Internal diameter of extension tubes

[Cumbrian]

I am pondering making my own extension tube for my .22 target rifle, perhaps more than one. Should I make the internal diameter as wide as possible, as narrow as possible, or does it make no difference to the behaviour of the bullet once it exits the muzzle? If pressed, and partly because I understand that cleaning the tube of powder debris is important, I would guess that as wide as possible might be desirable so as to minimise air turbulence, but what do I know. As wide as possible would also of course minimise the weight hanging off the muzzle. Grateful for more informed thoughts.

[Tim S]

My Barry Nesom is 20mm diameter internally, and 26mm externally

[Cumbrian]

My Barry Nesom is 20mm diameter internally, and 26mm externally

That's very helpful - thank you. Interesting that the Nesom tubes have quite thick walls at 3mm, sturdy but adds to the weight. I was looking at some off the shelf aluminium tube with 1.63 walls, which should be strong enough and will spare the exercise of my distinctly amateur machining skills. If I use such tubing I would end up with about 25mm internal, which I guess will be o.k.

[Tim S]

I think Barry chooses a thick inner wall as he cuts the dovetail into the tube - even the deeper 1400-1800 one. As your tube is thinner, are you planning to bolt/glue on a block for the foresight?

[gstarik]

From many tests and experiments,I can tell you that the minimum diameter of the inner tube should be more than 13mm.
Below that, accuracy goes down. All diameters above 13mm gave me the same results.
What really matters in my opinion,is the bloop tube weight. I have found out with Dr. Jeffrey Colbe(the previous owner of Border Barrels) that you can't tune for "positive compensation" if the weight of the tube is over 250gr.
Guy.

[Cumbrian]

I think Barry chooses a thick inner wall as he cuts the dovetail into the tube - even the deeper1400-1800 one. As you tube is thinner, are you planning to bolt/glue on a block for the foresight?

Good point about the dovetail - thank you. Yes, I wasn't planning on cutting a dovetail into the tube itself, though I have recently made one for a substitute BSA International rearsight platform that I constructed to take modern rearsights (seems to work fine). Instead I was thinking, as you say, of gluing or bolting one onto the tube, perhaps after milling a very slight flat into the tube. However, after your post I am now wondering about starting with a thicker tube altogether and deploying my limited machining skills to lighten it, except where the dovetail would sit. (I can achieve a fair finish on the outside of a tube but inside is much more difficult, I find.) If it all works out, I'll post the results. That will be some time off as currently I am working on a replacement fore end for a BSA Mk 5 to give it a floating barrel (except for the last 3 inches that are needed for attachment to the barrel). Anyway, all good fun and keeps me off the streets.

[Cumbrian]

From many tests and experiments,I can tell you that the minimum diameter of the inner tube should be more than 13mm.
Below that, accuracy goes down. All diameters above 13mm gave me the same results.
What really matters in my opinion,is the bloop tube weight. I have found out with Dr. Jeffrey Colbe(the previous owner of Border Barrels) that you can't tune for "positive compensation" if the weight of the tube is over 250gr.
Guy.

Very interesting data, for which I am most grateful. Probably tuning would not help me very much as I shoot at 25 yards only and I understand that tuners do not make much difference over this short distance, but I will definitely watch the weight after what you say. Many thanks.

[Tim S]

What really matters in my opinion,is the bloop tube weight. I have found out with Dr. Jeffrey Colbe(the previous owner of Border Barrels) that you can't tune for "positive compensation" if the weight of the tube is over 250gr.
Guy.

Guy,

would the 250g include the foresight, or just the tube + collar?

[gstarik]

Include the foresight.

[Tim S]

Cumbrian,

as a Luddite, gluing/bolting on a foresight block would seem easier than turning down the tube.

I think the inside should be quite smooth to reduce the build up of fouling, but can you not do this with metal polish?

[Cumbrian]

Tim,

I can be something of a Luddite myself, so I'm glad to find myself in good company.

