I am looking at shortening a 20" barrel to a 16" barrel.  From where do I measure to get legal length: "A", "B", or "C"?

None of the above?

The correct determination to make sure you dont create a SBR is with a bolt locked in the extension, firing pin inserted and protruding.  Measure your barrel length this way, then subtract from the current length to make it not less than 16.1"

I am sure you could get a reference from the exterior using this method.... just havent seen that.

You measure from the breach face of the bolt to the muzzle.

They don't make it easy do they?

Edited to add:
Screw it!  For that kind of hassle and legal balancing act, I'll just buy one or let a professional do it.  MILLER SHOULD HAVE SHOWED UP TO COURT IN 1939!

Quoted: if you have a 16" you can always use it as a gage

That's how I did it.  I took a 16" and marked the 20" where the 16" ended.  I added about an 1/8" for good measure and cut away.

Quoted: They don't make it easy do they? Edited to add: Screw it!  For that kind of hassle and legal balancing act, I'll just buy one or let a professional do it.  MILLER SHOULD HAVE SHOWED UP TO COURT IN 1939!hr

They do make it easy, FOR THEM.  

As stated above, it's from the bolt face, and I personaly would add 1/10th to 1/8" to be on the extra safe side.

I never thought of that: use a 16" for a model, and add 1/8" or a tad more.

This is an idea I can live with.

How about this for an alternate method?  Insert a dowel rod (wood or steel) marked at 16.25", and with it resting on the inserted bolt face, mark where it sticks out of the barrel.  Then, take it out and mark on the barrel the 16.25" mark, cut, and crown.  This is not fast, but it may be an idea, if one does not have a 16" barrel for reference.

Thoughts?

Quoted: I never thought of that: use a 16" for a model, and add 1/8" or a tad more. This is an idea I can live with. How about this for an alternate method?  Insert a dowel rod (wood or steel) marked at 16.25", and with it resting on the inserted bolt face, mark where it sticks out of the barrel.  Then, take it out and mark on the barrel the 16.25" mark, cut, and crown.  This is not fast, but it may be an idea, if one does not have a 16" barrel for reference. Thoughts?

The problem with that idea - is that once you take it out - how do you mark the barrel?  The correct way using this method is simple:

1.  Insert bolt and lock, with firing pin protruded (if you plan on it being real close to 16")

2.  Cut dowel rod to 16.25" and insert in bore.  

3.  Insert another piece of dowel rod, and mark it at the muzzle.  The amount to cut off is the distance between the inserted end of the dowel, and this mark.  Measure that, and use as a guide to mark the cut point of the barrel.

What I dont understand is - how are you planning to cut this?  It really should not be this complicated.  Only a machinist-gunsmith should shorten a barrel on a lathe, using a part-off tool.  You simply mark it approximately, part it off, then measure with a dowel.  Then using the part off tool, or if close, a turning tool, you "touch off" on the oustide of the new cut, back off, and use the lathe to measure exactly how much more to cut in the second pass.  Then re-crown.

The measuring is the simplest part.

1. Stick a 24" dowel in the barrel so that it hits the bolt face.

2. Take a pencil and draw a line on the dowel where it meets the end of your 20" barrel.

3. Take the dowel out of the barrel and subtract 4" from the 20" line and draw another line on the dowel. This is your 16" mark.

4. Hold the dowel on the outside of the barrel like you did in step 1 and put the 20" mark    where it meets the end of your 20 barrel. The 16" mark on the dowel can now be transfered over to the outside of the barrel. Leave yourself an extra 1/4 to be safe.

This is how I did mine.
And remember to measure a couple of times to be sure.

Lots of good ideas.  Thank you to all!  I am looking to buy a Krieger and cut it to 16" & contour to lightweight: not a new one, but one that has outlived its life for high power, and still shoots 1 MOA.

BTW, I missed a sweet deal last week.  A HP shooter had Hart and PacNor .30 cal bolt gun barrels that gauged at 3, which is not good enough for his level of competition.  I could have had them for free, but was too late: garbage truck beat me to them.  Most were 24-26".  All I want to do is take a Yugo M48 action, cut down an old match .30 barrel, thread, and chamber for a good shooting bolt gun in .308.  I figure that I could get a 1 MOA shooter for under $300 (without scope).  Its hard to beat the craftsmanship of a good Mauser action.

Meanwhile back at the ranch....

What I dont understand is - how are you planning to cut this?

