Can I reduce the muzzle velocity on my Beeman QB78D?

[Wayne52]

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.

I bought one of the early Xisico Sentrys in .22 that came with the tiniest transfer port I've seen, it's .0625  I didn't want to drill it out and used one that I'd already drilled out to the size that I wanted.  I have made them from delrin too, it works great.

[Ribbonstone]

Not talking you out ot it....can be intimidating the first time you take one apart. 
Sooner or later (hopefully much later) something will leak and you'll need to take it apart to fix it.

Lot of vids to search,but most of them are on boosting power.....the take apart sections are still useful. You'd NOT need to get the valve out to lower power level,so could just skip all that.

Design is so interlocked (partly by that cock on closing feature) that pretty much spend your time removing stuff to get to the stuff you want to change,

The safety interlocks to the stock...have to get the safety out before the stock comes off.
HAve to remove the cross screw of the barrel band and get the barrel out before you can get to the front breech screw.
Have to get the breech off before you can get to the transfer port.
HAve to  pluck the cocking pins out of the striker assembly before you can slide it out the back to get to the striker spring.'

Likly will chew the o-rings on the barrel on the first take apart....smooth the roughness inside the breech that chew the o-rings,replace the chewed ones, silicon grease them, and it won't happen the next time.

[Earl]

Not talking you out ot it....can be intimidating the first time you take one apart. 
Sooner or later (hopefully much later) something will leak and you'll need to take it apart to fix it.

Lot of vids to search,but most of them are on boosting power.....the take apart sections are still useful. You'd NOT need to get the valve out to lower power level,so could just skip all that.

Design is so interlocked (partly by that cock on closing feature) that pretty much spend your time removing stuff to get to the stuff you want to change,

The safety interlocks to the stock...have to get the safety out before the stock comes off.
HAve to remove the cross screw of the barrel band and get the barrel out before you can get to the front breech screw.
Have to get the breech off before you can get to the transfer port.
HAve to  pluck the cocking pins out of the striker assembly before you can slide it out the back to get to the striker spring.'

Likly will chew the o-rings on the barrel on the first take apart....smooth the roughness inside the breech that chew the o-rings,replace the chewed ones, silicon grease them, and it won't happen the next time.

WOW!  That is more difficult than I thought.
I think I will just shoot it like it is for now because it works good and is accurate.
When something leaks or breaks and it has to be taken apart, then I will change the velocity.

[Ribbonstone]

Patience is a virtue often noticed by its lack here.....I agree with you.  ALMOST (other than speed?) does what you want.

We forget. Been messing with these things for years,likely we could pull one completely apartand put it back togetehr again in the dark blindfolded.

But we forget how it was the first time....and forget all the little things we did to get it to get apart/back together without damaging any of the o-rings...or how many times in the beginning we pulled one apart, got it back together, and scrwed something else up in the process.

[rsterne]

As others have said, there are two basic methods to reduce the velocity.... restrict the flow of CO2 or shorten the time the valve is open.... The latter is more efficient, and QB's don't have a replaceable transfer port you can just change to a smaller one anyways.... Shortening the hammer spring would be the best way to go, and if you go too far, then shim it back up, or replace it....

One other advantage to shortening the spring is that you will reduce the sensitivity of the gun to temperature change somewhat.... ie the velocity won't drop as far in cold weather....

Bob