The difficulty I have encountered with turning the inside of a tube (on a perfectly decent school workshop lathe) is that the boring bar, quite apart from being tricky to set up and align, won't reach more than a few inches, so I have had to turn the tube around in the chuck, which means that a ridge results where the two boring actions meet. (As a guest in the workshop - I am long since retired from the school - I can't really complain about the supply of boring bars.) And even with the automatic feed on, the very slight flexing of the boring bar tends to leave ripples in the finished surface. I'm afraid metal polish would not deal with these. I am slowly improving my machining knowledge and technique, so I live in hope of better results. However, I must think hard about the whole problem before I embark. Thank you for your interest and advice.

Roger

[Tim S]

Ok, I was just thinking of polishing any rough bits, not boring it out. Can you not find the size you want ready-made?

[Cumbrian]

Ok, I was just thinking of polishing any rough bits, not boring it out. Can you not find the size you want ready-made?

I can get pretty close off the shelf but not precisely the right dimensions. Some machining is unavoidable. I am designing a tube that will fit snugly around the muzzle, with a degree of shimming, to be held in place partly/mostly by friction, though located through the existing holes for the fore sight. The existing screws will also keep the tube secure. (Though no engineer, I have heard of aircraft bearings being held in place by friction alone after they have been frozen, dropped into their matrix, and left for heat expansion to do its work.) I have not used the clamping method of locating the tube. I prefer the idea of using the existing holes for precisely lining up both the tube on the muzzle and the new place on it for the foresight. I plan of course to drill the two sets of holes on the milling machine without removing the tube. (Nothing wrong with clamping, by the way. I am planning to use that method for my Mark 2 version of a fore end to provide a free floating barrel on my BSA. I shall be busy for years at the rate that I work.)

Perhaps I would be better advised simply to buy an extension tube that has been professionally made, but I am hoping to make one that is lighter, as well as cheaper. Moreover, I simply enjoy making things. Very therapeutic after an academic career.

[Tim S]

So that,s why you want it larger. A friction fit tube would be new to me. FWB rifles have a slip-on single piece tube, but these are secured with bolts (there's a guide peg too). I'd be worried that the tube could work loose.

[Cumbrian]

Tim,

Yes, apologies for not making that clear earlier. I entirely agree that there is a potential weakness in my method but I think that friction plus the odd screw may do the trick. I am using the friction fit method on my Mk 1 new fore end for my BSA Mk 5, supplemented by the first of the three screws that holds the existing fore end in place. (That will still leave about 3 1/2 inches of barrel unfloated but will give an 80 or 90 per cent improvement on the original BSA set up.) With the pressure on the fore end that will be quite a severe test of my method. If it slips or wobbles in practice, I shall add an aluminium piece to connect the fore end to the cross bolt that secures the trigger block. We shall see.

Roger

[Tim S]

You could glue the fore-end hanger, but not the tube, well unless you plan on never cleaning it.

[Cumbrian]

You could glue the fore-end hanger, but not the tube, well unless you plan on never cleaning it.

By coincidence it was only yesterday, on one of my weekly visits to the school workshop, that one of the skilled technicians kindly welded the short tube I am using to encase the BSA barrel into the aluminum channel that will form the basis of the fore end. He made a very neat job of it. So far the fore end is a bit less than half the weight of the BSA original, though I have yet to add a short accessory rail and some thin walnut fillets to make it more comfortable to hold, so the weight will increase a bit. If it works, I will try to post photos here; if it doesn't, I'll keep quiet and add it to the list of my unsuccessful projects.

Sorry - I seem to have strayed some way from the point of my original post.

[D.Sigurdsson]

Regarding polishing the inside of the tube, have you thought about using a barrel hone?
http://www.brushresearch.com/brushes.php?c1=1

[Cumbrian]

Regarding polishing the inside of the tube, have you thought about using a barrel hone?
http://www.brushresearch.com/brushes.php?c1=1

Thank you for this helpful suggestion.

[HWN1011]

Include the foresight.

Hi Guy

So the weight of the tube not being more than 250g includes the tube, foresight and the tuning weight also???

Henry