I want to put a DPMS Bull Barrel and a 4 rail free float tube on my build.  The problem is that the barrel has a diameter at the nut of 1.051" +/- .001 and the tube's barrel nut has an inside diameter of 1.021" +/- .001.  Oops.  Since I already have both parts and am determined to make it work, I am going to take the nut to a local machine shop and have them open the inside diameter up to fit the barrel.  My question is how much larger than the barrel diameter should the hole in the nut be?  I want it to be large enough, but not too large and I am assuming that the machine shops around here do not work on guns regularly.  I need some expert opinions, and I frequently find them here.  TIA!

Another idea that I had was to get one of those 3 stone cylinder hones ( for lawnmower engines) and workin on it a little at a time with that until it just barely fit.

Quoted: Another idea that I had was to get one of those 3 stone cylinder hones ( for lawnmower engines) and workin on it a little at a time with that until it just barely fit.

No flame intended, but I don't think that's a very good idea.  The area that has to be cut is so narrow that the hone wouldn't be properly aligned and hone arms would be flopping around would prolly cut a tapered hole at best.  

This is an area that a person with good file skills could prolly work wonders.  If I had to do it with a file, I'd use a rounded swiss pattern fine cut (#0) file, and take small even cuts all the way round the nut.  One the first pass, I'd just file off the finish.  Then coat the raw material with Dykem or just use a sharpie to ink it.  Then file just enough to remove the ink.  Keep checking the fit.  Ink the part before you try the fit to see where it rubs.  Patience is the key here.  It may take a couple of hours making small cuts.

To answer the question.  Obviously, you'd need enough clearance to fit over the barrel. I'd think a couple of thousanths (.002") for the mechanical fitting.  Then you'd need to add more clearance for the material of the nut, to compensate for different rates of expansion when heated.  Another .002" for that if the nut material is steel or .004" - .006" if the material is aluminum.  Use .010" as a max in any event.  If ya don't have a way to measure the clearance, the nut should slide easily over the barrel and rattle a small amount when slid all the way down to the ring on the barrel while the barrel is off the receiver.  Once you get to the point where you think it's right, re-ink the contact area and try to screw the assembly to the receiver.  That'll tell you if you're off center.  If it's hard to screw onto the receiver (you'd needn't torque to spec at this point), check the rub marks , and that'll tell you where you need to remove a little more material.  I don't think this is a super critical clearance issue.  You just don't want so little clearance that a rapidly expanding barrel won't crack a too tight (in diameter) nut.

Yeah, I think you are right about the hone idea...it would be easy to make a hole that is not square(? I guess that is the word that I am looking for...)  I think that I will just let the machine shop handle it...it is such a small part, I do not think that it would cost a whole lot..thank you for your advice, I do not know what I would do without this board.

I had the same problem installing a Yankee Hill Machine free float tube on my RRA upper. I chucked the barrel nut up in my lathe and bored it out with about a 0.010" clearance. You want enough clearance so that the nut doesn't interfere with the barrel. The barrel is held in place by the barrel extension fitting into the receiver and the barrel nut compressing the barrel extension back against the receiver. At any rate, assembly was straightforward after I bored the nut out, and it has worked flawlessly ever since.

Well, I spoke to the Cmore Systems and the nut was in spec.  The fellow that I spoke to was very knowledgeable about the product and helped me figure out some possible solutions.  The local machinist wanted $65 to  put a reamer thorough it. So I decided to give my hone idea a try...got a 320 grit 3 stone brake cylinder hone from Advance auto($10)  put 'er in my drill, and went to work.  I used a block of wood to hold the nut against to steady it, and had the drill sitting on a block also.  I sprayed WD40 on the stones to keep them from clogging and would run the hone for about 2 minutes at a time measuring between runs.  It was slow going in that the hone only removed about .001" each run, but I was careful to keep the nut straight ( you can feel it when it is not) and after an hour or so, I had the hole widened to 1.056"  with less than .001" runout.  That gives me .006" slop, which seems to be the generally desired amount.  The hole is nice and smooth, and once I hit it with a light coat of flat black Krylon, you can't even tell.  Thank yall for your help.  This is my first build, and it is going quite smoothly, thanks to the folks on this board.  I will post some pics next week when it is finished. Thanks again, Yall rule!