Can I reduce the muzzle velocity on my Beeman QB78D?

[Earl]

My Beeman QB78D shoots JSB 8.44 gr pellets at 578 fps = 6.3 ft lbs.
Is there an adjustment to make it shoot about 500 fps?
I would like to get more shots per CO2 cartridge.
I only shoot targets at 12 to 29 yards so I do not need the added velocity.

[nervoustrigger]

You could try simply shortening the hammer spring a half coil at a time.

Or install a restricted transfer port.

These are the two easiest options.

The third and better approach is to do both.  If you have never made any changes to the hammer spring, it’s probably wasting CO2 so there would be an immediate benefit to shot count.  Shorten it until the velocity just begins to taper off to about 550fps.  Then if you want to reduce it another 50fps or so, install a smaller transfer port.

The reason for adjusting the hammer spring in the manner I described is for the sake of shot-to-shot consistency and less temperature dependence.

[Ribbonstone]

Sounds simple....QB's have no external adjustments (other than the trigger)...so are going to have to take it apart most of the way.

Are a lot of QB power up/take down vids.....just ignore the power up advice and look at the How to get it apart parts of the videos.

A lot of the power up process works well in reverse.  Stronger striker spring...translates into a weaker one when looking for less power.  Can't go so weak that the striker hit it inconsisenat,but can lighten that at least a bit.


Wider gas transfer port  for power...leave it alone if less power is the goal.

So it SOUNDS simple...take it apart,clip ONE coil from the striker spring...then put it all back together right and try it.  Clip annother 1/2 coil off if that still seems too fast/too few shots.

Go too far, and the groups get vertical (and the chronograph readings look like a saw-tooth graph) and you'd have to shim the spring back up.



Whole lot of more complicated things you could do....but at least the above way leaves you one new striker spring away from un-doing what you did.

[Frank in Fairfield]

Shoot 8.64 grain pellets.
Slower?
10 grain pellets.

[Fate]

Shoot 8.64 grain pellets.
Slower?
10 grain pellets.

That would slow it down... but wouldn't the shot count still suffer?

Don't have a CO2 gun but that's what I find on my PCP... same shot count with different speeds for pellets of different weights on the same settings.

[Fate]

double post

[nervoustrigger]

Yes for the most part if you don’t reduce the hammer strike (e.g. shorten the hammer spring), shot count stays the same.  There may be some improvement by restricting the transfer port, just not to the same extent. 

[Ribbonstone]

Like to see some numbers on that.

May not change efficiency (gas used vs. total energy out put) but even that is rare with co2.

So something like 10 foot pounds X 55 shots......vs 5 foot pounds X 110 shots.

[nervoustrigger]

Yes I agree with that as as a general guideline, I was just trying to simplify things for the current context.  Given we are discussing a modest detune and knowing the OEM configuration is oversprung, the biggest improvement in shot count will be obtained by getting it to close the valve while the pellet is still in the barrel rather than continuing to belch CO2 after Elvis has left the building.  That’s all.

[Earl]

You could try simply shortening the hammer spring a half coil at a time.

Or install a restricted transfer port.

These are the two easiest options.

The third and better approach is to do both.  If you have never made any changes to the hammer spring, it’s probably wasting CO2 so there would be an immediate benefit to shot count.  Shorten it until the velocity just begins to taper off to about 550fps.  Then if you want to reduce it another 50fps or so, install a smaller transfer port.

The reason for adjusting the hammer spring in the manner I described is for the sake of shot-to-shot consistency and less temperature dependence.

OK, it looks like I should shorten the hammer spring 1/2 or 1 turn?
578 down to 550 is not much of a reduction.
Where can I get another hammer spring if I don't like the results?

[Earl]

Sounds simple....QB's have no external adjustments (other than the trigger)...so are going to have to take it apart most of the way.

Are a lot of QB power up/take down vids.....just ignore the power up advice and look at the How to get it apart parts of the videos.

A lot of the power up process works well in reverse.  Stronger striker spring...translates into a weaker one when looking for less power.  Can't go so weak that the striker hit it inconsisenat,but can lighten that at least a bit.


Wider gas transfer port  for power...leave it alone if less power is the goal.

So it SOUNDS simple...take it apart,clip ONE coil from the striker spring...then put it all back together right and try it.  Clip annother 1/2 coil off if that still seems too fast/too few shots.

Go too far, and the groups get vertical (and the chronograph readings look like a saw-tooth graph) and you'd have to shim the spring back up.



Whole lot of more complicated things you could do....but at least the above way leaves you one new striker spring away from un-doing what you did.

Please give me a link to the "Are a lot of QB power up/take down vids....".

[nervoustrigger]

It’s been a while since I tuned a QB for CO2 but I think you’ll end up removing more than 1 turn to get the velocity to drop below the plateau.  If you overshoot a tad and remove too much length, it’s not a big deal...you can just put washers on the spring guide to bump it back up. 

[Ribbonstone]

As nervoustrigger (and myself) mentioned....the simple stuff doesn't really change the bqasic ratio of energy in vs energy out.

(OK...you have to take it apart,so call it semi-simple.)

Lets say you have 370- 400 TOTAL foot pounds (not a real efficienty co2 tune) for 2X12gr.

Could have that as 25 good 15 footpound shots....40 good 10 foot pound shots....or 60 good 6.5 foot pound shots.

Generally,heavier weright pellets are slower and usually earn a few more good shots per co2 fill...also tend to generate more energy although they are slower.

Considering the simple stuff being the striker spring tension (whacking the valve stem ligthter) and the transfer port (stagngling off the gas flow)....are pretty well limited to the same energy out...could be it many small  nibbles of the cookie or a fewgood big chomps.

A harder conversion would include a good bit of other work (like striker weight, antibounce,striker spring tension,valve changes, transfer port changes,etc.)...could make that up to 600 foot pounds of energy for 2 X12gr.

BUT those things limit the upper power range more than they limit the lower power range....and they get all picky-ticky about how they are done and combined.

So....tell me what you want in terms of shot count/energy,and I'll mention ways to get there (if possible).

[Nvreloader]

Guys

What about a SSG?, I don't know if it will work for Co2, I know it does for HPA,
plus you could tune it to fit adding more horse power if needed?

Don

[Ribbonstone]

Give it a try....hasn'tworked for me (in the total energy in vs. total energy out)...but it's worth a try.

[Earl]

OK, I just want to take 1 turn off the hammer spring at first.
I need instructions to get the hammer spring out.
It looks like:
Take the stock off. I know how to do that.
Then remove the rear trigger screw.
Then remove the end cap.
Then remove the hammer spring.
Is that correct?
Is there a video of someone just removing the hammer spring?

[Ribbonstone]

No....going to have to go all of that....but also  take the barrel off and the receiver off as well.

[Earl]

No....going to have to go all of that....but also  take the barrel off and the receiver off as well.

WOW!  That is more work than I thought.
I changed the hammer springs on my Maximus and Discovery and when I removed the end cap, the hammer spring slid right out.

[Wayne52]

Restricting the transfer port would be my first choice then adjust the hammer accordingly.

[happymecanic]

Hi Earl, on my Canadian-detuned QB78 .177, the hammer spring is the same as the ''full-power'' ones, but the exhaust port in the valve is TINY, something like 2 mm, about 0.078''. Haven't shot it in a while, but IIRC I was getting in the neighborhood of 80 good shots @ 450-460 fps with 7.9 grains pellets.