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CMP Modern Military AR is over weight, Questions for machinists. Calculating weight of material

Looking at the drawing here:
http://riflebarrels.com/wp-content/uploads/h-bar-ar15.pdf

It looks like the barrel could be profiled on a 7x12 lathe with some setup work with a spider and steady rest.
Taking 0.100" off is slow on a small lathe but even at 0.010" per cut it is only 10 passes - it will take longer to setup than to actually turn.

Cutting the crown - I'm not certain enough of the barrel could fit through the headstock to fit the crown through the chuck.
 
McReef said:
Both A and B are kind of the same question, and the answer is, it depends. It is all about heat. Your cutting method, and how it is executed determine the heat added to your work and the way it is added. The effect of that heat on any given part can have several effects, the severity of those effects largely determined by the material in question. The main effects you are concerned with (simplified) are the strength of the material, and distortion of the part you are working on.

Long story short, don’t “take a grinder to it”. You could grind it, if you took light cuts and kept the part cool. However you do it, apply this same idea. Take your time, and keep it cool.

Honestly, not knowing what your final plans are, I would be most concerned about you removing any material at or near the chamber. Without strong working knowledge of the physics involved there, you should absolutely make no modifications in this area. Information on barrel contours is available on some manufacturer’s websites. That is a good place to start for some parameters on the rest of the barrel.
Click to expand...
IF I was actually going to take a grinder to it I would use a air grinder and keep the barrel under a constant flow of water from a hose! This would be a last resort GIT'R DUN stupid....

I basically need to profile a Hbar to a A2 to shed weight. If its going to cost me $$ I might as well buy a A2 barrel and chop the threads off. I dont want?need a muzzle device.

here is a A2 profile barrel
PhZ4z5Qh.jpg


The A2 barrel under the hand guards are .645" Im not even going to go that thin!

If I can find the time to get over to the the shop I would not mind doing something a little different. Just because.
The "idea" is to Flat side 4 sides and leave a little "radius" then maybe do the same to the barrel forward of the sight block and cut the flats 90' to the flats on the rear of the barrel. With the hand guard I plan on using you will see some of this detail?

Scale Drafting is not my thing but I gave it a shot.
bExysVeh.jpg

Originally the barrel weighed 58oz I shed 4.34oz with the loss of the MD and 1" of barrel =53.66oz if I can squeeze another 12oz?
 
Last edited:
pastera said:
Looking at the drawing here:
http://riflebarrels.com/wp-content/uploads/h-bar-ar15.pdf

It looks like the barrel could be profiled on a 7x12 lathe with some setup work with a spider and steady rest.
Taking 0.100" off is slow on a small lathe but even at 0.010" per cut it is only 10 passes - it will take longer to setup than to actually turn.

Cutting the crown - I'm not certain enough of the barrel could fit through the headstock to fit the crown through the chuck.
Click to expand...
Im not worried about the crown, my DIY kit has done well by me
 
The problem with the 4 Flat Sides approach is you can't remove enough material to get anywhere close to the weight reduction you want.

The barrel is a pressure vessel and it's only as strong as its minimum wall thickness, which will be at the center of the flat sides. This is why the ideal shape for a rifle barrel is a cylinder.

If you want to achieve your weight reduction you will have to either turn the barrel on a lathe or use a mill to put multiple sides on the barrel. 16 flats would approximate a cylinder close enough.
 
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mac1911 said:
IF I was actually going to take a grinder to it I would use a air grinder and keep the barrel under a constant flow of water from a hose! This would be a last resort GIT'R DUN stupid....

I basically need to profile a Hbar to a A2 to shed weight. If its going to cost me $$ I might as well buy a A2 barrel and chop the threads off. I dont want?need a muzzle device.

here is a A2 profile barrel
PhZ4z5Qh.jpg


The A2 barrel under the hand guards are .645" Im not even going to go that thin!

If I can find the time to get over to the the shop I would not mind doing something a little different. Just because.
The "idea" is to Flat side 4 sides and leave a little "radius" then maybe do the same to the barrel forward of the sight block and cut the flats 90' to the flats on the rear of the barrel. With the hand guard I plan on using you will see some of this detail?

