Loss of accuracy after tight bore upgrade... why???

[KenTsui]

On top of volume-matching, what kind of bb are you using? With a tight bore, high quality bb should be used.

[KenTsui]

Originally Posted by Blackthorne

It has nothing to do with compression.

Compression = Seal

Non Ported Cylinder = more air down the barrel

If your barrel is bedded properly, then what you are experiencing is "barrel suck". The bb isn't at the end of the barrel when the cylinder is full forward and all air has been pushed out of it, there is a tiny back vacuum when the piston starts its return trip. THis is what is likely ruining your groupings.

Full cylinder = air movement sooner = bb out of barrel before the piston starts its return.

I'm no expert but I'm curious about this on a mathematical standpoint. Shouldn't the air nozzle be disconnected from the hop-up before the piston's return stroke (spring starting to compress for the next shot)? It looks like the post on most sector gears are about what would be about 2 teeth ahead. If that's the case, wouldn't any vacuum in the barrel be broken?

Furthermore, the OP said the gun shoots 400 fps with M100 spring. That equates to about 121,920 mm/s. Inside a 509 mm barrel, it would only take the bb 0.00417s for it to exit. Granted, I have no idea what is happening in the barrel and the bb needs to be accelerated, but even at 1/3 of that speed, if the bb took a really long time to accelerate and averaged, say, 133 fps inside the barrel (accelerate from 0 - 400 fps non-linearly and very very slowly), it would still only take 0.0126s for it to fully exit.

The OP said the gun is relatively stock. We have no info on the battery type, but an average gun could do about anywhere from 13-16 rps. Let's give it the benefit of the doubt and say it's doing 18 rps. That would mean each cycle takes about 0.0556s to complete, during which more than half of this time the spring is being compressed. Doesn't it seem unlikely that the bb is in the barrel long enough for it be affected by the piston's movement on its return (spring compression stage)?

Having all that said, I don't know how fast the piston is moving forward once it is released so I don't know if the bb would be gone by the time the piston hits the cylinder head but I would imagine any kind of vacuum would be reflected in the fps (slowing bb).

Just to be clear, I totally agree on having a good cylinder-to-barrel ratio for best efficiency. There's no question about that, but I'm curious about this barrel suck theory.

What are your thoughts on the subject?

[Stealth]

From a practical standpoint, the phenomenon of "barrel suck" does not exist.

This myth has been dispelled quite some years ago.

[Hectic]

^that
No suckback exists even on stupid high rates of fire at lower velocities.
I say the issue would be lack of volume causing the bb to start its decel while in the barrel. As the air runs out behind the bb it starts to slow and bounce more in the barrel due to lack of air cushion around the bb that exists when the air behind (and escaping around) the bb is still being compresed.

[ThunderCactus]

FYI suckback only occurs with low fps and unvented piston heads.
Barrel length isn't really critical, I had suckback on a 310mm barrel
With an unvented piston it's theoretically impossible. If you greased it heavily enough it might do it though

[MaciekA]

At an extremely conservative and totally useless exit velocity of 100ft/sec, your BB is travelling fast enough to clear 310mm in 10.17ms.

Unless your setup was firing over 100 rounds per second, I'm going to have to demand some scientific-ish evidence that you can determine without a shadow of a doubt that barrel suck exists. A lot of gun techs smarter than our little kiddy corner here have debunked it. It's nonsense

[ThunderCactus]

you dont believe it until you see it happen in your own gun lol

[KenTsui]

Originally Posted by ThunderCactus

you dont believe it until you see it happen in your own gun lol

How exactly did you know that barrel suck was happening in your gun?

[ThunderCactus]

Because you could hear it getting sucked back, and I'd then have 2 exploded BBs come out the barrel.
It was a misfeed issue, where the BB didn't actually leave the barrel before the next one fired, the unvented piston sucked the first one back a bit.
Suckback isn't impossible, but it requires more than one failure to occur

[KenTsui]

Originally Posted by ThunderCactus

Because you could hear it getting sucked back, and I'd then have 2 exploded BBs come out the barrel.
It was a misfeed issue, where the BB didn't actually leave the barrel before the next one fired, the unvented piston sucked the first one back a bit.
Suckback isn't impossible, but it requires more than one failure to occur

But that's not really a problem with the volume:barrel ratio which is where this thread topic led to. That's a combination of different problems, and who's to say that the ratio had something to do with it?

[Hectic]

Agreed with such a short barrel houd need to be runnin an mp5k cylender to be under volumed

[Azathoth]

Originally Posted by Hectic

Agreed with such a short barrel houd need to be runnin an mp5k cylender to be under volumed

Don't laugh, I saw a "custom" gun through my bench 4 years ago with a 580mm EDGI barrel and one of these, SHS high speed gears and I can't recall the motor.

[Evil_yd]

i know this isn't exactly on topic but i recently upgraded my 416 from a 275mm barrel to a 363 6.03 Madbull and not it consistenly shoots to the right, is this a bucking problem or could the barrel be crooked ?

[Hectic]

Likely twisted a lil. Try turning the barell a lil to the left. A good way to check it is to take the barell and hop unit out turn the hop way up and loon through the hop up. Then twist the barell side to side youll see the nub/bump going side to side as well. Center it and put 1 drop of super glue where the inner meets the hop up to lock it down. Itll be tough to remove it but it is still removeable if you just put one or two drops. I have some m4s that the barrel just doesnt sit tight in the hop unit i do this to keep it centered. Other thing it could be is a bent barrel. Put the barrel on a flat surface and roll it. Youll be abke to see if its bent.