Error in Kuhnhausen Shop Manual

[ssgross]

I sat down last night to continue adding to my own shop notes. After a few double takes and head scratching, I sent the following letter to Heritage Gun Books, publisher of the Kuhnhausen-VSP shop manuals. Being this is such a great book (as are the others) and a go-to reference for so many of us here, I thought I would post for others to find. It's not hard to observe for anyone paying attention to or wondering about max/min barrel draw when swapping or installing a new barrel.

Dear Heritage Gun Books,

I am writing to bring to your attention an error in some data contained in Jerry Kuhnhausen’s The U.S. .30 Caliber Gas Operated Service Rifles: A Shop Manual, Volumes I&II, Bench Edition, 1995. Specifically the table at the bottom of Figure 87, page 97, concerning Final Barrel Shoulder Draw incorrectly reads:

Degrees Rotation ...|... Final Barrel Shoulder Draw
15 degrees ...|... *5/16" rotation
12 degrees ...|... *1/4" rotation
10 degrees ...|... *3/16" rotation
*Rotational draw figures rounded off to nearest 1/16”

The Final Barrel Shoulder Draw should instead read 5/32”, 1/8”, 3/32”, respectively, rounded to the nearest 1/32”.

Indeed, the exact distance between scribe marks is given by 1.100*sin(?/2), where 1.100 is the barrel shoulder diameter recorded on page 95 and ? is the angle of rotation. Perhaps Mr. Kuhnhausen mistakenly used 1.100*sin(?), which gives the values in the table as published.

EDIT: apparently our forum does not like to render thetas

[browningautorifle]

The Final Barrel Shoulder Draw

I never bothered with the science, just use an angle meter. Stripped receiver(Of course) and use the flats, stripped gas cylinder and use the front sight flats. If you're careful it doesn't even take long to achieve TDC and have sights perfectly timed. I've never had trouble installing a barrel like that yet. I had a bent one messed with me until I straightened it, that was different.

[ssgross]

The spot in the book I was referencing isn't about timing for sight alignment, it was referring to achieving proper torque. Whenever I install a used barrel, I always use the bench vise first - level off the bayonet lug with the barrel upside down, then use a dial level off the bottom of the receiver with it hand tight. The spec for the rest of the take up is min. 8deg, nominal 10-15 deg. No need to scribe marks and measure with a cheap dial level, but as I was reading 3 to 5/16" take up seemed like an awful lot, and it was too easy to confirm. The point of the exercise in my process is that a used barrel will often have little to no draw, and will need the shoulder's edge lightly peened. It's much easier to take it in and out of the bench vise for this than to spend the time getting the barrel vise all setup just to realize it has to come out and back again. The barrel I was about to install had less than 5ish deg. to go. This too can be eyeballed close enough with enough experience. New barrels can sometimes draw 20-25deg or more and seem impossible, but this too can be fixed by drawing up tight, backing up, tight again, rinse and repeat until you are in range.

I was gifted a good machinist level and the fancy badger ordnance jig last year. Jig is still in the unopened box. I have always used a 24 inch drill rod through the rear sight ears, and another across the front sight flat and then align by eyeball. Range tests have consistantly shown this is good enough, but I was looking forward to breaking out the jig after using my drill rods to see just how perfect or imperfect I have been.

Also, the sight alignment spec is said to be max of 20' (minutes) of true. Seems a lot at a glance. If I had sight height and radius in hand I wouldn't be able to resist translating that to downrange impact.

[ssgross]

Also, the sight alignment spec is said to be max of 20' (minutes) of true. Seems a lot at a glance. If I had sight height and radius in hand I wouldn't be able to resist translating that to downrange impact.

I was way wrong. It's nothing at all.
Sight height spec is 0.728 - 0.005
Sight radius is 29.5
So 20 minutes rotation gives 0.0042 lateral displacement of the front sight
This gives a downrange error of 0.522 MOA from true.

[browningautorifle]

it was referring to achieving proper torque.

Gotcha... The rest of post #3 makes perfect sense to me too. I just get fiddly when it comes to TDC because of the factory fit barrels that came past me with a rear sight adiustment of several index lines left or right. When you shoot close shots fall different than far. To me you can't group that one let alone zero reliably.

I was way wrong. It's nothing at all.

Sounds like you just did the math yourself...

[ssgross]

Sounds like you just did the math yourself...

Yes. Although I cheated by looking up the sight height and radius rather than going back down to the shop to measure myself.

when it comes to TDC

AR's are even worse with their high sight post. I think the tolerance spec for zeroing is something like 7 or 8 clicks from mechanical center. It's in a TM somewhere. I bought a brand new barrel from a shop on which they pinned a front site post for me. I went to zero and I needed 15 clicks. Tried to tell me at first it was normal so I did my own homework looking up the spec. I won't name the shop because he did make it right and exchange for one done more carefully.

[browningautorifle]

AR's are even worse

I have had a couple of those out of TDC and prefer the set screw front sight base if I have to deal with something or was making a custom length barrel. It was usually a custom job to begin with. The barrel and extension can be hard to get apart, if that's what one tries. Once the set screw sight base is located correctly they can be set up so the don't move again.

[ssgross]

prefer the set screw front sight base

me too. On my match builds I ordered Douglas heavy barrels with the flats milled for the front sight post, and they also have a dimple under the gas port. Additionally I threaded the lower hole for a 8-32 set screw. hand tight the bottom one to align the gas port, eyeball the other 4, then adjust after a trip to the range. On my A4 I noticed a lot of carbon buildup when switching to a low profile gas block for an optic. I added the screw on the A2 build, and checked after a few hundred rounds, and it works much better.