How to move the peak on a bell curve?

[rkr]

I went and shot a chrony string with my .177 SE using crappy Gamo pellets that I had no other use of. While the variation was almost acceptable I got no bell curve, just a string with lowering fps. Now how does one move the peak of a bell curve to lower pressure area? I can either change the spring tension or if needed add some mass to the hammer, what's the correct way of doing it?

[darryl]

I suggest either more start fill pressure or less hammer spring or maybe a combination  of both. If I had to choose just one option, I'd lessen the hammer spring tension. As I understand it the hammer (spring) won't be as effective in opening the air valve until the ar pressure starts to drop, so initially the FPS should start lower, then slowly rise as the hammer overcomes the pressure. The peak should move towards the center that way. That  should do it. If you lessen the spring tension and still have the same incline you have now, see if you have room to increase the start pressure. If not, lower the spring tension a bit more. BTW, the flatter the curve, the more consistency you'll see in FPS between shots, and possibly a bit more overall accuracy. The whole world loves a flatter shot curve, just a horizontal one. Try it and post your results.
have fun
darryl

[lloyd-ss]

rkr,
I think darryl nailed it.  To get the inverted bathtub curve the valve needs to struggle to open when the tank pressure is high.  Looks like your gun had no trouble opening at high pressure, so as darryl said, either lighten the hammer spring or increase the fill pressure.  But you are already at 200 bar.  Is that your max? 

Lightening the spring will give the string the full curve, and more shots, but will drop the average velocity.
Another cause for the one sided curve might be too light of a pellet.  Going up a couple of grains in weight AND increasing the spring pressure ought to give you a nice curve, but in a slightly shifted location. 
Lloyd

[rkr]

Well, it seems you guys were right. I eased the spring tension a bit and got flatter curve as shown in the graph. However, there's still no peak in the curve. I'd like to have the peak around 170 bar since I'm using handpump and usually only fill to 200 bar. This is a 12 fpe gun and since I'm now at 12 fpe I don't want to lower the spring tension much more. I thought I'd add a washer to the hammer next and then see how much more I can ease up the spring tension and whether I'll get the peak further down in the curve. As a side note, I had this SE and my Dynamax apart at the same time and they use exactly the same hammer spring. Only difference is the weight of the hammer and I believe the transfer port diameter. Go figure, 12 fpe to 38 fpe with those two things.

[lloyd-ss]

I still think a heavier pellet might be the answer.
If you look up some of rsterne' s old threads about his Hayabusa orQB78 mod, he talks a lot about cause and affect of shot string characteristics.
I sure hope Bob is doing ok.
Lloyd

[Ribbonstone]

When the "curve" starts at the peak and slowly declines, can think of it as getting the last hqalf of a bell curve.  Would be useful to shot count to have some of the front half (so there is a slight rise, peak, then fall).

To get there, will either increase fill pressure or slightly reduce the force of the striker's hit to the valve.  IF you're already starting at max pressure, that option is out.

Striker hit can be reduced by reducing the spring tension slightly or by lightening the weight of the striker slightly.  Of the two, spring tension is the more controllable (and reversible in case you go too far).

Are so close as it is...the suggestion to try a heavier pellet might be worth doing.

[darryl]

I agree that a heavier pellet might be a very good option. BTW, what is your shooting preference with this rifle (hunting, plinking, FT-shooting, etc)? Either way, I believe you'll get the best overall and consistent results with as slight a curve as possible  (think theoretically flat; shows best efficiency, imo). If you can get that peak in the middle of your shot count, and it's a gradual rise to, and fall from that peak ,like Rib says, I'd say stick a fork in it; it's done (as long as you stay with that pellet weight). It's fun to try to find that optimum set-up,  but it's reassuring to know the predictability once you do find that optimum. That's just me thinking out loud. About that heavier pellet, LLoyd makes the case that you can preserve your FPE and even gain a bit more efficiency from your rifle by going to a heavier pellet. So how much do those crappy Gamo pellets weigh? Also, I've come to adopt rsternes expression of looking at how many shots fall within 4% of the fastest recorded shot; although sometimes I'll go to 5% to sooth my ego. Looking at shots through that lens (of 4%) shows where the most efficient range of shots (within the total shot count) falls; for example maybe shots #4 though #24 out of 30 total shots fall within 4% of that fastest recorded shot. Now, of course, you can select any percentage that suits your accuracy requirements. Don't sue me. I'm still learning.
darryl

