Canadian Airgun Forum

I hope it comes up to 21fpe in the spring. After a few hundred shots the piston seal looks pretty well broken in. For the winter it will only see basement duty, killing cans and what not with the small spring. It's remarkably accurate with the iron sights, well accurate for me (2.5cm@ 25m). Overall very happy. Thanks everyone for your help!

So the gun started losing power over last week. It was down to 14.5fpe. I was worried something broke so I opened it up and everything looked good. I decided to check the piston seal which came on the new piston. It was marked as 28.1mm. Pulled the seal off of the original piston 28.6mm. The current seal has no burnishing on the OD. I was worried it wasn't sealing well so I swapped it for the 28.6 seal. It immediately came up to 21.5fpe, Yay! A week later and it's down to 14.5 again.

What is going on? I know the temps have gotten lower here going from 15 when I got the thing to about 5 now but that shouldn't account for that much loss. I even put a 5.4gr pellet through it last night ( for.my B18 I don't ussually run light rounds in the magnum). Subsonic 100%. The day before it was putting 8.49gr H&N's downrange at 1100fps.

I'm so confused.

I bought a .177 Diana 350 pal rated rifle about 13 years ago. It is very powerful. I’ve actually detuned it down to about 14 foot pounds because I could never get it to group. I was all about max power back in those days. I’ve since learned more accuracy and a smoother shot cycle are way more desirable in a spring gun. I also got a .22 barrel for it and now it groups very nicely. It is still a harsh gun though when I compare it to others I now have.
I’ll probably never get rid of it but I’ve since gotten much better spring guns that I use way more often. It is fun to try all sorts of different springs and lubes etc. on it to see what sort of difference it makes each time. Have fun with it!

Your gun might be dieseling and giving more power and then settling down after a few shots. Especially if you take it apart and reassemble. It is common to get a few powerful shots at first until it burns off the little bit of grease or lube that might have gotten in front of the piston. They will usually also shoot better once they settle down as well.

first off as mentioned you might have some dieseling going on...
do you smell any burnt oil after shooting, flowed by some smoke in the barrel ?
also your running way to light of a pellet for a magnum not a good idea
not sure why everyone is so set on speed fps...especially in .177

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"A candle loses nothing by lighting another candle"

Mine in .177 does really well with the Jsb 13gr Pellets. Still cranks them out at around 900 fps. Light stuff does around 1200 fps. Fun to go supersonic but yeah it's not good for much.