Adjustable vs non-adjustable regulated pcp

[solder]

What are the advantages and disadvantages of adjustable, regulated pcp guns. What is the best value in an adjustable rifle?

[nervoustrigger]

Just to clarify, you mean the regulator's setpoint is adjustable, right?

Otherwise, adjustable can mean many different things.

[solder]

Nope. I mean adjustable power at the regulator set point.

[LDP]

They are all adjustable. Some just easier than others.

[Gerard]

I hadn't heard of a non-adjustable regulator. Perhaps you are asking about externally adjustable versus usual sort which requires the gun being dismantled to adjust the regulator? If that is the question then I would think the advantage is quite obvious; dialing in the tuning with an externally adjustable regulator setpoint would be a lot simpler than opening up the gun to add or remove spring elements in the regulator.

[nervoustrigger]

Nope. I mean adjustable power at the regulator set point.

So adjustable power without changing the regulator setpoint?  The most common way of doing that is by adjusting the hammer spring preload.  Changing the energy level can allow you to trade off lower velocity  for a higher shot count.  That can be advantageous if you want to downtune a fairly hot PCP to plink at modest ranges of, say, 25-30 yards.  Taking a 30fpe .22 cal down to 15fpe should yield more than twice as many shots on a fill.  And because the lift and dwell will be much reduced, it should also be much, much quieter. 

[rsterne]

In a nutshell, the advantage to an adjustable PCP is that you can use the gun for more than one purpose.... The disadvantage is that when you change the tune you will almost always have to resight it for the new configuration.... The only exception is if your low power setup is for plinking at close range at relatively large targets for what the gun is capable of.... I have never seen a PCP that didn't change POI with a major change in power.... I prefer to find the optimum tune for a given gun and leave it that way.... If I have another purpose in mind, I build another gun specialized to that job.... Then when I have a specific task to do, I grab the appropriate gun for the job....

Bob

[anti-squirrel]

What are the advantages and disadvantages of adjustable, regulated pcp guns. What is the best value in an adjustable rifle?

Here's an easy way to break it down:
Adjustable and Regulated means you can adjust power for the need of the moment.  Regulated means a steady output of pressure for consistency (and thus accuracy)

If you include pistols in your "best value", the SPA PP700W pistol by far wins this.  Available for $225 delivered to your door.  The QB-79 on a Ninja bottle is probably a close second, but requires a little more work.  There are plenty of other guns also meeting these- but if you say "best value", my idea of best value is "cheapest affording good accuracy"

The regulator, hammer spring tension,  and trigger are all very adjustable on this pistol.   Theb trigger and regulator can be tuned down for giving a long shot string (and with an air reservoir under 70 CC this is important) that is quiet and consistent... Or it can be tuned up for a more-powerful and shorter shot string, more "bark", and squirrel-head-drilling potential for pesting.

Bob is dead on the money regarding the POI shift, though.  And- if you are looking for the most compelling reason to go with an adjustable power level regulated gun, it offers you two of the primary components required for optimal tuning on your airgun right from the get go. 

[solder]

nervoustrigger,rsterne, and anti-squirrel all pretty much nailed it. Does the POI change horizontally and vertically, or just vertically when power is turned down externally(no disassembly to adjust the regulator) ?  The QB79 keeps coming up, but so does the BSA Buccaneer and Gamo Coyote. Just a rifle, no pistol. The QB seems to be a favorite for tuners, but BSA/Gamo barrels seem better and maybe quieter. Mostly backyard friendly shooting up to 45 yds.

[Taso1000]

You may be able to minimize the change of point of impact drop if you use heavier pellets at full power and lighter pellets at low power.  Sight in for the heavy pellets though.

It won't be exact and you'll have to actually test it but it should be close, I think.

This wasn't my idea.  I read about Sumatra owners doing this in combination with the power wheel as they shot their rifles down from max fill.

But, I do agree with Bob.  Different rifles for different tasks.   

Taso

[anti-squirrel]

nervoustrigger,rsterne, and anti-squirrel all pretty much nailed it. Does the POI change horizontally and vertically, or just vertically when power is turned down externally(no disassembly to adjust the regulator) ?  The QB79 keeps coming up, but so does the BSA Buccaneer and Gamo Coyote. Just a rifle, no pistol. The QB seems to be a favorite for tuners, but BSA/Gamo barrels seem better and maybe quieter. Mostly backyard friendly shooting up to 45 yds.

In theory, the POI should change vertically only, but I have found I have a slight shift horizontally on my pistol- and by slight I mean 1/4" shift at 20 yards.  At longer ranges it might be more, but at my pistol's power levels, I see it at my closer ranges.  This is using my externally-adjustable regulator, and... let's be honest, my POI change on my regulated pistol is probably within my personal margin for error. 

