Inaccurate at higher pressures ?

fishinwrench · Mar 30, 2019

Is it normal for a PCP to spray shots in a 3" circle until you get down into the sweet spot? And if this is happening should I switch to heavier pellets ?

From 3000psi down to 2800psi my P-rod groups lousy at 30 yards, but once pressure hits 2700 the groups tighten up to 1/2". I could understand the high/low shots....but the right/left flyers concern me.

sonny · Mar 30, 2019

What caliber and fps? I know smaller/lighter pellets get squirrely at higher speeds.

Jorgo · Mar 30, 2019

Same by me

Hatsan Bully cal 25 unregulated jsb 34gr 850 fps

If i pump it above 170 bar it start grouping bigger

I think it comes from vibrations or turbulences

Vetmx · Mar 30, 2019

Yes, that's why they call it a sweet spot. That's when you listen to what your chronograph is telling you. It will say filling above XXXXpsi is a waste of effort and pellets. If you want to shoot consistent from your max pressure, buy a regulator and go down that rabbit hole.

Eaglebeak · Mar 30, 2019

Your velocity will be higher at 2700 than 3000 so it can't be the speed that is messing up your accuracy. Probably more to do with balance and harmonics. Try a bit more HST and see what happens.

Vetmx · Mar 30, 2019

The gun I am currently messing with starts to shoot left when it comes off the regulator. Am I concerned about it? No. Tuning a gun to shoot below the regulators set point doesn't even make sense. Your gun and my Prod like to be pumped to 2,800. If you add enough hammer strike to make it like 3,000psi, you will likely lose the 3 shots you gained on top off the bottom. If you post how many shots you are getting with what grain pellet and at what speed, Prod owners will let you know if you are good or not.

Bullfrog · Mar 30, 2019

Of course it could be because the pellets are shooting too fast at a higher pressure. Too fast to be accurate. Depends on how your gun is tuned.

Sometimes a transfer port or the barrel port can be misaligned or the barrel not seated properly back, causing the seated pellet to sit over the transfer port when fired (a defective bolt prob can also fail to push the pellet past the tp). If that is happening, higher pressures will warp the pellet when shot (because the skirt is catching a sideways blast of air) but lower pressures may not. I had a .25 Mrod do this to me once.

Neomorphy · Mar 30, 2019

You need a chronograph to see what is happening. You will see that the pellet velocity varies with pressure and that it follows a curve. You can't easily tune or troubleshoot an air gun without a chronograph.

fishinwrench · Mar 30, 2019

The gun I am currently messing with starts to shoot left when it comes off the regulator. Am I concerned about it? No. Tuning a gun to shoot below the regulators set point doesn't even make sense. Your gun and my Prod like to be pumped to 2,800. If you add enough hammer strike to make it like 3,000psi, you will likely lose the 3 shots you gained on top off the bottom. If you post how many shots you are getting with what grain pellet and at what speed, Prod owners will let you know if you are good or not.

I am shooting CPHP 14.3's at about 715fps (calculated from my drop @ 40-50) and am only getting 2 magazines (16-18 shots) that are good (between 2750-1600).

I have a 42 hammer spring/.105 TP and the current tune is 6.5 turns HS/ 2 turns of throw.

I have one of those little infrared chrony's ordered but it won't be here for another week or two I guess.

Vetmx · Mar 30, 2019

We will all still be here when you get your chronograph and can give us real numbers from 3,000psi down to 1,600psi. If your velocity is truly over 700fps, you are a little hot to be getting 3 mags. Like I said before, you are just chasing your tail without a chrony.

davidsng · Mar 30, 2019

Sometimes, a regulated gun will shoot worse at high pressures. This is usually because of the barrel band attached to the air tube. The pressure at 250-300 bar changes the pressure put on the barrel.

New designs are avoiding barrel bands and using tension nuts instead.

AirNGasman · Mar 30, 2019

A man could easily drive himself to distraction trying to figure out all the foibles and idiosyncrasies of a PCP. Sometimes I'm not beyond writing-off some of them as, "Must be my karma", or "It's magic"... because I really don't have enough sanity to spare!

fishinwrench · Mar 30, 2019

We will all still be here when you get your chronograph and can give us real numbers from 3,000psi down to 1,600psi. If your velocity is truly over 700fps, you are a little hot to be getting 3 mags. Like I said before, you are just chasing your tail without a chrony.

I do realize that, but right now it is all that is on my mind.....until the White Bass start running up the creeks. And I am having fun so of course I wanna talk about it.

