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

[ao_tenshi]

Hello all,

I would like to get some advice from the community about the possible reasons why an inner barrel upgrade would cause a loss of accuracy. This is my first attempt at upgrading an aeg...

I have a stock KWA M4, which was shooting very accurately (1/2" groupings). I've recently upgraded the inner barrel to a 509mm madbull 6.03mm tight bore (stock was 395mm). No other mod except a suppressor extension to accommodate the longer inner barrel.

Now with this upgrade, Im getting extremely inconsistent shots and groupings as bad as 3-5"... any suggestions on the cause of this and how to rectify this?

Thanks in advance!!!

[Ben.Davis]

It seems you might need a new cylinder! I'm not sure if kwa m4's come with non-ported cylinders, but I don't think they do. A barrel as long as the 509mm needs a non-ported cylinder because with a ported cylinder, more air can escape therefore making the piston head hit the cylinder head after the bb hits leaves the tip of the barrel, which causes lack of compression (fps/accuracy degrades). If you already have a non ported cylinder in your m4, my second best guess would be getting a new hop up bucking, it could have ripped, or lost elasticity during the inner barrel swap!

[mike_sinyard]

Thats a huge upgrade!!! I believe your cylinder does NOT have enough compression + volume in it to use a barrel that long. Getting a bigger cylinder = more volume
better piston, piston head, cylinder head and nozzle.

im no gun doc, but this is my guess.. Hopefully someone else chimes in on this

[zone 69]

inner Barrel is to louse in the outer barrel take the suppressor off and check the inner barrel for any play.

[ao_tenshi]

Hey thanks for the quick suggestions everyone..

In terms of compression, Im no expert, but i think the stock kwa is pretty good with compression. Prior to the upgrade Im getting 360fps on a M90, and 400fps on a M100 spring.

I've just checked the inner barrel, and its not wobbling inside the suppressor or outer barrel.

Have not checked the hop up rubber yet. Should i be looking for cracks or rips on the rubber?

[Chiba]

Sounds like you aren't matching the right cylinder for your barrel length.

You should be using a full cylinder (no holes) for a 509.

[zone 69]

This may help

http://arniesairsoft.co.uk/?filnavn=...andbarrels.htm

[ao_tenshi]

Originally Posted by zone 69

This may help

http://arniesairsoft.co.uk/?filnavn=...andbarrels.htm

That's really helpful! So if i install a no-hole cylinder to bring up the compression, i should see more consistent accuracy?

What I dont understand is, if the compression is low, i should just get lower fps.... why does the accuracy drop so significantly? (just curious)

[Hectic]

It drops because you run out of compression befor the bb exits the barrel and so gthe bb starts to slow down while its in the barrel and will be bouncing around in the barrel befor it exits.
And yes also check the inner for play in tbe outter barrel and shim it with some tape or orings if needed. in the mean time you can get away with a 430 or 455 no problem on a 4/5 ported cylender as(most m4 these days have that. also going to 509 or 550 won't make much improvement over a 363 its the quality of the bore barrelthat make s the difference

[Kozzie]

Originally Posted by ao_tenshi

That's really helpful! So if i install a no-hole cylinder to bring up the compression, i should see more consistent accuracy?

What I dont understand is, if the compression is low, i should just get lower fps.... why does the accuracy drop so significantly? (just curious)

It's not the compression (air seal) that changes, it's the volume of air that changes. The idea is to match the volume of your cylinder to the volume of your barrel. Your cylinder is running out of air (propulsion) before the bb clears the barrel, the theory is that this may cause your bb to start rattling around in the barrel because there isn't a cushion of moving air between the barrel and your bb. Too much volume can cause excess stress on your mechbox. Some people also believe too much volume will cause inaccuracy because the bb is still being pushed some after exiting the barrel.

Another thing to consider which was already mentioned by Zone 69, is what I would say is more likely to be the problem, how securely is your inner barrel being held. If your inner barrel has any movement then where it's pointing can change independent of your sights. I'm personally not a fan of hanging inner barrels long for that reason. A suppressor used to cover up offers no stability to the barrel. IMO you're far better off getting a longer barrel or just using a "stock length" inner barrel.

If your gun was already shooting "very accurately" then why change barrels. The quality of the barrel is more important than the length. Insert obligatory dick size joke here. Aftermarket parts are not necessarily upgrades.

edit: Hectic beat me to it.

[pestobanana]

At what distance are you getting these groupings?

[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.

[ao_tenshi]

Thanks for all the explanation. This makes sense now.

So based on your experiences, is any standard non-ported cylinder enough to push air for a 509mm or are there some special cylinder that I should consider?

[Styrak]

Standard un-ported cylinder, also known as a "full" cylinder.

[ThunderCactus]

FYI a 509mm barrel won't offer much improvement over a 395 anyway, you're better off getting a new 395mm barrel instead of opening your mechbox and risking messing something up.
Also the hop rubber should be your first upgrade, not the barrel