fps gone wild!?

[phayonyx]

Took my Aug to a game last the weekend and for some reason it was firing at a constant 380fps and I have no idea why. Here's the story,

Originally as bought (second hand) it was firing at around 290fps.
I replaced the old tb barrel (madbull 6.03 590mm) with a shorter one (madbull 6.03 509mm)
Having replaced the barrel but not having a chance to shoot it the hop was fully off.
The temperature on the day in question ranged between -6 and -1 (I have no idea if that would actually affect fps but it could be worth mentioning)
Other than those few details the gun is exactly the same as the day I got it.

I really have no clue why the fps rocketed so high so any help here would be appreciated .I should be able to re-chrono in a few days in a better temperature just to be sure. If there's anything else any of you think could be affecting it or need to know please, please, please post and I'll get back to you.

[voodooTheMarshmallow]

Re: fps gone wild!?

Well the old barrel might have been fitted badly giving bad compression and letting a lot of air escape and therefore lowering fps ? I have also heard that a longer barrel can result in lower fps as the bb starts to bounce around in the barrel ?

[josh426]

Re: fps gone wild!?

Probably due to the length of the barrel mismatching the cylinder size, with such a long barrel a cylinder with a larger air capacity is needed, most likely the previous user had just shoved the new barrel in without a matching cylinder, whereas the shorter barrel matches the cylinder better, creating a more efficient use of the compressed air, giving a stronger fps.

[Skolob]

Re: fps gone wild!?

i'd agree with a leak around the bucking/ hop unit causing a problem. It could also be a worn or dirty barrel, but prbably not for that fps difference.
A cylinder mismatch isn't as big a prolem as many think, go to that mechanics site, many think that bb sucking is a myth & an oversized cylinder will just result in a lot of wasted air and wasted battery power. In proportion to the overall barrel length differences not that high. The hop fully off will also make a difference ( i fnd about 10 fps or so compared to set up for a .2 bb )
I had a very similar problem with a mates gun last week. Replaced the bucking and it jumped 60 fps. Turned out there was a tiny tear in the bucking nub causing air blow by. In the end we down graded the spring and its a nice consistent fps).

[julesbloke]

Oddly enough I had a similar experience with JG Aug. It was advertised for sale as doing 350fps. On my chrono it registered 290ish so I was a tad disappointed. Changed the hop rubber and cleaned the barrel and it now does 370!

Must have been a dodgy seal on the hop and possibly a filthy barrel.

[phayonyx]

Re: fps gone wild!?

So consensus is its probably either the hop or the barrel making the change, good to know. When I go get it chrono'd again next week I'll get a new hop rubber and nub as I was going to replace them any way and then adjust internals accordingly (lower rated spring methinks). I'll keep you all updated, thanks for the help thus far.

[phayonyx]

Re: fps gone wild!?

Well here's the dealio, couldn't wait so I went ahead and got it chrono'd earlier. Over ten shots fps remained pretty constant (381, 383, 379, 382, 379, 379, 383, 380, 381 and 382) and after talking to the gun tech have decided that I'm going to leave it as is for now whilst upgrading/downgrading the gearbox internals ready for big ole 11.1v lipo's.

Smaller spring, high torque gears (32:1), 6mm bearings, silent piston head and metal toothed piston. At that point I think I'm ready to level yes fps out. Thanks again guys.

[Quibble]

Re: fps gone wild!?

I have had a very similar problem!

My M4 Stubby used to chrono at about 340 which was perfect. In an effort to make her a bit quieter and more accurate, I replaced the piston, piston head and installed a new tightbore barrel. Last Sunday she chrono'd at 410 fps!!! Did I simply have a really bad air leak before that I have now unintentionaly fixed or can those new parts really make that much of a difference to the FPS?!

Guess I'm going to have to downgrade the spring now...