I'm combining a DPMS gas tube with a YHM flip-sight gas block.  When it'sassembled & lined up for the pin, the gas hole doesn't line up perfectly with the hole in the gas block--they're maybe 1/3 mm off.  Is this normal, or likely to be a problem?

I see this kind of thing all the time.
My solution is to drill through the gas block and into the gas tube with a 0.081" drill bit.
That is 0.002" undersized for the roll pin still.
The drill will cut away the offending metal and you will have a much easier time installing the pin.
The 0.002" press fit on the pin will keep it in place just fine.
This is best done on a drill press so that the bit does not grab/break/oggle out the hole larger than 0.081"
Any of that would be bad...

Randall Rausch
www.ar15barrels.com

Quoted: The drill will cut away the offending metal and you will have a much easier time installing the pin.

I already have them pinned together, but the gas holes don't line up perfectly.  You recommend I re-drill the pin hole instead of the gas hole?  I don't have a drill press  

Is there a standard distance between the pin hole and gas hole?  Or is this another thing like the pins on the front sight base, where the thing is just drilled--wherever the hole ends up is fine?

Oh, you mean that the large gas hole in the gas tube does not align inside the gas block?
That's OK, those holes are usually 1/8", but the gas port hole in the barrel is more like 1/16" to 0.090" so ot's ok if they are off 1/2 mm.

How is that for the intermixing of measurement systems?

Randall

I had problems with a bunch of DPMS tubes I bought. They did not have enough offset in them and the offset was not true. When I put them against a straight edge it was easy to see the problem I had. I sent them back and got a few from Bushmaster.

Quoted: How is that for the intermixing of measurement systems?

thanks for the help!