Seems it would be easy to find, but I can't find a barrel drawing anywhere.

I'm building a large frame AR and need a barrel blueprint.  I found one with the external dimensions but not of the machining to the tenon.  I'm sure I can just copy another one and make it work, but I'd prefer to do it to a blueprint if possible.

Your terminology is incorrect for an AR.
I am guessing you want a blueprint for the barrel extension??

His terminology is fine.

I've looked around and the best I can find is 1"x16tpi with a 1.325" tenon length.  I'd have to guess at the cone cut, 30deg with a dia of .650"ish.  Get and extension in your hands and check the minor dia and thread pitch though cause the information on these extensions are limited on the internet.

I'd try to contact Bat Machine and DPMS for their dimensions.  They both sell extensions.  If you get any good information from an authoritative source please post it.

If you have a barrel extension and bolt you can assemble it as it would lock up and use a depth mic to measure from the extension shoulder to the bolt face (not the breach face but the highest point of the bolt).  Subtract .010" and that's your tenon length.  That should give you .008-.009" end spacing clearance when you torque the extension.

The Bergara barrels that were clearanced from Sportsman guide had a blueprint posted with them...  Bout a year ago???

Looks about like I remember...

Thanks stiles, that link is helpful.  I'm using a Bat extension, I just sent them an email and will post whatever they send back.

I'm looking for the blueprint for the barrel tenon.  What would the proper terminology be?  

This what I need, but for a LR308:

Sounds like good advice to me.  I have to wonder about the cone, though.  I make barrels for personal use and have not cut the cone, instead I put a small radius (~0.025" or less) and highly polish it.  I have not had chambering problems with 6.8, 5.56, or 300/221 barrels.

Be careful about more than 0.010" bolt clearance.  I recall reading GI specs for clearance were about that, but did not save that post for posterity.  My last 6.8 barrel I cut very close as a test, on the order of 0.002 or less, with no functioning problems.

I made a barrel way back when with a lot of clearance, say 0.025" or more, and I consistently found brass I was resizing that had very short headspace.  I think the cause was the bolt unlocking forces were driving the bolt forwards into the brass and shortening it.  I already pulled the barrel before I tested this theory as it had other problems, not chamber related.

I'm with ya on that.  For the most part I was extrapolating from published ar-15 specs which is something like .570" @ 30deg.  As long as you have enough area for the bolt face to barrel face contact and you don't cut too deep into the chamber you should be good.  The ride up the extension's feed ramp is pretty forgiving.



IIRC the tolerance range on a ar-15 is .006-.011" with a nominal bolt.  I think the bolt's tolerance is like +/- .002 so that's like .004-.013" total variation in end spacing.  I guess that makes me a typical machinist, .0085" is nominal which is about what you'll get after torquing the extension.  You for sure can go lower, some do so to keep the bolt from sizing the brass when chambering the round.

Very interesting about your experience with the .025"+ end spacing.  Makes me wonder if the brass' elastic limit is something like .015" or so when at the temperature of extraction but in your case the brass had plastic deformation.  I better stop, thinking can be dangerous...

Never got a response from Bat.