HW100 tuning question

[bchannell]

I'm trying to tune my neighbors HW100 for him and am having some head scratching moments. I have both a .177 and .22, so this should be fairly easy.
First, it is shooting 8.64gr H&N pellets at ~940fps with the hammer spring backed out nearly flush with the rear of the hammer itself.
Now for the head scratcher....turn the hst adjuster in one turn, chrono shows 846, hammer is noticeably harder to cock.
Turn the hst in another one turn, chrono show 809fps, and again, harder cocking for sure.
Back out the hst to flush with the rear of the hammer (original position), and sure enough 940ish fps.
Put reg gauge on the action and it is factory set at exactly 80bar.
When shooting at 940ish fps, the chrono shows high 20's ES and maybe 40-45 shots, I didn't really count as I knew it was low.
Try 10.6gr pellets, 40 shots maybe, high 20's es.

Anyone makes sense of this?

 

[nervoustrig]

Regarding why the velocity goes down as you increase hammer spring tension...if the hammer strikes the valve stem so hard that it drives the stem flush to the back of the valve, the hammer will rebound. The result is less valve dwell—in other words it snaps closed sooner than it would have otherwise—and the velocity is diminished.

I’m not familiar enough with the HW100 to comment on how likely it is on that particular model but that is the usual cause.

Regarding the low shot count, that’s a clue the hammer spring tension is already set too high for the current regulator’s setpoint, which is consistent with the preceding symptom.

How does it respond when you back it off a turn? Two turns?

 

[elh0102]

I just looked at my HW100 hammer adjustment, and the back of the hammer is about perfectly flush with the adjustment screw. I tuned mine down to sub 12 fpe, so the comparison may have no relevance. But, you have plenty of room to back off the HST more, and I think you should try it, as it sounds like the tune is not well balanced at the current velocity. If 940 fps is about the maximum velocity at the current reg setting, then somewhere in the 880-890 range should be a decent balance with a lower ES. Just a guess, but maybe worth trying.

Also, I'm not sure that your shot count is all that low. Tuned to FAC power level, the rifle doesn't have a great shot count, which is one reason I tuned mine down.

 

[bchannell]

You nailed it. I figured it out, this rifle has a much finer hst adjustment than my two HW100's. It is flush or maybe just a thread or two inside the hammer sleeve when at maximum hammer tension. It is out of the sleeve when set at it's lightest setting. I backed it out to where it barely fired and worked my way up and it worked perfectly. I actually thought it would hit the back action cover when it was installed at it's lightest setting, but all was well there too. I did 5% below max velocity, and the results are nothing short of amazing. I got 905fps max, with maybe 3-4 mags max, which would be ~50 shots with a horrible ES. With the hst adjusted to 5% below max or 850fps, I got... wait for it...137 shots with an ES of 16fps and a %ES of 1.88fps. To me that's incredible improvements. If i wanted to take just over a one hundred shots, my extreme spread would be less than 10! What a gun! I think I'll look at accuracy and maybe bump the speed up to 880ish and I bet I still get well over one hundred shots on the reg. These are great guns to tune!

 

[elh0102]

I'm impressed with your shot count. That's close to what I get with a 12 fpe tune! I've never actually counted, but I know it's well over 100 good shots.

 

[bchannell]

These guns are amazing to tune, and they reward you with shot counts that are well over anything I expected out of their smallish cylinders. I know as I go up in power, air usage will go up exponentially, but you have a gun well in the "useable" range of pellet speed that is deadly accurate. Even with a terrible tune at 33 yards, I was getting entire magazine full groups of one large hole, maybe 3/4".