[lloyd-ss]

Darryl,
Good point about Bob's 4% velocity rule.  He's a real stickler for a flat curve and high air usage efficiency, and he worked hard at it.  I do know that very often he would back off on the spring pressure and reduce the fpe to get the longest string at the highest air efficiency.  For me, I would sacrafice a few shots to keep the fpe up, but that's just me.
Lloyd

[darryl]

lloyd i just finished testing my B50 with a smaller custom transfer port. I can't seem to post a decent sized image here, but whereas my prior test was pushing a JSB 18.1 grain pellet at a max of 998.2FPS at 38 FPE (good for only 20 shots) it's now pushing that pellet at a max of 896 FPS @30FPE, over 30 shots, with 25 shots falling within 4% of the fastest recorded FPS. I need to tweak it just a bit more and  move the peak FPS 4 shots to get to the middle of the peak. Otherwise I'm quite pleased. I won't be shooting at anything that can resist 30FPE. 
darryl

[Scotchmo]

darryl,
It looks like you reduced the FPE by about 20% with a corresponding increase in shot count at the same fill pressure. That is exactly what I need to do for my .177 B50. What was the ID of the original transfer port and the new transfer port?

[coax]

Doing some good work Darryl.  Nice improvement!

Keep going rkr.  It looks like your tracking in the right direction.  When I did my Mrod, I used the Hammer spring tension to flatten the curve the way I wanted.  I then got the velocity back by alter the amount the transfer port was opened.

[lloyd-ss]

lloyd i just finished testing my B50 with a smaller custom transfer port. I can't seem to post a decent sized image here, but whereas my prior test was pushing a JSB 18.1 grain pellet at a max of 998.2FPS at 38 FPE (good for only 20 shots) it's now pushing that pellet at a max of 896 FPS @30FPE, over 30 shots, with 25 shots falling within 4% of the fastest recorded FPS. I need to tweak it just a bit more and  move the peak FPS 4 shots to get to the middle of the peak. Otherwise I'm quite pleased. I won't be shooting at anything that can resist 30FPE. 
darryl

Darryl,  Very nice results, and the new lower velocity might even be easier to shoot accurately.
Lloyd

[rkr]

Power curve number 3. This time I filled to 230 bar and gave it 60 shots. I released the spring tension even more and this is the lowest fps I want to go as I want to stay close to 12 fpe. I guess I'd get 40 good shots if I were to start filling to 220 bar, good enough for HFT I think. I'd still like to move the curve to a lower fps but I guess I'd have to start working with the hammer then. Say, if I were to add a washer to the hammer - could I then back up the spring a bit more and retain the fps while moving the power curve? If I were to lighten up the hammer then I would have to add the spring tension and that would then steepen up the curve?

Those gamo match pellets are really poor quality. Some just drop in the magazine while others need to be pushed in, I guess that explains those few drops in the curve. I usually use H&N FTTs or JSB Exacts which are 8,4 grain versus the 8 grain of these gamo pellets.

[lloyd-ss]

Power curve number 3. This time I filled to 230 bar and gave it 60 shots. I released the spring tension even more and this is the lowest fps I want to go as I want to stay close to 12 fpe. I guess I'd get 40 good shots if I were to start filling to 220 bar, good enough for HFT I think. I'd still like to move the curve to a lower fps but I guess I'd have to start working with the hammer then. Say, if I were to add a washer to the hammer - could I then back up the spring a bit more and retain the fps while moving the power curve? If I were to lighten up the hammer then I would have to add the spring tension and that would then steepen up the curve?

Those gamo match pellets are really poor quality. Some just drop in the magazine while others need to be pushed in, I guess that explains those few drops in the curve. I usually use H&N FTTs or JSB Exacts which are 8,4 grain versus the 8 grain of these gamo pellets.