I can't buy the "unstable at higher velocity" theory because I have springer's that shoot 14.3's faster than that, and they hold a decent enough group.

Vetmx · Mar 31, 2019

It has little to do with velocity. It’s about the tune. That’s why they call it a tune. If you want to shoot from 3,000psi down, you need to change things. Once you do, you may see pellets doing whacky things at 3,200psi or below your sweet spot. Right now what you are experiencing before your guns sweet spot is probably a combination of too much pressure for your hammer spring, too much pressure closing the valve after the hammer strike and lack of balance with the transfer port. It’s almost like taking a real gun like a .223 and changing the powder charge. The bullet doesn’t just shoot higher or lower when you add or subtract powder. It goes left or right also because you changed the harmonics of the gun.

Bullfrog · Mar 31, 2019

I can't buy the "unstable at higher velocity" theory because I have springer's that shoot 14.3's faster than that, and they hold a decent enough group.

But those aren’t the same gun. Every barrel is different in how they perform with X projectile at Y velocity.

For example, my 2 Flexes can’t shoot .30 Polymags well past a muzzle velocity of 875fps or a set pressure higher than around 2250psi. Faster or higher pressure then that the Polymags start to tumble. But those two same guns can shoot .30 JSBs 45 gr well at any speed or set pressure up to 1000fps or a bit faster. And then my .308 Texan SS shoots .30 Polymags well at over 1200fps at any pressure.

So yes, particular barrel lengths, twist rates, internal designs, and cleanliness or last thereof, can dictate how well a particular pellet does at a particular speed and your second airgun shooting that pellet well has nothing to do with how well your first airgun performs with that pellet.

Big HOWEVER. Do you chrony your springers, or are you just going by their advertised speeds? Advertised speeds on springers are often inflated over real world results by 200fps or more.

MileHighAirGunner · Mar 31, 2019

I just got a Prod a few months ago also. It’s my first and only pcp so far. I haven’t really had a lot of time to play around with it. Im just starting to learn the ins and outs of tuning a PCP. But I have noticed that my gun groups much better when I’m in that 2700-2800 psi range and under as well. I don’t do so well the the CPHPs. The Ultra Mags so much better for me. JSB 14.3 and 15.89 also do good, probably better and the 21 grain H&N Baracuda also did very well. I’m limited to only 15 yards in my backyard however.

With the Ultra mags my chronograph told me I was averaging 665 FPS for 14 fpe. This is my best group from last weekend. I’m sure others can do much better at 15 yards.

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fishinwrench · Mar 31, 2019

I just got a Prod a few months ago also. It’s my first and only pcp so far. I haven’t really had a lot of time to play around with it. Im just starting to learn the ins and outs of tuning a PCP. But I have noticed that my gun groups much better when I’m in that 2700-2800 psi range and under as well. I don’t do so well the the CPHPs. The Ultra Mags so much better for me. JSB 14.3 and 15.89 also do good, probably better and the 21 grain H&N Baracuda also did very well. I’m limited to only 15 yards in my backyard however.

With the Ultra mags my chronograph told me I was averaging 665 FPS for 14 fpe. This is my best group from last weekend. I’m sure others can do much better at 15 yards.

Nice! ImI in the same learning stage that you are, as this is my first PCP also. I knew right away that I wanted more FPE because the stock gun, although very accurate, wouldn't blow through a soup can at 30 yards. She'll blow through one filled with water at 60 yards now

I just need to settle on a tune that I can live with. I wish my chrony would hurry up and get here. Once I decide on a tune I plan to disassemble and fix the spring and throw settings to STAY PUT for good.

fishinwrench · Mar 31, 2019

Well here's an update ..... 1 1/2 turn CCW on the throw brought everything into near perfection. Velocity appears to be unchanged and I now have 19 good clean accurate shots from 2750-1650

No more Right/Left craziness at higher or lower pressures. Now it is either BALLS ON or 2" LOW regardless of whether pressure is above 2750 or below 1650

brink · Apr 1, 2019

OK? So just what is a Prod? Hate to be a dummy but curious minds want to know?

RB

MileHighAirGunner · Apr 1, 2019

OK? So just what is a Prod? Hate to be a dummy but curious minds want to know?

RB

Prod = Benjamin Marauder Pistol.

Mrod = Benjamin Marauder Rifle with wood stock

Synrod = Benjamin Marauder Rifle with synthetic stock

I don’t particularly like the nicknames myself. It’s just easier then writing out Marauder Pistol. If you just said you had a Marauder you’re likely to get the follow up question Which one?

Inaccurate at higher pressures ?

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