Here is a FWB from later '60's that looks new condition wise, almost unfired, but the breech/piston seals are shot and lots of spring twang. Do I need to replace the spring as well as the seals? 
Tuning Do springs age out?
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I would not replace the spring. There's no load on the spring other than preload which isn't much. That would last longer than you'll live. I have lightly used BSA prewars twice as old and the springs are still fine. One runs almost 12 ft lb.
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I just replaced the breech and buffer seal on this newly acquired FWB 150. 1968 model. The gun, spring and piston ring are like new. A lot of new springs these days seam of lesser quality. Why chance it. I’m not sure why you have spring twang. Won’t know till you get inside it. Congratulations, you have a beauty. 
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Technically yes they can, on a brand of your quality I wouldn't worry about it at all. Maybe a cheapo Walmart springer I'd worry, and my nitro piston leaked down
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I think they lose a little bit of power over time. I had a Daisy 111 I sent to Dennis Baker to reseal and put in a new spring. He just did the reseal, and he got about 210 fps. I asked him to put in a new spring, and he got 249 fps. Granted, the Daisy 111 was about 60 years old, so I'm not sure if this is applicable to newer, more powerful springers. I don't think my 40-year-old FWB 124 had any significant velocity decline before I did some maintenance on it.
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I've got a 30 year old daisy (gamo contracted) that still cronos with the factory spec. I can't remember the model number and it's at work, but it's had thousands of cycles and is still going strong in spec. If it's the theory of spring memory, that's basically a non issue too if it's not a super budget gun ime. I've had powder burner mags loaded for decades and they haven't suffered memory either (I do annual checks)
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but you have to understand the rifle first off
a 150 or 300 have a breech seal if it is blue it is just fine if the piston buffer is blue also fine
seals over time
white or cream-blue-green-red
both have steel piston rings and will last as they state 100,000 shots
the springs plural are short and in the 300 but the 150 did come with one as shown at Feinwerkbau
https://www.feinwerkbau.de/en/service/downloads/technical-information/exploded-drawings
a 150 or 300 have a breech seal if it is blue it is just fine if the piston buffer is blue also fine
seals over time
white or cream-blue-green-red
both have steel piston rings and will last as they state 100,000 shots
the springs plural are short and in the 300 but the 150 did come with one as shown at Feinwerkbau
https://www.feinwerkbau.de/en/service/downloads/technical-information/exploded-drawings
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Beautiful gun. I got two of those earlier this year. One had documentation where it had been resealed and new spring a couple of years back. Visible seals are of proper blue color. The other needed resealing so I sent it out to a reputable airgunsmith and he did the seals and said it needed a spring. Kind of hard to argue from 500 miles away.
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Most gunsmiths will want to replace all internals. Not being certain of past use or abuse, they have their reputation to protect. If you learn to do it yourself then you can maintain it as you go. With the help and encouragement from other AGN members and videos out there, I found the task of replacing the seals quite straight forward.
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