Air Gun Home :: View topic - QB78-AR2078 Power Adjuster/effects on accuracy

SMP · Silver Status Member

I was forced to make a power adjuster up because all of my attempts with transfer ports left me 25 fps above or below where I ideally wanted to be.

My power adjuster is much more rudimentary than Al's work of art

The parts on the bottom replace the solid spring tensioner on the top. This allows the spring guide to adjusted in or out just under 1/2" Via a tiny hole drilled in the end plug at a small angle that just misses the breach retaining screw running through the middle of the plug.

Using an 0.059" transfer port gave me a stock velocity of 548fps. With the adjuster installed I was able to adjust the power from 446fps to 565fps. I tried other transfer port sizes and the adjustment range was about the same except there was a slightly longer range of adjustment with the larger ports.

However it was what I basically stumbled onto next that has me completely bewildered.
Originally I had found that CP field targets gave very good ragged hole groups between 515 and 540fps. Meisters and hobbies would not group better than a nickel and JSB somewhere in between

After I installed the power adjuster I tried a group of CPFT with the adjuster all the way out (mean 455fps) and it was awful an quarter sized group. Just for kicks I tried Meisters and discovered they gave a true one hole group. I shot two more groups just to be sure. After that I tried Hobbies and JSB's and they were not as good but better than CPFT.

I played around a bunch more adjusting the power up a bit at a time and shooting a group of everything I have. What I found roughly is that every 40fps +- there is a harmonic or a "sweet spot" where a different pellet becomes the real winner in group size.

It would obviously take weeks to properly test and record all this data but I thought I would post this interesting bit of info. Currently my head is spinning from looking at targets with pellet info/velocity and group sizes scribbled all over them

rsterne · Moderator

Welcome to the wonderful world of barrel harmonics.... When you get finished deciding which pellet is best at what particular velocity.... Then add a muzzle weight of an ounce or so and you get to start all over again.... Rolling Eyes

Bob

SMP · Silver Status Member

There are words for ppl like you Bob

Laughing

oddtodd · Silver Status Member

Mad Airgun Scientist? Twisted Evil

rsterne · Moderator

It's actually a very handy thing to know.... If you have a pellet that is deadly accurate at one velocity and not so good at another.... you can often "tune" your barrel for that specific pellet and velocity with muzzle weight....

A skinny Crosman barrel is affected by changes of only 5 GRAMS....

Bob

Alstone · Moderator & Site Supporter

I like the adjuster Sean just one bit which I’m not sure what it does? ( See Photo)

I’m not sure what you are running Co2 or Air? But I know that with Co2 I tend to use the power adjuster to get the best performance depending on the outside temperature, the only problems I have with accuracy is if the Co2 pressure is to low, and this I can usually get around by turning the power up.

Al

rsterne · Moderator

Leftovers from lunch?.... Mr. Green

Bob

SMP · Silver Status Member

Haha silly me

Thats my secret to getting any accuracy at all. That is one of two shims I use to take the side wiggle out of the barrel/breach assembly. I put them in on either side of the loading ramp.

They are going to be replaced by a pair of set screws once I convince someone to mill a breach for me.

Edit: Al, I'm running air, which is what started this whole exercise. I had to convert because it is already starting to be to cold here for CO2. At 1 pm it is 10C/50F