Advice Please Gents.

My seals arrived from Archer, so put back together my little 78, no problems, but then when tested it in the garden, shots are all over the place, power seems to change from shot to shot (I have no chrono). Stripped, rechecked, rebuilt but to no joy - still the same problems.
The mods I have carried out are: Bolt probe mod (drilled out to increase gas flow) exhaust valve mod including drilling the washer, Piercer body mod - drilled 4 holes at angle. I also recrowned the barrel. I removed the MuzzleBrake to make sure pellets weren't catching. My next mods I was going to do was change the hammer spring and exhaust spring, might also turn it into a carbine just as I like carbines! I also would like to do breech mod where you enlarge the transfer hole(?not sure on the correct term - sorry)  but not sure what you use as a seal.
Any help advice/info would be great as I've a new shoot but no gun!
Many thanks Guys.
Joe  

[DanoInTx]

Hey Joe,

I am assuming you are following this tuning guide?
 http://www.gatewaytoairguns.com/library/Charlie’s%20QB-78%20Tune.htm

First thing to check would be the barrel crown.  I know you said you recrowned it, but go after it with a q-tip and make sure there are no burrs.  Next the transfer port seal you are talking about can be replaced with the tubing you can get from Home Depot for ice makers.  It's 0.25" O.D. x .170" I.D. poly tubing, I bought mine from Ace Hardware for about $0.10 a foot, buy a couple feet because you will undoubtedly cut a bunch before you get one that is the right thickness and square on both ends(use a razer blade to cut it as square as possible).  The other thing effecting your groups could be a leaking bolt/breach o-ring, make sure it's a good one, or just replace it for good measure.  You can also try to detect a leak in the breach area by putting a tissue paper over that area and then shoot, if the paper moves, you have a leak.  I would actually load and shoot a pellet for this test because the pellet will create back pressure and make the leak more apparent.  A leak will produce vertical stringing in your group/inconsistant shots.  Also, have you shot the gun with open sights or another scope to make sure the current scope is working right?  Try these things out and see what you come up with, may also try pushing a pellet through the barrel, from breach to crown to make sure you don't have some odd rough/tight spots, but it sounds like the barrel was ok prior to any mods.  Good luck!