multiple upper to single lower gun help.

[Novak]

So, I decided to try a multiple upper to single lower reciever project which is going to go on a jg metal body. Now I know that wasn't the smartest choice brand for this project but I had bought the gun then after I got it thought of the idea. What I need the most help with is the cylinder porting. The longest upper is going to have a 509 inner barrel length and don't know about the shortest yet but need to know if I should put an unported cylinder in there to support the volume of air needed for the max length, and then let the shorter barrels do the work of decreasing the fps to separate engagement levels. The target fps im looking for for the long range is about 410-420, will i be able to shorten the barrel enough to get a noticable difference in velocity for say a cqb upper and still have an unported cylinder for the long barrel. This may sound confusing how i wrote it but I am new to the part where you need to make one lower that will fit the needs of different uppers for different engagement styles. I am experienced enough with the internal workings of the gearboxes and have done many upgrades, mods, rebuilds, gearboxes from scratch. I would just like the opinions of people who have done this before and have experience this since this seems like it would take a certain and different take on just a one gun gearbox. I appreciate all the help that I may get in advance.

[Squirrel]

A lot of people say you get lower RPS out of an unported cylinder. This isn't necessarily true at all spring rates.

It is pretty tough to armchair tech a gearbox as there is so many variables that come into play. From my experience with unported cylinders, I suggest sticking with a polycarb piston / head to keep the weight down and retain your RPS. Stick with an m110 spring. You will be hanging around 370-400 depending on your air seal and all that.

[Novak]

Im not really worried about rps, having had plenty of hi speed set ups and have kind of lost interest in them. Ive actually grown to like more realist firing rates as they don't eat up as much ammo and are more stable in my experience.
Let me ask this another way, would I be able to supply the air capacity needed for the longer barrel(509 in this case) with a ported cylinder which has the port located at the rear, instead of having to make the change to full cylinder.
From just looking through the selection on several online airsoft sites, all of the cylinders that are saying that they are for the length that I need don't have ports in them.
If someone who uses the longer length barrel and has experimented with the best porting of their cylinder would like to share their findings I would appreiciate it.

[Phantom G3]

The ports in the Cylinder will differ for each length. You can have a small port for that length, or no port at all. I run a G3A1 and I do not have a Ported Cylinder. Why? This is because of the max air capacity to push air because of the longer barrel. It needs that air. If you have a shorter barrel, you will need a port (depending on size, of course). If you would like me to elaborate more, I can PM you on a very important guideline on this.

[Squirrel]

To clarify Phantom a bit, you will not have the cylinder volume with a 363mm ported cylinder to efficiently push a BB from a 509mm barrel. The inner volume of the barrel will exceed the volume of air being pushed. The piston will also cycle faster with the shorter ported cylinder, so it will have a negative effect on the BB that may still be in the longer barrel when the piston gets to the backstroke.

It would be better for you to stay with the unported as the effect on the shorter barrel will not be as detrimental as a ported on a longer barrel.

[Phantom G3]

Secretsquirrel hit it right on as well. The ports are there to regulate how much air is being used. Don't think that you want the most amount of air possible so you get a Bore-up Kit and then get a Type 0 Cylinder (unported) because you want more FPS. This would actually contradict what you are trying to do if you have a smaller length barrel like the 363mm M4-type barrel. Just be aware before making a purchase and you will be A-OK!

[Squirrel]

Don't bother with a bore-up kit. You will not be pushing enough air to use it properly. I really don't see much merit in the bore-up system unless you are running something in the 650mm range or a very heavy spring.

Only so much air can flow through a given diameter at the given rate, which is basically dictated by the pressure exerted by your spring.

[Phantom G3]

Oct 3, 2012 11:29:13 GMT -5 Squirrel said:

Don't bother with a bore-up kit. You will not be pushing enough air to use it properly. I really don't see much merit in the bore-up system unless you are running something in the 650mm range or a very heavy spring.

Only so much air can flow through a given diameter at the given rate, which is basically dictated by the pressure exerted by your spring.

I agree 200.1%.

[Novak]

Thanks for the help guys, had all the parts for the rest of the gearbox but needed that last bit of help with the cylinder because I was using a gearbox that was ported for a 363 length inner barrel to build from.

[X]

I think the biggest question of your post that hasn't been answered is whether the shorter barrel will bring your target of 410-420 back down to <350. I can't really answer that either, but I would like to know. If you find out make sure to update this thread. My suggestion would be to get the smallest barrel you are comfortable with and try to tune your FPS to ~340 then see how much velocity you can pick up with the longer barrel.

[Novak]

Well I already have tight bores that are 509 and 363, I was hoping if I went down to about 280 or less it should be able to drop it enough, cause if your playing cqb how accurate do you really need it to be, besides I used to run a jg cqbr with a pretty short barrel and I had no issues with accuracy, it was actually suprisingly accurate. If I need to go shorter I could always try making one of those really super short uppers.