Gunsmithing Barrel Machining for weight/accuracy

[Quicksilver]

Here's my question- and its long...

I was thinking about having 6 flutes cut into the barrel that is underneath the floating handguard. This is a AR-10. My barrel is 20" long- the first 13" is a 1.000 diameter, the gas block is 1.5" long and .875 diameter, and the last 5.5" is .800 in diameter.

When fluting a barrel, do you ONLY go to the "minimum barrel" size, IE only machine the 1" diameter section of the barrel to a maxiumum "plunge" depth of .100?

When I plugged the numbers into solidworks, and used a theoretical 7/16 ball end mill with a plunge depth of .090 and had 6 featured flutes that were 12" long (.500 away from each end of the section) and calculated the mass per square inch of the stainless steel used, my barrel weight "fluted" is 3.14lbs. My weight of the barrel unfluted is 3.54lbs-

If this is correct, and you only use a ball endmill to go the "minimum depth" according to the smallest section of the barrel, were talking a weight savings of ONLY 1/3 of a pound.

So, how deep do people usually plunge the endmill, and also- what size do you commonly use for 6 flutes?


My second question is:
my barrel has a 90 degree face cut at the end of the barrel. Are there any benefits to doing a 11 degree cut concentric to the bore of the rifle? If so why/how?

Thanks! Sorry its so damn technical, but theres no other way I could say it.

 

[Quicksilver]

Re: Barrel Machining for weight/accuracy

Let me rephrase this to simplify.

When you cut flutes into a barrel, what size end mill do you use and how deep do you typically go.

... or is this top secret.

 

[elephantrider]

Re: Barrel Machining for weight/accuracy

QuickSilver,

I just pulledout a .30 cal heavey barrel (diameter varies from over 1.25" to under 1") and measured the flutes. The flute width is 0.325". Flute depth where the barrel diamter is 1" seems to be about 0.1" to 0.125 (hard to measure because my depth guage doesn't measure the tapered flute are very well).

As for your other questions:

- Muzzle crown. You will find varied opinions on the "correct" angle for a crown. My rifles have a recessed crown with a 90 degree face on the barrel and a very small 45 degree crown on the exit hole. Whichever way you go, make sure it is a recessed crown to protect the lands/grooves at the exit hole of the barrel. You could aslo have it threaded and add a flash hider.

- If you are trying to put your barrel on a diet, also consider having the O.D. turned down instead of fluting. You can precisely calculate the weigh savings on reducing the barrel diamter. You can also swap the gas block for a lighter weight model. You can have the gas bock seat area turned down as well ( It will slighly increas port pressure, but not enough to be a problem). While you are at it you could get the gas block pinned on as well.



-

 

[krw]

Re: Barrel Machining for weight/accuracy

I think your answer will vary barrel maker to barrel maker. For example Brux will offer a 30 cal barrel in a smaller contour than Kreiger will, and also they will flute a smaller contour bbl in 30 cal than Kreiger. And Brux's fluting is considerably deeper than Kreigers. Nothing derogatory towards Kreiger at all, as they make a great bbl and offer outstanding customer service. I just don't think your answer is carved in stone.

 

[Quicksilver]

Re: Barrel Machining for weight/accuracy

Didn't think about the gas block weight- will start looking into that.

Decided not to flute the barrel- I could not find a definitive answer on the effects on accuracy and didn't want to risk it. I would rather have a rock solid POI than carry 1 less pound. If it can't make weight on certain classes, so be it.

Decided to go ahead and have an 11 degree crown and barrel threaded for 5/8-24 TB 30BA suppressor. It sounds like a great package that allows you to have a brake and use a suppressor when you want/rules allow!

 

[elephantrider]

Re: Barrel Machining for weight/accuracy

I wouldn't worry about fluting or re-profiling degrading the accuracy. If there is some sort of degradation, I doub't it would be anything you would notice. You just need to be careful about who you allow to do it. Also be equally careful about who threads/crowns your barrel as that can be a disater if done improperly.

If you add a suppresor your rifle, it will be overgassed. Decreased bolt unlocking time, increased carrier velocity, increased chamber and action fouling. None of which are good for accuracy or service life. There is a good thread in the semi-auto section started by George from GA Precision with his thoughts on putting a suppressor on a semi-auto.