View Full Version : Increasing barrel OD
December 4, 2001, 10:35 PM
Greetings.... I need to increase the outer diameter of a pistol barrel to improve lockup. I was thinking along the lines of a barrel sleeve. Could something like this be press-fit onto the existing barrel, like a shaft bearing? I don't think there is enough difference in the current barrel and the desired diameter to take threading. If this would work... how do I make it work? How much bigger should the ID of the sleeve be than the OD of the exsisting barrel for press fitting purposes? .001? I don't really have a clue. Anyway, this is the other part of my .31ACP (Automatic Chipmunk Parabellum) project I've been toying with in the Reloading forum. The end result would be a Hi-Power clone that launches 60gr JHPs at about 1300 - 1500 FPS from a .30 Luger barrel. Probably good for absolutely nothing, but if nothing else it gives me something to do. Should be flat shooting, virtually no recoil, and straight hell on small game. (Hence the ACP designation, with apologies to JMB) Thanks in advance for the help.
December 5, 2001, 10:21 AM
DCloudy777, I've done this a couple of times over the years for one reason or another. I found the best way for me was to turn down the barrel a little and solder on the sleeve. George
December 5, 2001, 03:48 PM
OK ... so after you realize that a press fit has to have an interference fit (the ID of the sleeve actually has to be SMALLER than the OD of the barrel), you will probably give up on that idea. Wait!!! because there might be a clever way to get around this problem. If your barrel is made of a non-stainless steel, it will grow much less than stainless steel when they are both heated up (think Kenmore oven here ... not heat treat kiln!)
At 375 degrees and a barrel OD of .5", 304 stainless will grow 0.0005" more than a standard cold rolled steel ... I don't have the numbers for 4140 alloy. This means that if you can get the sleeve turned in a lathe so that it is 3 ten-thousanths of an inch smaller than the barrel it will BARELY clear the barrel at 375 degrees and it WON'T budge at room temperature. You could put the precise ID in the sleeve while it still had a much larger OD and then turn the whole assembly down after it cooled. There might be another material with a higher thermal coefficient of expansion than 304 stainless, but one with acceptable wear characteristics (aluminum is out!) doesn't immediately come to mind.
Just a thought,
December 8, 2001, 02:40 AM
Use the same theory of expansion. Make a sleeve. Preferably of the same material turned to a point the it would be possible to press fit it on. JUST BARELY. Then heat up the sleeve(in your hand) and slightly cool the barrel. The sleeve should slip on. If you want it off you will have to cut it off. My machinist made several parts like this for me and it is like they are locktite'd on. They will never budge unless hammered upon.
December 8, 2001, 04:27 PM
Riss's idea will work. If you go that way, you will NEED to get an appropriate TCE (thermal coefficient of expansion) match for this particular application because your barrel WILL heat up and you don't want it to get loose as you are using it. You might be OK using a material with a lower CTE ... it would get tighter as it heated up, but I'd go for the perfect match and heating the outer and icing the inner part. The bigger the difference in temperature you can accomplish, the easier your job will be ... but work fast because if these two pieces get cozy, they will get to the same temperature in a hurry and you'll be in the cutting mode! I guess timing is the advantage to my original method.
December 11, 2001, 08:37 PM
Went to the machininst today... sleeve should be produced and pressed on tomorrow. Cost was pretty reasonable, I'm pretty excited. I'll post results as soon as I have some, live fire test (with factory 93gr, still no load development) should come Sunday. Thanks for all the help guys!!
December 13, 2001, 03:23 PM
Finally picked up the barrel. Everything looks very nice, locks up nice and tight. Live fire should be Sunday, will keep everyone posted.
December 17, 2001, 11:07 PM
Worked like a charm. Locks up tighter than the 9mm barrel now, and shoots tighter groups too. As soon as the dand reloading stuff gets here its on to load development. 60gr JHP sounds like varmint death to me... the .31 Auto Chipmunk Parabellum will indeed see the light of day!! :D
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