Ported or non ported.

[plastikpyro]

I just picked up a King Arms M7 which I plan to use for outdoors skirmishing. The gun measured around 300fps stock with .20 on the chrono and I'm looking to raise it to 400fps and use .23. The stock barrel length is roughly 8 inches, noticeably shorter compared to a standard M4 and the cylinder was non ported. To my understanding from the "stickied" article above, if I upgrade to a 120 spring then I should change the cylinder to a ported one? Can someone please confirm or elaborate on this for me? Thank you in advance

[TriChrome]

If your cylinder has no-holes with a barrel that short, either KA messed up, or they're using tapered cylinders. Most likely the latter. If that's correct, you don't need to upgrade the cylinder when you upgrade the FPS.

[plastikpyro]

the KA cylinder isn't tapered. I compared it with my KWA M16 cylinder and they're pretty much the same. Must've been put together on a Monday morning. Should I put an m4 ported cylinder in there?

[The Saint]

Yes, if it's non-ported and non-tapered, putting in a cylinder with the correct porting will improve your performance.

[plastikpyro]

Just for the hell of it I went ahead and chronoed the gun with the stock non ported cylinder and the systema irreg. m120 spring and it measured 350. I assume the FPS is low because the bb has already left the barrel before the piston fully hits the head?

[TriChrome]

Originally Posted by plastikpyro

the KA cylinder isn't tapered. I compared it with my KWA M16 cylinder and they're pretty much the same.

You can't just look at them and tell if there's a difference. You would need to measure it with a caliper, or at the least do a compression test and see if compression is held for the entire length of the cylinder (or if I'm right it'll only make a seal about half way in).

[Styrak]

Originally Posted by plastikpyro

Just for the hell of it I went ahead and chronoed the gun with the stock non ported cylinder and the systema irreg. m120 spring and it measured 350. I assume the FPS is low because the bb has already left the barrel before the piston fully hits the head?

That's most likely the case. A ported cylinder would probably help that.

[ILLusion]

A velocity of 350fps is not out of line with a Systema M120 shot through a barrel as short as that. I've seen velocities as low as 360fps with an M120 through a 363mm inner barrel.

[plastikpyro]

I installed a ported M4 cylinder and chronoed the gun again and it still shot 350fps. I guess with a barrel that short being ported or non ported doesn't come into play? Would installing a tight bore increase fps or would it be pointless at that 8" length?

[The Saint]

How's the existing consistency and accuracy? If it's satisfactory to you, you probably don't need to bother hunting down a CQBR length inner barrel. Instead, you might be able to get a slight fps increase (and better hopup effect) by getting an aftermarket hopup sleeve. But really, you don't need to unless something isn't performing up to your expectations. Generally, upgrading only to get a few fps isn't worth the money.

[CDN_Stalker]

Ported vs. non-ported is based 100% on length of the inner barrel. It's about (air) volume really, nothing to do with the spring or any other part actually. I once tried a full cylinder in my 350fps MP5 and saw my fps drop to 320 using 0.20g BBs. Back went the ported cylinder and I got my 350fps back.

The MP5K has a short inner barrel and uses a ported cylinder, with the hole in the middle of the cylinder. The MP5 (and others) with a longer inner barrel has the port near the rear of the cylinder, representing a larger internal volume due to the longer inner barrel.

[zone 69]

Here's a guide to match your cylinder to your barrel length.
http://arniesairsoft.co.uk/?filnavn=...andbarrels.htm

[plastikpyro]

The gun is accurate and can produce a nice grouping at about 50ft. I'm seeing dropoff at about 120ft-130ft. I'm mainly looking to get increased range/accuracy of about 130ft-150ft using .23 without increasing barrel length. Is this possible? My other gun is a stock internal KWA M16 converted to CQB using a JBU front conversion, Madbull 363mm tb and 70% guarder bucking. It shoots 437fps and is accurate to about 150ft. The KWA barrel 14" w/non ported cylinder vs. 8" in KA w/ported cylinder , would the 6" difference = 80fps? Should I use a soft type bucking @ 350fps even though the spring is a 120?

[m102404]

Stalker, I get what you're saying and would tend to agree...but a question or two spring to mind.

Isn't the traveling speed of the piston a direct factor in resulting FPS as well?

Don't the ports serve more purpose other than to break the airseal and fill the cylinder with "fresh air" quickly on the piston head's return stroke?

When the spring is fully compressed, the piston is at it's most rearward position. In a full cylinder, it's still in the compression zone (i.e. if it moves forward, it'll start forcing air and start building up pressure behind the bb). In a ported cylinder (port mid-way, 3/4 back etc...) the piston isn't in the compression zone yet and has some distance (maybe half the cylinder, maybe less...) to travel before it starts moving air.

I'm not a big physics guy...like you I'm more of a learn with hands/eyeballs type...so I'm probably going to butcher this explanation...

If a piston is at rest and starts to move forward, the pressure builds behind the bb and eventually reaches a point where it starts to move the bb past the hopup nub. As it continues to move forward, the piston speed will increase to hit a max speed largely based, I'd figure, on the expansion/compression characteristics of the spring/coils/material/temper. Perhaps the acceleration curve is steep...maybe more linear.

Now, if it's a full cylinder, the piston will start the bb moving before the piston has reached full speed...and in a very short barrel the bb could very well exit the barrel before gaining the full compression benifits that the spring could provide...resulting in a lower than optimal velocity.

In a ported cylinder...the piston has time to reach a higher speed before starting to compress the air in the compression zone. So the pressure behind the bb is higher than that of the same setup but with a full cylinder...resulting in a relatively higher FPS.

Taken to an extreme...where the port is very much forward...so that the piston is traveling as fast as it can...but the compressible air volume is tiny...you'd end up with a very low FPS because the volume of air in the compression zone may not exceed the volume within the inner barrel.

I hope I explained things right...I probably didn't.

I'd appreciate your opinion,

Tys

[The Saint]

Originally Posted by plastikpyro

Should I use a soft type bucking @ 350fps even though the spring is a 120?

Sounds like you want a new hopup sleeve, rather than a new inner barrel. My personal favorite is the Prometheus soft sleeve, good up to 400fps. You pick your sleeve type based on actual fps of the BB, rather than the spring.