I don't know how I would cut an O-ring groove around the exhaust port, even if there was room to do so.... The surface of the valve, and the tube it fits into, are curved, so the O-ring groove would have to be much deeper on the sides than in the middle, relative to a flat plane tangent to the valve at the port.... I suppose a CNC could be programmed to do it, using a 1/16" or smaller end mill, but I don't happen to have one of those handy....

I am getting a blast of air out of the side valve holes in the rear half, but when I am finished development I will squeeze some silicone into the recess around the screws before I install them.... I hope that will eliminate that leak.... The volume between the 2 O-rings on the rear valve half is only about 0.2 cc, (figuring a 0.010" gap all the way around the cirfumference and a 1/2" width between the O-rings).... With a 10.8 cc valve, that is only going to cause an additional 2% pressure drop before the air gets to the pellet, so I don't think that is a big concern, once I get the leak stopped.... A fair amount of air is blowing out past those side screws....

….The MRod valve stem is a pretty good fit in the new insert I made for the back of the valve, so I don't think that is a major loss....

Today I fitted the lightweight hammer that I made the other day.... Now that I have reduced the operating pressure to 1250 psi, it has plenty of adjustment for the 22 pump initial charge.... At that pressure I was getting about 807 fps with the 15.9 gr. pellets (23 FPE)…. With 1 turn of gap on the SSG with the 18.1 gr. Heavies, I got 788 fps (25 FPE), which I'm pretty sure is maximum power at that pressure.... It doesn't really matter anyways, because I will be dialing it down until I get 2 equal shots.... Here is a photo of the two hammers....

The hole for the hammer spring is drilled 0.16" deeper than the stock hammer, so that I can set the SSG for zero gap with the adjusting bolt a turn clear of the back of the hammer group at zero gap, instead of 0.20" behind it.... That shortens how far the SSG protrudes.... The rear of the hammer is about 7/16" OD, and the front portion is only 5/16", which reduced the weight to 40 grams.... In addition, I machined the sear catchment collar on an angle to match the remachined sear, so that the load won't just be on the corner.... I made the hammer out of 1144 stressproof steel, but I didn't bother to harden it.... We'll see how long it lasts....

I started out at 22 pumps, with the SSG set for 1 turn of gap, and then increased the gap, recording the velocity of all 3 shots, and the pressure after the second shot.... Here are those results....

As was the case the last time I tried this lightweight hammer, the report is noticeably quieter, particularly on the second shot.... The first 2 shots were equal at 3.5 turns of gap (instead of 7 turns), and at that setting the remaining pressure had increased nearly 100 psi with the lighter hammer.... The velocity of the 3rd shot increased from 585 fps with the heavy hammer to 615 fps with the light hammer, proof positive it is saving air....

It still takes 6 pumps to replace the air used by the first shot, but instead of 7 pumps on the 2nd shot, that has dropped to 6, so to refill after two shots only takes 12 pumps.... This also means that instead of filling to 22 pumps and getting 2 shots, I can just use 16 pumps for the initial fill, and shoot a 21 FPE shot and pump 6 times to refill, and do that for as long as I want to shoot....

I then repeated the process using 16 pumps as the initial fill (1000 psi), and dropping down to the 14.3 gr. Express pellets.... I started at 5 turns of gap on the SSG, and gradually increased that to 8 turns, recording the results as follows....

The results are similar, just at 16 FPE instead of 21 FPE.... The first 2 shots are equal at 1 flat less than 7 turns of gap, at 712 fps (16.2 FPE) average.... Just like yesterdays results, I can shoot a mixed bag of single or double shots.... It takes 9 pumps to refill after 2 shots, and 4 or 5 after a single shot.... As long as I fill to the high side of the 1000 psi mark on the gauge, all shots are within a 3% ES.... I also have the option of simply filling to 750 psi (12 pumps) and shooting a single shot with a 4 or 5 pumps recharge between shots.... I then repeated the process again, using only 12 pumps for the initial fill (750 psi), again with the 14.3 gr. pellets.... Here are those results....

Two equal shots occur at 2 flats past 9 turns of gap, at 620 fps (12.2 FPE)…. It only take 6 pumps to refill after 2 shots, or just 3 pumps after each shot.... I also found a setting that gave a decent 3-shot string of 585, 611 and 574 fps (11 FPE with a 6% ES) from that 12 shot fill.... It takes 7 pumps to refill to 750 psi to repeat that 3-shot string....

I tried something interesting at this setting as well.... I pumped just 10 times, took a shot, and then refilled with 3 pumps.... I shot a total of 15 shots like that, averaging 608 fps (11.8 FPE) and the ES over all 15 shots was less than 2%.... This means as a retained air pumper I am getting nearly 12 FPE on just 3 pumps....

I also checked the efficiency, based on the amount of air pumped into the valve at the three power levels where I was getting 2 equal shots.... At 12.2 FPE I was getting 1.11 FPE/CI, at 16.2 FPE I was getting 1.06 FPE/CI, and at 21.1 FPE I was getting 0.99 FPE/CI, based on the air pumped into the gun.... Calculated using the pressure drop in the valve over those two shots (like you would a PCP) it was between 1.3-1.5 FPE/CI....

So, I can use the heavy hammer and about 30 pumps and hit over 30 FPE with a 25.4 gr. pellet.... Alternately, I can use the light hammer and tune for 2 equal shots of 21 FPE at 22 pumps with 18.1 gr. pellets, or 16.2 FPE at 16 pumps with 14.3 gr. pellets, or 12.2 FPE at 12 pumps with 14.3 gr. pellets.... In any of those three tunes, I can get a single shot at only 16 pumps, 12 pumps, or 9 pumps respectively.... Lastly, I can get a 3-shot tune at 11 FPE filling with 12 pumps, or shoot indefinitely at that setting from a 10 pump fill (just over 600 psi), with 3 pumps per shot giving 11.8 FPE....

Bob