How straight is straight enough?

ranb · Post by **ranb** » Wed Sep 06, 2006 3:19 pm

I am making a form 1 can for my ar15. The piece of 304 stainless tubing I have is 1.35x8", but it is about 0.013 inches off from being perfectly straight as measured when in the lathe between centers. How straight should it be? I guess I could just open up the baffles and end cap, but I would rather be no more than 0.275" bore.

I do not have a boring bar that can turn the inside. Am I better off just finding a better piece of tubing? What size bore do you guys use on 223 suppressors? Thanks.

Ranb

HandyMan · Post by **HandyMan** » Wed Sep 06, 2006 4:37 pm

Do you mean that there is a noticable bend in the tubing? Or are you saying that the ID is not concentric with the OD? Did you take measurements from one end to the other?

Without knowing more, I can tell you this; stainless tubing has a certain amount of stress in it. If you bore to correct a bend in the tubing, you may end up unbalancing the stresses in the metal and cause an even more pronounced problem later. Similar to how a rifle will walk it's group as it heats up.

What is the wall thickness of your tubing?

ranb · Post by **ranb** » Wed Sep 06, 2006 7:17 pm

There is a noticeable bend in the tubing, I can see a gap when I lay it down on a flat surface such as my lathe bed. When I put it on the lathe and measure deflection with a micrometer while rotating, I get 0.013 inches. The tubing has a wall thickness of 0.010", I was planning on reducing thickness to about 0.065" after welding on the endcaps.

Ranb

David Hineline · Post by **David Hineline** » Wed Sep 06, 2006 7:28 pm

Get new tubing something that did not come from a muffler shop.

HandyMan · Post by **HandyMan** » Wed Sep 06, 2006 9:32 pm

I agree with David. Get some other tubing. If it has that much runout between centers, you will never get it straight enough for a can. If you buy new tubing, I would upsize to 1.5" dia. Or maybe even 1.625" with a .120" wall thickness. The thicker wall will help keep distortion down when you weld on it. I have bought small quantities from both of these online stores.

http://www.mcmaster.com/

http://www.microgroup.com/

Oh, and I took a look at your website. Very nice work.

ranb · Post by **ranb** » Wed Sep 06, 2006 10:20 pm

Thanks for the advice guys. I currently get my tubing from www.onlinemetals.com , and the barstock from anywhere I can get it cheap. I will check out the other sites.

So if I use a thicker wall to weld up with, can I turn down the wall after welding to reduce weight? How thin can I go if I limit myself to 223rem or 22-250 pressures? Thanks.

Ranb

HandyMan · Post by **HandyMan** » Thu Sep 07, 2006 4:08 pm

How thick you leave the walls will be determined by your planned use of the suppressor. If you want rapid semi-auto, or full-auto, then I would leave them pretty thick. If, on the other hand, you plan on fairly sedate useage, then you could go all the way down to .050". This is all assuming you are not threading the end-caps on before welding. I personaly would not go below .065". Robert has said that the M4-2000 uses .065" tubing with .035" spacers. This adds up to .095-.1" thickness for a full auto capable can.

A .22-250 with a 20" barrel will be about the same as a short barrel 5.56mm as far as exit pressure and abuse goes. If you are looking to have a dual-use can, I would leave the walls as thick as possible. Are you going to use spacers in your design? They can reinforce the outer tube.

ranb · Post by **ranb** » Thu Sep 07, 2006 5:27 pm

My current design screws onto a modified ar15 flash suppressor, so it is a two-point mount, with the points a little over an inch apart. There will be spacers up to the blast baffle to hold things in place. This will not be a full auto can, and I rarely shoot the ar15 fast enough to make the barrel hotter than 200 F. Am I asking for trouble if I use aluminum spacers? Or are steel spacers called for here? I would guess that steel is needed, but I like to use aluminum on the lathe much more.

The caps will be a press fit prior to welding. I hate to make threads on my lathe, so I wouldn't make them if I was welding the caps on.

Ranb

HandyMan · Post by **HandyMan** » Thu Sep 07, 2006 7:10 pm

Well, I think you know the answer on the aluminum spacers.

Sounds like a neat design. Post pictures when you finish it.

Third_Rail · Post by **Third_Rail** » Sat Sep 09, 2006 12:53 pm

Odd, I've never had a problem with Online Metals' tubing being out of true by that much.