Under over or inline?

[SpudUke5]

Ok well i am making a large bore combustion with a possible 4 foot chamber of 4" diameter pipe, with interchangeable barrels of 3", and 2".

Now i was just thinking that since this is gonna be somewhat big and have significant mass, im not so sure if i should do a under over design because i dont want any snaps in the U bends (yes i am a believer of using dwv in combustions) so i was wondering if i should just make it inline? If i go with a 2 foot chamber of 4" diameter pipe, would it be safe to make it over under?

Different question: Would it be okay if i used screws to attach a fitting to the pipe? I want to make my cannon serviceable and i want a 2" ball valve for vent, so i had to buy a 4" to 2" flush bushing which has no OD threads. In order to make it serviceable, i would need to use screws. So what do you all think?

[SEAKING9006]

Go for it man. We need a bit of inspiration right now, it seems.

[SpudUke5]

Go for it man. We need a bit of inspiration right now, it seems.

Inline or under over? You have not addressed the question.

[SEAKING9006]

Oh, right.

Well, It depends on what you want, really. If you want it compact, go over-under. I know It sounds childish for me to say that, but hey. DWV is cheap, right?

[SpudUke5]

Well its cheaper than NSF-PW.

[Gippeto]

I prefer the looks of the in-line style.

If you were to incorporate a cam-lock setup to attach the barrels, any storage/transportation issues would be resolved, as well as presenting you with the option of using a burst disk like Starman uses.

As far as attaching the fitting to the pipe with screws, I used that method on a 2" tee pneumatic.

Six 1/4 x 20 pan head, phillips drive, screws equally spaced does the trick on mine, you may want to use more with the 4" pipe.(a garbage grade STEEL 1/4 x 20 screw will require more than 200 pounds of force to shear it)

Be sure you drill the holes at least 2 screw diameters from the edge. This will help spread the load carried by the individual screws, and lower the risk of tear out.

Use the proper size drill bit, and tap the holes. Drill, tap, and install the first screw. Then do the rest. Having the screw in place, will keep things aligned.

[starman]

You won't need 4' of chamber. I would say 2' for a practical max of 4" dia pipe. Plan on at least 2 centrally located spark gaps preferably 3 or 4 for a larger chamber.

I prefer the inline designs although I can see a few over and under advantages, none really performance based....mostly overall size and handling related. However, you can make a nice gun from either type.

It really depends on your goals for performance as to the sizes of things....you are familiar with all this I'm sure. If you haven't already, download HGDT to work your parameters for design.

Take a look at both of my combustions for inline examples.

[jook13]

I Prefer over unders personally. Like others have said, it compacts the design to allow for a larger chamber and barrel. Imagine a cannon with a 4 foot long chamber and a 7 foot long barrel. the inline would be about 11 foot long, versus the over-under just over 7 foot long. Others like the looks of inline's, but I think the over under looks more complicated and I like that.

[psycix]

An over-under design allows you to use longer barrels and therefore, it can have more power.
When using DWV, I'd say stick with an inline, or just spend those few bucks to use NSF-PW.
Surgery is more expensive then pressure rated pipe you know.