap, and the cocking force, even more....
These vents did one more thing for me.... They increased the available hammer strike to the point that I could now test the gun at 2900 psi.... I checked it with the 34 gr. JSB Heavies, and the results are shown as the black line on the graph above.... This was shot with the preload on the spring still at 0.40", and to get to the plateau I had to set a turn of negative gap (actual preload on the valve stem).... I didn't try it, but I am confident that by going back to the 0.60" preload I could operate at 2900 psi and hit the plateau with a small gap.... In any case, these results proved that this valve is capable of cranking out 90 FPE at 2900 psi with the 34 gr. Heavies.... I didn't have the proper sized 51 gr. BBTs for this barrel, the ones I had were sized to 0.250" (so they were likely leaking a bit in the bore).... but by cranking in some preload I managed to find the plateau for them as well.... I was able to hit 963 fps (105 FPE) at 2900 psi with those bullets.... I have ordered an NOE Mould for the 51 gr. BBTs in 0.253/0.255", and the sizing bushings for them.... so at some point in the future I can experiment with them....
In conclusion, I am extremely glad I vented the area behind the hammer to prevent the vacuum from slowing the hammer.... I am frankly shocked at how much hammer strike was lost because of that vacuum, and the horrible ES that it caused.... You can bet I'm going to have a long, hard look at all my Disco based guns.... There may be more consistency, and easier cocking, lurking amongst any of them....

Tim, the LW barrel is the standard, 23.8" length they sell....
Bob