QB78 FPS with Standard Rifle

[London177]

Hi everyone,
Has anyone tested the FPS in .22 cal with their un-modified QB78's? I'd appreciate any info' you have.
best wishes,
Alan

[ribbonstone]

Last three I've got numbers for (using 14.3gr. pellets):
515fps
545fps
578fps (QB79)

Have read on-line of others who had guns that shot as low as 425fps (obviously, had some problem or other) and as high as 600.

Mark it up to friction and that often mashed out transfer port seal.

[threex4]

My stock QBD  22 cal shot 485. unmodded. removed the barrel breech changed out the breech seal , fps went up to 545.
pic of the oem breech from my gun

[mikeiniowa]

Most of them get around the 500fps as stated in the factory specs. replace the transfer seal and it jumps to 550 on an average, open the probe inlet to the size of the hole in the reciever and the probe its self (approx. .170 inch) and you get 600fps..........

[London177]

Well I bought a used QB78 from a seller and he told me he had used an Archer's seal kit and opened up the transfer port a little. He did not have a chrono but said it was a little more powerful after the new seals and work. Ahem    I should have realised....
It can spit out a 20.5 grain .22 pellet 580 FPS. That's around 15 odd ft/lbs. Way over the UK legal limit. I had better bury it in the garden and forget about it!
regards,
Alan

[PeakChick]

It's Ok Alan, we won't tell........  

[ribbonstone]

Actually, you are in good shape.

Need to open the rifle up again (use the tune step-by-step on this site in the Library).  Will new REDUCE the strength of the hammer spring. Can grind off one coil (be careful not to get it too hot) or replace with a lighter spring (will have to hunt one up).  Ideally, the shortest spring that should work is one that just barely has contact with both bolt pins when you reassemble....not short enough to "rattle" once assembled but not long enough to demand any real compression to reassemble.

(And if you get it too short by a little bit, can shim it back up...and vary the strength by using different thicknesses of shims...shims are usually plain old washers.)

Chronograph again.  If you have no chronograpy or easy-access to one, then test penetration BEFORE, and then test after (use pellets from the same tin, not just the same brand, and somthing like a big block of clay).

I think you can get it down to 12 foot pounds this way...and end up with an easier to cock rifle in the bargain.  Had my Qb78 shooting in the low 700's with 14.3gr. (16 foot pounds) then detuned it (mostly the hammer spring talked about above).  Now a little over 12foot pounds (12.3) but is smooth and very easy to cycle.  Could drop it another 10% if needed.
-------
You could put in a more restrictive gas port seal...but I hate to do that.  By the time the gas reaches the gas port seal, it's released from the valve...so if restricted, it's "wasted"...get a low shot count and low vel.  By adjusting the hammer spring, will use less gas, higher shot count for the vel.

[mikeiniowa]

you could pull the barrel and fill the opened port with an epoxy glue (JB weld here in the states) and redrill the port to the standard 2mm or 2.5 mm, I've done it and it holds....

[London177]

Thank you Stephanie. This message will self destruct in five seconds....

[London177]

Thanks for the info' Rib. I'll try that as I think I can manage the work. I left the QB in a warm cupboard yesterday for a few hours and fired it through the chrono. It was putting out over 16ft/lbs ! Who'd have thought a QB78 could do that?
I'll work on it within the next few days.
regards,
Alan

[London177]

Hi Mike,
Thanks for that too. Good to have more options and info' from the tuning wizards

[mikeiniowa]

I use a wire size drill bit with some cello tape wrapped around it them stuck into the loading end of the barrel when redoing the port, get the bit and tape as snug as possible then put the epoxy into the port and let it harden, pull the bit out of the barrel and the tape should come with, keeps the work on the inside to a minimum, can be a bugger getting it smooth inside there....

[Stephen Archer]

I've tested many hundreds of "factory standard" QB78s for Archer Airguns "Gold Service" testing, here is a summary of the results for the .22 models:

All figures are corrected to 65 degrees F.

The lowest muzzle velocity (other than obviously defective guns) is about 480fps.
Maximum muzzle velocity about 540fps - attained by very, very few guns in factory trim.
Average (mean) muzzle velocity is in the range between 500 and 510 fps.

The mean muzzle velocity agrees well with the factory specification of 500fps.

Muzzle velocity of QB78s increases (or falls) by about 2 fps per degree Farenheit. So, for example, that 500/510fps at 65 degrees F becomes 540/550fps at 85 degrees F.

All this testing was undertaken with wadcutter pellets. pointed pellets may give slightly higher muzzle velocities.

I hope this helps.

Stephen Archer
Archer Airguns Inc.
http://www.archerairguns.com

[lillysdad621]

i got a 78 non deluxe and it shoots the rws hobbies (11.9 gr) at 540 avg. I have seen some shots in the 548 fps level and my lowest was 536. also get close to 65 shots at that power.

[geewhiz380]

if mike tunes them on the .22 you get 700 plus i have two qb78d.22 and they both chronied to reach those fps and they r .they r going through wood 2inch thick maybe not cherry wood but its wood . i use super-h-point and they attack ,i have group well at 37 yds with those qbs accurate at anything you shoot they r fixed barrels anyway.im gonna buy the reapters this week from mr. stephen archer ......jorge wow this is 2008 thread i feel dumb sorry for late entry.......