FPS climbs when falling off the reg?

[UCChris]

Jason, as usual, can put it more clearly than I can. I just wanted to say one thing, which Jason did touch on. But restricting the transfer port is not the ideal way of adjusting the rifle. It won't change how much air is used per shot, on restricts how quickly it can flow into the barrel. Again, Jason touched on it and mentioned that it was fine for small adjustments and not cutting a rifle's power in half, but I just wanted to reiterate.

[beachgunner]

Interesting thread. I'm rather new to PCP's and this is very helpful information.

There is a difference, or maybe I should say a preference, when finding that optimal velocity for accuracy. Bob, you seem to want to be in the 950 range, while Dillon mentioned the 860 range. What determines this? Is it caliber dependent? Pellet weight? Pellet shape?

I have an unregulated rifle (Regal XL in .22), and I am in search of the optimal velocity for accuracy at the distances I'm shooting. I can adjust the hammer spring pre-load, and other than picking various starting fill pressures, that's it for adjusting velocity (of course using pellets of differing weights is also an option).

How do you view maximizing efficiency when dealing with unregulated guns? Is there a way to increase shot count??

Thanks,

[UCChris]

It largely has to do with accuracy, Michael. 860 fps happens to be the sweet spot with JSB 25's in a lot of guns. Bob probably used 950 because it's about the upper limit of diabolo stability. As the pellet enters the transonic zone, it will quickly destabilize. Generally 900-950 fps is the limit, though testing your pellets in your gun is the only way to know exactly where the pellets will destabilize.

[nervoustrigger]

There's no hard and fast rule but in general, the best accuracy at distances >50 yards will be obtained with a domed pellet leaving the muzzle somewhere in the 800-950fps range, with the edge going to heavy pellets in larger calibers.  This happens to be a range where the ballistic coefficient is best (again, speaking of domes) but it would be overly simplistic to boil it down to just one characteristic.

The optimal velocity will be dependent on several factors, the most significant of which are probably the pellet and barrel harmonics.  Ultimately just have to try different velocities if you are trying to wring out very best from your rifle.

[beachgunner]

Thanks Christopher. I'm shooting JSB 15.89's at around 900 fps over a 40-shot string. Accuracy could improve for the distances I'm shooting. The previous owner had it set up for 100 yard bench work, but I'm generally shooting inside 45 yards for pest control and hunting. Not saying accuracy is bad, but I think this rifle can do better. I'll have to lighten the hammer spring pre-load and see what I get over the chrony.

[beachgunner]

Thanks Jason. Yes, I've got some testing to do at varying velocities. Better order some more pellets 

[Dairyboy]

Yeah for me 860fps just seems to be the sweet spot where I like my guns to shoot. Good power and velocity but still normally very efficient and quiet. When I tune my rifles this is just the pellets in there respective calibers that I like the 860fps range.

.177 10.34gr
.20 13.73gr
.22 18.13gr
.25 25.39gr
.30 44.75gr

However the AA 16gr I prefer at 900fps. Probably because I don't need to change my tune between the 16 and 18gr .22s 

No real rhyme or reason and not saying that's the best speeds but that just seems to be where I've settled.

[beachgunner]

Thanks Dillon. I appreciate the information. Sometimes rhyme and reason is just experience over time.

[rsterne]

The least wind drift with round nosed pellets occurs at a muzzle velocity of about 850 fps.... IMO the only reason to shoot much less than that is to increase the efficiency.... ie reduce the amount of air used per FPE of energy produced.... Being a hunter, I am interested in the most energy downrange without picking up too much extra wind drift, so that is why I push towards the 950 fps range.... I get a flatter trajectory and more energy downrange, without a lot of increase in drift.... As long as the pellet I am using is accurate at that velocity, that is my target velocity range.... Some pellets when pushed past about 900 fps tend to spiral, particularly if the twist rate is too high.... I think the MRod (GM barrel, 15.5" twist) with the 34.2 gr. JSBs suffers from this if you push it too fast, and a lot of guys stay just under 900 for that reason... On the other hand, my Hatsan, with a 17.7" twist, has no tendency to spiral with the 34.2 gr. even at 1000 fps, and the way I have the gun tuned, the best part of my string is 950-965 fps, which gives me 70 FPE at the muzzle and over 40 FPE at 100 yards.... with groups at that range between 1.0 - 1.5".... I am willing to put up with less efficiency and a lower shot count to get that kind of long range smackdown.... I get 18 shots within a 4% ES (200 bar fill), and the best 9 shots (1 magazine) are within a 1.6% ES, and that is unregulated, with a 190 bar fill....

Bob

[UCChris]

Don't mean to hijack, but why do pellets have the least wind drift at 850 fps, Bob?

[Tweeter]

Don't mean to hijack, but why do pellets have the least wind drift at 850 fps, Bob?

I think it has to do with BC.  I remember Brian (EdgunWest) showing in one of his videos that in his .22 matador the Jsb 18.1 had a better BC when slowed down and that it beat the .25 cal in his test for BC.  It varies from gun to gun I believe.

The BC test starts at about 12:45....

[toku58]

I think your air volume between the reg and the valve is too small for what the reg is set at. You can either increase the volume by adding spacers between the reg and the valve to increase it's size, or turn down the regulator to maybe around 120 bar to keep your 880ish FPS.

[rsterne]

As you approach Mach 1, the air resistance goes up by at least a factor of three.... This means that the pellet slows very quickly from 1000 fps to 950, a bit less quickly from 950 to 900, less again from 900 to 850, and since the wind drift is proportional to the difference in time of flight in a vacuum and in the real world, the least wind drift occurs below 850 fps.... depending on the pellet the minimum drift may occur as low as 500-600 fps for a wadcutter.... Most good round-nosed pellets, at 50 yards, have the least drift when they start at about 850 fps.... Here is an example....



A similar graph for 100 yards would be shifted slightly towards higher velocities, because it is the average velocity that matters.... Notice that at 950 fps the drift hasn't increased much (about 6%), and at 100 yards the percentage increase above optimum would be even less.... which is why I feel OK about using velocities in the mid 900s providing they are accurate....

Bob