FPS and voodoo sorcery

[CacklingHawk]

So I bought an M16 from Hong Kong, and my first skirmish it was 350fps bang on the dot with a 0.2g bb. The gun was great! Fast forward two weeks of unuse, and the next skirmish it was shooting at 366fps. Well, no biggie, atmospheric conditions can change readings and such. So I used a rental for the day, fired the gun on auto, made sure that the spring in the gearbox was compressed, and left it for four days. Today I chrono'd it again...and it was up to 400fps.

What the eff?

I thought spring compression was supposed to drop the fps, at the very least not change it at all. Why would it INCREASE?

On a tangent--another gun of mine, bought second hand, is shooting at 270fps. To troubleshoot something like that, in what order should I check the parts?

Thanks.

[a_flea]

Re: FPS and voodoo sorcery

For the low fps one, take it apart and check compression, thumb and forefinger over the two holes on the hop unit and blow down the barrel, see it it leaks... Then blow backwards through the nozzle and listen for air leaks (nozzle leak will sound much louder than a leak within the gearbox). If theres a leak within the gearbox you can take it apart and test the individual parts... If all is fine, it may just be underpowered so consider a new spring. (BTW silicone oil is supposed to be non-toxic, but to be super safe give it all a clean before testing compression with your mouth...)

As for the new one, might just be bedding in! What brand is it? You might find you have to downgrade it...

[Fizzy]

Re: FPS and voodoo sorcery

Sometimes you can find FPS increases one a gun has bedded in, rather that the FPS dropping. It could be it was heavily greased, and needed a few mags through it to clear and bed in better.

As for the 270fps gun, if it's an old standard TM it could be how it should be.