Lower FPS after a delay

[madeintheuk]

Thanks @HogKiller Looks like I need to get the IHS correct for the reg pressure and go from there. I have other tunes from AEAC that I can try also.

It’s not all about the ihs. The power wheel will make big adjustments for you. Setup your chrony? And shoot a couple shots at each power setting. See if the power continues to rise all the way through the settings. If and only if it’s still rising at the highest power setting should you start playing with the ihs.
If you get to a point where the power levels off, or decreases before then, go one click down, and start increasing your ohs till you reach the knee. Once that velocity levels odd, back it off a quarter turn or so, and you’ve found balance. At this point, if your speed is too high for your liking, lower reg pressure, and start over.
It’s certainly time consuming, but once you find the harmonic sweet spot, you’ll be amazed at how well the gun shoots, how quiet it is, and you’ll probably pick up a few shots per fill too.

 

[thammer]

Sounds a lot like reg creep to me. That is when over an extended time a small amount of air continues to pass through the reg raising the pressure past the 150 bar setting you have. Typically a small defect/debris with the reg seal. Since you are already at a 150bar setting it can cause partial valve lock holding the valve closed and not letting it open as much as it did at the 150bar. Even a 5 bar increase with creep can cause the slow shot at that high of a reg setting. You're on the edge of the limit without additional hammer weight/spring tension.. When you shoot faster you don't have time for the additional pressure to get in to the plenum but as it sits it continues to rise. Rebuilding/replacing the reg should fix that but also lowering the 2nd reg pressure and tuning to the knee as described above could also fix the issue. There may still be a little creep of the plenum pressure but it wouldn't really be noticed because the hammer can over come a slightly higher pressure as long as it isn't towards the max of the tune settings.

 

[scottg0814]

Just bear in mind if partial valve lock is indeed the source of (or at least a significant contributor to) the wide extreme spread, dialing back the power wheel to the same 850-900fps range will put you right back where you started. There is only one combination of HST and regulator pressure that will yield the very best consistency so if you plan to do a lot of shooting with the 25.4gr, you may want to consider changing the regulator pressure to something more suitable for that particular pellet.

OK, the pieces are starting to come together. After taking in the advice and comments here, I've come up with some thoughts and the advice you and others have given are confirming those thoughts so I'm getting a little closer to understanding the platform.
I'm gonna try a Maverick Sniper tune from AEAC which he says is ideal for the 25.39 JSB. And I think I see how this tune is much more inline with what I'm hearing here in this thread.
Reg1-95, Reg2-70, PW-2 IHS-0. This yielded an average velocity of 883, ES 11, SD 2.18 and 44fpe for him.
I'll try this tune and see if I come up with similar results. Steve's (AEAC) tunes did not have the slug power kit so I'll have to see what affect that has on the tune.

 

[scottg0814]

It’s not all about the ihs. The power wheel will make big adjustments for you. Setup your chrony? And shoot a couple shots at each power setting. See if the power continues to rise all the way through the settings. If and only if it’s still rising at the highest power setting should you start playing with the ihs.
If you get to a point where the power levels off, or decreases before then, go one click down, and start increasing your ohs till you reach the knee. Once that velocity levels odd, back it off a quarter turn or so, and you’ve found balance. At this point, if your speed is too high for your liking, lower reg pressure, and start over.
It’s certainly time consuming, but once you find the harmonic sweet spot, you’ll be amazed at how well the gun shoots, how quiet it is, and you’ll probably pick up a few shots per fill too.

Thank you. Now that I have a working chrony I can start digging in. Appreciate you outlining this for me.

 

[JimD]

Sounds like Jason got you pointed in the right direction. I think he was one of the guys on GTA that helped me when my 25 caliber P35 velocity was low on the first shot after a rest. I increased the hammer spring to give max velocity for that reg setting. The velocity variation stopped. The resulting velocity was a little high for the H&N FTT it likes so I decreased the regulator 1/4 turn (no gauge on the P35). That gives me about 875 fps and a nice flat shot curve. It smacks the squirrels very nicely.

As a result of this experience I do not try and decrease velocity with the hammer spring, I turn the regulator down. Or I find a heavier pellet the gun likes.

 

[Emu]

Two topics:

A) 145 bar in the regulator is adequate for a 30 Cal. And105 - 115 bar for a 25 Cal. The higher preassure restrict the travel of the hammer to open the valve. So at higher preassure you need the hammer to hit strongerly to get the same fps that you can get with lower regulated preassure.

B) the plenum need to be refilled to the target preassure after each shot, so, according to the logic of my first point, if you shoot to quickly a second shot you do not give enough time for the re-filling of the plenum and will have less preassure. As you have tunned your regulator too high, then the second shot will have more fps than when on over preassure.

Suposse that you have the regulator in a theoretical preassure in which the rifle gets the highest possible fps without a regulator, Lets say 105 bar..... The plenum will need less time to refill after each shot and you will get a flater string of fps.

There's a common error in thinking that the higher the regulator is set the harder the rifle will shoot. That is not the case.

It is needed to understand that there are two forces in fight: the one of the preassure of the air in the plenum and the one of the hit of the hammer. The equilibrium among them will make you reach the optimal open of valve and release of air.

Best theoretical and practical way to get to know the maximun power you can get from your rifle is to remove the regulator, set the hit of the hammer at maximum and fill the rifle to lower preassure, Lets say 90 bar and measure the power, then increase to 95 and measure, then to 100 and measure again. You will get to a certain preassure in which the fps will start to decrease. That point will give you the maximum power of your rifle.

The regulator is useful to send to the valve always the same theoretical best preassure to work with the adequate hammer tensión in order that they can armonically open the valve in the best and constant way shot after shot. The higher the preassure in the cilinder or bottle will provide more refillings of the plenum at the optimal preassure.

 

[B_y_r_o_n]

If you are loosing fps if there is a delay in firing then you have either an air leak or a temperature change that is dropping the pressure. You choose.