GTA
PCP - HPA - C02 Gate => QB-7X Gate -W/Co2 & HPA => : IJL June 14, 2006, 03:27:33 AM
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Hi Charlie,
I just wanted to drop you a line to say thanks for the tips on tuning the QB78, I'm SO impressed!
As my QB is possibly a little unusual you may be interested in the whole story? Well here goes:-
I bought the gun new in autumn 2005 as a companion for my girlfriends BSA Superstar Mk1.
Out of the box it was accurate and just powerful enough at a little under 10 fpe (This is England, where the Powers frown on air rifles that exceed 12 fpe, unless you have a Firearms Certificate).
As a provider of dinner (Derbyshire rabbits are soooooooooooooo tasty) the QB was doing well but was eating CO2 at an alarming rate. Then a friend offered the services of his acquaintance who for £70, was offering a PCP upgrade. I jumped at it and eagerly awaited the guns return. Imagine my disappointment when it came back with a maximum power of 8.6 fpe, I was gutted! But I struggled on with it and got hungrier as the weeks passed. Messages to the "upgrader" to return the other 2 fpe believed lying on his workshop floor were eventually responded to with a "Get it tuned"
Ok so here we have a £70 rifle with £70 conversion +scope, moderator and all the usual "bits"and now this bloke was suggesting I spend more on it.............
So I did some research and found your article on tuning the QB. Now I have only my garage as a workshop which doesn't even have electrics, therefore armed with a cordless drill, a new set of needle files, and your article all printed out I made a start.
On stripping the gun I found a restrictor screwed into the replacement valve, just below the breach seal, a 1.7 ish mm hole cannot pass much gas I thought, so that was removed! I followed your instructions to relieve, open and polish the rest of the parts but left the valve assembly alone. the other ports and bolt were all "adjusted" and given a polish with new, lighter spring for the trigger. Re assembled and on the chrono ................. 5.6 fpe! That's getting worse, not better! last chance, a 3/8 shim behind the hammer spring, I could not find a .25 at the time,
TOTAL TRANSFORMATION!
974 fps with a 15 gn pellet!! WOW! now we're cooking! another test with a 29.5 gn bunny basher gave over 38fpe! Now this is goooooooooooooooooooooood!
Now a week later, after many hours of tinkering and destroying breach seals I have a PCP rifle that can deliver a regular and reliable 23.88 fpe and group to under .5 inch at 20 yards.
It loves Air Arms Field 15.7 gn pellets where most others will give a 2"+ group
The restrictor was replaced with a larger bore version (approx 3mm)
There is still a shim on the hammer spring but only a little under 1/8"
I'm getting 15-20 powerful shots from a 150 bar charge.
That's good enough for me but how it needs that silencer!
On the subject of breach seals, the best thing I have found so far is a piece of outer sheath from a mains lead! Yes I know! but necessity breeds invention and it is the longest lasting seal yet!
And so to dinner, well the complacency that I had managed to instill into the local rabbit population has worked a treat! 60yard+ knockdown, usually first time kill. Any variance to that I'll accept as my shortcoming.
Again, many many thanks for the excellent article, it has turned a ruined rifle into what has become affectionately known as my QB Fireball.
Now where did I put that Firearms Certificate application form..............................................
Sincere regards
IJL