Scale Drafting is not my thing but I gave it a shot.
bExysVeh.jpg
Click to expand...

delete
 
Last edited:
mibro said:
The problem with the 4 Flat Sides approach is you can't remove enough material to get anywhere close to the weight reduction you want.

The barrel is a pressure vessel and it's only as strong as its minimum wall thickness, which will be at the center of the flat sides.

If you want to achieve your weight reduction you will have to either turn the barrel on a lathe or use a mill to put multiple sides on the barrel. 16 flats would approximate a cylinder close enough.
Click to expand...
you dont think I can shred 12oz off this stick? the heavy section of the barrel is .950" The A2 barrel profile is .654"
I just hate to be so plain and remove it round . but that will be the way if I need to shed the weight.

im to tired to figure out how much weight I would shed from the domed section lost ?

ok so maybe just trim down the .950" x 10.5" area to .750" round. then for looks do something odd with the barrel forward of the sight block, chop another inch or so on muzzle end for another 2oz ?
 
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mac1911 said:
you dont think I can shred 12oz off this stick? the heavy section of the barrel is .950" The A2 barrel profile is .654"
I just hate to be so plain and remove it round . but that will be the way if I need to shed the weight.

im to tired to figure out how much weight I would shed from the domed section lost ?
Click to expand...

mibro said:
Not with 4 flats, no.
Click to expand...

Right. OP, you'll be shedding only about half the weight (0.67oz) per inch of length reduced if you cut to a .75" square profile instead of a .75" round.
 
mac1911 said:
im no machinist but im pretty good at using tools to get what i need done.
I have been looking at thrift stores and libraries for older (free) books though
Click to expand...

44marty said:
You won't find a free Machinery's Handbook
1) They are not inexpensive
2) Any machinist will never part with their copy
But, yes, the formula for volume of a cylinder is in there, as is steel density
Click to expand...

Dont forget the library! My local library has the large version on the Machinerys Handbook. Its probably the best tool I keep in my box. Just reading the index sometimes you can find cool things to calculate that you never thought of.
 
NickLeduc said:
Dont forget the library! My local library has the large version on the Machinerys Handbook. Its probably the best tool I keep in my box. Just reading the index sometimes you can find cool things to calculate that you never thought of.
Click to expand...
Was there Saturday had to put 2 on hold. The daughter and I had fun looking up the formulas for the math sunday. Not just the online calculators.
 
One online trick I use to calculate volume quickly and easy is using an aquarium calculator.. You just adjust your measurements as if the solid material is water. and once you have the volume in ounces, you can just convert and calculate for the weight of the material.
 
mac1911 said:
Was there Saturday had to put 2 on hold. The daughter and I had fun looking up the formulas for the math sunday. Not just the online calculators.
Click to expand...
I'm old school, I like fingering thru a book. I have had instances where I didnt know WHAT I was looking for. So Google wouldn't help.
 
So I did a quick “recrown” I’m just going with a 11’ crown . I will most likely cut another 1” or so off the length of this barrel so I just did a quick cut and cleaned it up some. I will do a final polish after I cut it back and loose some weight.

wlfoD96l.jpg

O1WBtKPl.jpg

n8u7bI5l.jpg


If I dont loose power I will post pics of the original crown that was buried in the pinned muzzle device. I though I got this clean last time around...

Just a quick look with the $14 bore scope from Amazon. This barrel has about 3k rounds of all various ammo types including some Norinco silver box for about 800 rounds or so. Actually pretty good ammo better than most XMblahblah these days. amm. The focus on this scope is crap but its good enough for my needs. You can still make out the boring marks on the lands....looks like Railroad ties.

View: https://youtu.be/kgNhTQKrpxo
 
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jayhitek said:
That's great stuff.
I also didn't realize how thick the walls are.

Love the bevel.
Click to expand...
when I do the final trim?cut/crown I will run the bevel out to about .075" from the edge giving it a slightly deeper profile but with a "edge" around it.
 
cut a few more inches off and started the recrown... I cut it back just a smidge more after this to get a thinner "edge" (blue ring) polished it up.
c392o8Em.jpg

Xy3m9GFm.jpg
 

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