Well I took the opportunity last night to install JM's full power kit w/piston and breech seal. Thank god I have a spring compressor. This thing was a pita to get up and running and there's no way I could have done it by myself without one. I wont go into details about how I did the tune. I just wanted to report on how well my 48 shoots after the install.
Before the install my 48 was a horrible twangy inconsistent mess. The spring that came out of it looked like a hockey stick. I mean it was canted into the same shape. The breech seal was torn badly throughout. It looked as though someone had used a dental pick to either try to get it out or install it. The piston seal was in good shape so I saved it. But I did check the velocity before I installed the kit and tried to get an average. Forget it. The velocity spread from shot to shot was anywhere between 30 and 60fps! This was for every shot. The best I can say is that it was shooting in the 800's.
With the JM full kit installed (both breech and piston seal also) I'm getting a consistent 896 fps. the extreme spread was 22 fps and the standard deviation was 12 fps. My 48's shot cycle is very quick now with no spring noise or twang. Its got a fair amount of recoil but it happens so fast that it's managed easily. So far I've only shot it at 10m and I'm getting ragged one hole groups.
This is going to be my long range (out to 60yds) tree rat, crow and woodchuck blaster. I believe with the higher power (my english made Xocet with JM kit is shooting at 12fpe) I now have a good solid gun for that job. Although the extra energy shows itself (through recoil) when compared to my Xocet the flatter trajectory and quicker shot cycle offset what could be perceived as a disadvantage.
All in all I recommend the Full power 48 JM kit to anyone looking for a really good upgrade for their 48/52. But be warned its not going to be easy to install this kit without a spring compressor. I would highly recommend either making one or purchasing one.
I'd like to thank everyone who responded to me about my 48's trigger. I'd also like to thank all the guys that posted pics of the tear down and tune of the 48/52. These were invaluable in helping me with my tune and I probably couldn't have done it without this information/help.
Making the world a safer place...One red squirrel at a time!
Best Regards,
You will find that it will settle over time. Do check the performance of a number of pellets in your gun. Guns are female, each is a law unto its own. They all like different things.
After a tin of pellets, you should be getting extreme spreads of about 10 fps. or less and energies in the 20 ft-lbs region (I assume your gun is a 4.5 mm's/0.177" cal. If it is not, then it is running too hot).
Just keep shooting that beauty and she will bring a LOT of smiles to your face.