Those two rifles you mentioned are both great shooters from the reviews I've seen.  The Gamo Urban might be another possibility for you to look at. 

And, if you don't mind some shipping costs, the Cometa Orion and Orion bullpup are both regulated based on the descriptions for each at Aceros de Hispania.  I'm looking very very closely at that Orion 'pup as a potential next purchase... along with the Kral Puncher Breaker Evo, QB-79, SPA P-10/12 and ...

[terry mc]

My Air Arms with an added regulator would probably be a good example of what you are describing.
It uses an adjustable port size to restrict the airflow and the regulator that I have added gives a consistent 135 bar.

So the plus is I can shoot at a lower power to lower the noise level, if I so choose.
Very nice for backyard shooting, however if practicing for competition it does you little good as other factors are changed.
The other advantage is a greater shot count. I get 40 shots from my AA S510 between 230 bar and 135 bar so at minimum setting I would most better than 80 shots on a fill. If you have a small tank or a 3000 psi. recharge tank this count be a big deal.

The negative is that as everyone else has said you change your point of impact vs. point of aim.
Another negative is lower power means more wind sensitivity. BIG TIME.
Change in point of impact vs. poa. for me has mostly been verticle with a small sideways shift.
It depends on wind speed and direction.
This explains a lot......
A "phenomena that affect spinning projectiles in wind, also known as spindrift. This happens as the projectile spins around it's axis and the easiest way to think of it, is as either climbing up or climbing down depending on the direction of the incoming wind."
So my thoughts is that the greater the velocity of the pellet the greater the spin of the pellet so the greater the spin drift. Keep in mind this will happen in all axis including updrafts and downdrafts.

Since I am trying to prepare to compete in a local club, I for one have given up on lowering the power for quieter shooting because it changes to much for
shooting an event.

[Taso1000]

Terry,

I was not aware of spin drift.  Thank you, I will have to look it up.   

What I do know is that higher velocity means smaller time of flight.  The shorter the time of flight the less time external forces, (gravity, drag, wind, etc.) can affect the pellet trajectory.

Taso

[Gerard]

Bob Sterne wrote an excellent summary of wind drift as it impacts airgunners recently:
http://www.airgunforum.ca/forums/topic74106.html

[rsterne]

Yep, when it comes to velocity, more ain't necessarily better, in terms of wind drift.... Thanks for posting that link....

Bob

[solder]

My Sentry is VERY accurate at 850fps with .177 Crosman Ultra Magnums. Not so much at FPS above 950 or below 800fps. What would help besides regulating to 850fps? I don't really understand this phenomenon.

[terry mc]

Bob Sterne wrote an excellent summary of wind drift as it impacts airgunners recently:
http://www.airgunforum.ca/forums/topic74106.html

Thanks for posting up that link I did not see that summary I look forward to reading that thread this evening when I get home from work.
I enjoy learning the technical end of our hobby.

[nervoustrigger]

My Sentry is VERY accurate at 850fps with .177 Crosman Ultra Magnums. Not so much at FPS above 950 or below 800fps. What would help besides regulating to 850fps? I don't really understand this phenomenon.

Very likely has to do with barrel harmonics.  The barrel vibrates during the firing cycle, producing nodes and antinodes along its length.  See this illustration:



If the vibration happens to produce a node at the muzzle when the pellet reaches the end of the barrel, its flight path will be consistent.

Conversely, if the vibration happens to produce an antinode at the muzzle, the location of the muzzle is wagging, so to speak.  So when the pellet emerges, it is headed in a seemingly random direction each time.

Most anything that touches the barrel will influence these harmonics, sometimes for the better and sometimes for the worse.  A barrel band or a muzzle brake are typical examples, and some guys will experiment with a "harmonic tuning" device which is a weight placed on the barrel.  They move the device to different locations on the barrel and shoot groups to find the location which produces the smallest groups.   Sometimes this device is elastically coupled to the barrel (rubber), sometimes not.  The stiffer the barrel, the less of an effect it will have.  So in general, a thinner, longer barrel will be more susceptible to harmonic variations.

[Taso1000]

Just to add to Jason's post.  I have read that it is best to tune the barrel for the higher peak.  Due to the fact that there is more time spent at the peaks versus zipping through the middle areas.  And the barrel pointing up to help with pellet drop versus making drop worse in the lower peak.

Taso

[nervoustrigger]

That is true, the velocity of the vibrational movement near the zero crossing (node) is comparatively high.  However as a practical matter, it isn't feasible to perform a tune where one is actually measuring the vibrational nodes and anti-nodes.  Unless you just happen to have a laser interferometer setup in your workshop    (Now watch, somebody will step in and describe a McGyver solution with a rubber mallet and some baby powder).  For the most part, you just have to perturb things and shoot groups to see whether things improve.