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9/22/2017 12:11:25 AM
Posted: 9/22/2005 7:27:20 PM EDT
I have a twenty-inch DPMS HBAR barrel that I would like to modify. My first thought was to have a gun smith/machinist turn down the heavy mass under the hand guard not to a government profile, but to turn down in rings leaving 1/8” vanes with ¼” spacing between vanes for cooling fins. Also, I understand a twenty inch barrel can successful be shorten to eighteen inches without little effect to accuracy on a 1 to 9” twist rate.

Am I all wet or is this worth a $100 bill to reduce weight?
Link Posted: 9/23/2005 10:44:51 AM EDT
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Randall
Link Posted: 9/23/2005 10:53:28 AM EDT
Turn it down til it looks like a MacDonalds beverage straw!
Link Posted: 9/23/2005 12:53:39 PM EDT

Originally Posted By markm:
Turn it down til it looks like a MacDonalds beverage straw!





Link Posted: 9/25/2005 10:43:23 AM EDT
I think it's worth it. When I get my paperwork, I'm going to have my 11.5 heavy turned down to an M4 contour under the handguards.
Link Posted: 9/25/2005 10:46:45 AM EDT
why the heck would you want cooling fins? Adding surface area under the handguards isn't going to do jack for cooling.
Link Posted: 9/25/2005 11:44:52 AM EDT
More surface area = improved cooling abilities, even under handguards.
Link Posted: 9/25/2005 11:47:19 AM EDT
It'll give the effect of a heatsink, and yes, even under the handguards, though not to the same effect that it'd give outside where there was good airflow.
Link Posted: 9/25/2005 11:48:18 AM EDT

Originally Posted By MisterPX:
More surface area = improved cooling abilities, even under handguards.



I'm going to have to pull out a thermo book...there is going to be no air movement so any fins isn't going to be beneficial if I can remember back to undergrad engineering
Link Posted: 9/25/2005 12:13:31 PM EDT

Originally Posted By 1Gunner:

Originally Posted By MisterPX:
More surface area = improved cooling abilities, even under handguards.



I'm going to have to pull out a thermo book...there is going to be no air movement so any fins isn't going to be beneficial if I can remember back to undergrad engineering



This guy sounds smart enough to have already figgered out that he will have a solar powered cooling fan!
Link Posted: 9/25/2005 7:52:09 PM EDT
You guys have really hit a nerve with many good responses. I don't know how you figured out the blower so fast.

I would like to ask the head of the class another question?

In a 24” barrel with a 1 twist per eight inches makes three revolutions down the barrel. And a 21” barrel with a 1 twist per seven inches makes three revolutions. The rate of twist is a factor of three to one.

Considering a single 55-grain or 62-grain FMJ bullet. Is the rate of twist optimum for both loads and what factor does propellant and speed play?

What twist rate does a 55-grain bullet require for optimum performance?

What twist rate does a 62-grain bullet require for optimum performance?

Are two revolutions adequate?


Link Posted: 9/25/2005 8:02:53 PM EDT
The handguards are not a sealed item; if you get rain through the vents, you'll get air. Granted, it won't be great, but there will still be flow.
Link Posted: 9/25/2005 8:34:14 PM EDT
Link Posted: 9/28/2005 10:54:58 PM EDT
[Last Edit: 9/28/2005 10:55:31 PM EDT by Randall_Rausch]

Originally Posted By catusbill:
I would like to ask the head of the class another question?

In a 24” barrel with a 1 twist per eight inches makes three revolutions down the barrel. And a 21” barrel with a 1 twist per seven inches makes three revolutions. The rate of twist is a factor of three to one.

Are two revolutions adequate?




The number of twists that the bullet actualy does in the barrel does not matter.
The actual twist RATE gets the bullet spinning the proper rate in only a few inches of barrel travel.
55gr needs a 1:12" twist rate to stabilize at carbine and rifle lengths.
62gr needs about a 1:11" twist, but 1:9 is the next most commonly found twist rate that's faster than 1:12

Cutting the barrel down significatly will influence the velocity enough that you MIGHT (but probably not) need to be a little bit faster than 1:12.
A short 1:9 barrel would easily handle the 62gr bullets.

The propellant does not matter to bullet stability as long as it makes the proper velocity and the right amount of gas to run the action.

Randall
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