That's a darn nice looking curve, really, don't you think?
Are you out of the good 8.4 gn pellets?  I think that going from 8.0 gn to 8.4 gn ought to drop the vel by about 25 to 30 fps.  Then you can tighten up on the hammer spring to bring the FPS back up, and that will alter the curve.  What about going smaller on the transfer port tube?

When you say add a washer to the hammer, do you mean to get more weight, or to compress the spring more? Where would you put the washer?
Lloyd

[rkr]

Yes, that's decent curve for a carbine size gun. A 40 shot string in the middle is almost as good as regulated guns get and I think those few low velocities would be eliminated with good pellets. The washer would go in between sf134 and tophat in this drawing http://www.gunspares.co.uk/shopdisplayproducts.asp?id=24857&cat=Scorpion+T-10+PCP thus adding more mass to the hammer assembly and hopefully allowing less spring tension while retaining the fps. Do you think that would move the power curve to lower pressure region?

[lloyd-ss]

rkr,
Yes, that clears up the picture and it could very well have the effect you want.  I was noticing in the exploded view what looks like a set screw near the green washer and sf134.  Is that possibly a hammer stroke length adjuster?  Shortening the hammer stroke will move the peak to a lower pressure and then up can tighten the spring back up to get the fpe back up. So it looks like you have several things you could try...... which sometimes is too much of a good thing.  A lot of pumping, and a lot of pellets. 
Sometimes in a testing situation like you are in, and hand pumping,  you can test on the way up.  Pump a little, take a shot or 2,  pump it up another 500psi, shoot a couple, etc, etc.  Then you can change something and shoot on the way down.  Sometimes I will just shoot air if I am looking for gross results and not total shot count.  It saves time and pumping, and pellets.  Also, if you are testing several pellets, you can shoot one of each in rotation, all in one string.  It will show you the trends and will save a lot of time.
Lloyd
PS. Did you see the dynamax graphs?

[rkr]

rkr,
Yes, that clears up the picture and it could very well have the effect you want.  I was noticing in the exploded view what looks like a set screw near the green washer and sf134.  Is that possibly a hammer stroke length adjuster?  Shortening the hammer stroke will move the peak to a lower pressure and then up can tighten the spring back up to get the fpe back up. So it looks like you have several things you could try...... which sometimes is too much of a good thing.  A lot of pumping, and a lot of pellets. 
Sometimes in a testing situation like you are in, and hand pumping,  you can test on the way up.  Pump a little, take a shot or 2,  pump it up another 500psi, shoot a couple, etc, etc.  Then you can change something and shoot on the way down.  Sometimes I will just shoot air if I am looking for gross results and not total shot count.  It saves time and pumping, and pellets.  Also, if you are testing several pellets, you can shoot one of each in rotation, all in one string.  It will show you the trends and will save a lot of time.
Lloyd
PS. Did you see the dynamax graphs?

That screw there is holding the pin that connects the hammer to the trigger unit. Very good ideas Lloyd, saves time and pellets - thank you Yes, I saw the Dynamax graph and now I'm trying to find a deal for 15" barrel The plan is to have a shrouded barrel that's the same length as the airtube. I think it will look quite nasty with synthetic stock.

[darryl]

darryl,
It looks like you reduced the FPE by about 20% with a corresponding increase in shot count at the same fill pressure. That is exactly what I need to do for my .177 B50. What was the ID of the original transfer port and the new transfer port?

I don't know the original port size, but the one I have in now is 0.119. I have a smaller one at 0.109 that I may try to test. Maybe I can increase the shot count and keep the same FPE?

rkr, you're on your way. good luck!

[coax]

rkr, I'm probably not saying anything you don't know.  I think you're close enough that you will need to start testing your strings with exactly what you plan to shoot now.  Otherwise, you might start chasing your tail or pass your optimum adjustment.

[rkr]

Added two washers (1.2 grams) to the hammer and eased up the spring a bit to keep the power the same. The power curve stayed the same, no change in peak position   Coax, I think you are right. I did some preliminary testing with H&N FTTs and the power was down below 11 fpe. I think the barrel needs cleaning up and the power upping a notch. After that it's time to do a string with FTTs.