I mark, make a file cut, then use a fine tooth hacksaw.  I have a tool that resurfaces the barrel to 90 degrees, then another for an 11 degree crown, and, lastly, one to radius the outside diameter.  I have a collet for .22, .30, and 8mm.  This is worth every penny.  I have saved two barrels and improved three others.  At $65 each crown job, it paid for itself.
PRECISION REAMERS MUZZLE CROWN REFACING TOOL

I cut mine with a hacksaw too. OH, the HORROR.......

Mine need threading so I faced them off while they were in the lathe, but I have the Brownell 11 degree facing tool and it works like a champ. Used it on a couple rifles and it paid for itself in no time.

Thezman, I just missed your post.  Check out the tool I use.

Hey, I used to make rocket parts with hacksaws.    Lathes and mills are good equipment, but it is hard to beat good hand tools with the right skill.

Damnnnn, that's one expensive cutter.

I got the 79 degree muzzle crowning cutter, it's was only $50.

I'm sure the one you got works better than mine.

Damnnnn, that's one expensive cutter.

I didn't pay that much for it: cut about $100 off that price (if memory serves me correctly).  I bought it years ago from the inventor, before Manson Reamers got to be the distributor.  Like I said, it has paid for itself with six crown jobs.  Its fast too.  After hacksaw surgery, I can have a barrel finished in 30 minutes, and that's taking my time.  I don't like to rush my work, because I want to do things right the first time.  I crowned a friend's .308 Rem 700 at the range, while shooting: decreased group size by 1 MOA....took less than 10 minutes.

BTW, I am out of luck on match barrels: all sold out.  Maybe a few more will turn up before the season starts.

Quoted: I am looking at shortening a 20" barrel to a 16" barrel.  From where do I measure to get legal length: "A", "B", or "C"? www.beastwerks.com/barrel.gif

The down&dirty refenence is that the breech face should be 3/8" from the end of the barrel extension, position "A" in your picture.
Of course you should add an 1/8" to your 16" barrel length to ensure you are over 16" long.

Edited to add:
Don't shorten a 20" barrel all the way to 16" as you WILL reduce reliability.
Go to 17" as it gives you significantly more dwell time.
Read about gas systems here
You will likely need to increase the gas port diameter as well, but try it first before you do.

Randall Rausch
www.ar15barrels.com

Randall,

Thank you for the information: good idea.  I had trouble with a DPMS barrel (16" with a full length gas system).  FINALLY, I had to ream the front gas port to .125" to have it work with all types of ammo.  I have three WOA SPR 18" barrels (two installed) and these run flawlessly; I do not know the port size.  So, your idea of shortening to 17" would improve functioning, AND give an extra margin of 'legal'...just in case...measure with a micrometer, mark with a ruler, and cut with an axe....or something like that.  hahaha

I don't necessarily need to replace my DPMS barrel, but it doesn't group all that well: 2-3 MOA with M193 bullets, and 2 MOA with my 69 and 77 grain handloads.  That is to say, I can shoot better than it can.  Other than this, its a good barrel.  BTW, I've shot 'match' barrels that did not group this good.

I thank everyone who has contributed to this thread.  I would never have found this information or thought of these measuring methods.  I greatly appreciate everything.

The problem with that idea - is that once you take it out - how do you mark the barrel?  The correct way using this method is simple: 1.  Insert bolt and lock, with firing pin protruded (if you plan on it being real close to 16") 2.  Cut dowel rod to 16.25" and insert in bore.   3.  Insert another piece of dowel rod, and mark it at the muzzle.  The amount to cut off is the distance between the inserted end of the dowel, and this mark.  Measure that, and use as a guide to mark the cut point of the barrel. What I dont understand is - how are you planning to cut this?  It really should not be this complicated.  Only a machinist-gunsmith should shorten a barrel on a lathe, using a part-off tool.  You simply mark it approximately, part it off, then measure with a dowel.  Then using the part off tool, or if close, a turning tool, you "touch off" on the oustide of the new cut, back off, and use the lathe to measure exactly how much more to cut in the second pass.  Then re-crown. The measuring is the simplest part.

I modified your method I used a whole dowel rod, marked at 16.25", and clamped a popcycle stick at the end of the barrel.  Then, took it out, put popcycle stick at end of barrel and marked next to 16.25" mark on the dowel rod.  Hacksaw and crowned.  I shot it today.  For what it is, it shoots like a house-a-fire.  Will try to post photos and thread later tonight.

Thank you all for you thoughts and ideas.