Barrel Profile

[maverick5582]

I want to re-profile my barrel to improve the balance of the rifle. I have the equipment to do it. When I bought the barrel blank I decided to profile it close to Lillja AR24 profile. At the present it is 0.975” behind the gas block and 0.830” in front of the gas block. Since this was my first AR15 project I did not know what I wanted for all of the components at the time.

I now wish to change it to something closer to the AR740 profile. I am thinking 0.850” behind the gas block, 0.750” for the gas block, and 0.740” in front of the gas block. I want to keep the 24” barrel length.

But, I do not want to do anything to hurt the accuracy of the barrel. I know that the AR740 barrel is 20” long and the AR24 is 23” long. I don’t foresee a problem with the reduced diameter and keeping the 24” length I have now. So I am asking for opinions before I go this route.

The barrel weighs about 4 lbs. now. I figure I can lose about 1 lb. in weight with the new dimensions.

My gas block is an old AA gas block with an odd-ball diameter of 0.900”. I want to be able to switch out the gas block for a Syrac Ordnance Click-Adjustable Gas Block AR-15, LR-308 Standard Barrel 0.750" Inside Diameter Clamp-On Low Profile gas block. This is another reason to change the diameter of the barrel.

[Tedward]

I would talk to a barrel manufacture. You might want to be cautious due to the stress relief that was done to the barrel after contouring. I know you can flut them afterwards, that is what barrel manufactures due, but double check when in the process the profiling is done in relation to the stress relief.

[maverick5582]

I would talk to a barrel manufacture. You might want to be cautious due to the stress relief that was done to the barrel after contouring. I know you can flut them afterwards, that is what barrel manufactures due, but double check when in the process the profiling is done in relation to the stress relief.

I turned the barrel from an 1.200" blank. There has been no stress relieving done at any time. I have turned 3 barrels now and never had any problem with them. Profiling just changes the diameter. The only thing right now I am concerned with is the longitudinal stiffness of the barrel. I understand the flute issue, but I am not a big fan of flutes anyway.

[dmsims21]

Turning the barrel down will reduce the stiffness. However, it should be fine.
To put things in perspective, my 24" .308 barrel is between .75 and .65ish for most of its length.
What you are planning should be fine.

[maverick5582]

I finally remember where I had seen this little bit of information:
Barrel Contour: You want some meat on the barrel. Between the upper receiver and the gas block don’t go real thin with a barrel (we like 1″ diameter if it’s workable weight-wise). When you touch off a round and the bullet passes the gas port, the gas system immediately starts pressuring up with a gas impulse that provides vibrations and stress on the barrel, especially between the gas block back to the receiver. A heavier barrel here dampens that. Staying a little heavier with barrel contour through the gas block area and out to the muzzle is good for the same reasons. ARs have a lot going on when you touch off a round and the gas system pressures up and the carrier starts moving (all before the bullet exits the bore) so the more things are made heavier and rigid to counteract that the better — within reason (I’m not advocating a 12-lb barrel).

[LRRPF52]

Different barrel manufacturers do drilling, rifling, profiling, fluting (if applicable), and stress relief at different times. Each has their preferred methods.

Most start with drilling of course.

Then rifling, although not all rifle at that point.

Some stress relieve earlier than others.

Some profile before other steps. What make of blank is it?

[maverick5582]

Different barrel manufacturers do drilling, rifling, profiling, fluting (if applicable), and stress relief at different times. Each has their preferred methods.

Most start with drilling of course.

Then rifling, although not all rifle at that point.

Some stress relieve earlier than others.

Some profile before other steps. What make of blank is it?

Green Mountain

[Tedward]

I finally remember where I had seen this little bit of information:
Barrel Contour: You want some meat on the barrel. Between the upper receiver and the gas block don’t go real thin with a barrel (we like 1″ diameter if it’s workable weight-wise). When you touch off a round and the bullet passes the gas port, the gas system immediately starts pressuring up with a gas impulse that provides vibrations and stress on the barrel, especially between the gas block back to the receiver. A heavier barrel here dampens that. Staying a little heavier with barrel contour through the gas block area and out to the muzzle is good for the same reasons. ARs have a lot going on when you touch off a round and the gas system pressures up and the carrier starts moving (all before the bullet exits the bore) so the more things are made heavier and rigid to counteract that the better — within reason (I’m not advocating a 12-lb barrel).

Well like I just read from LRRPF52, every manufacture does things different.

JP Rifles uses .650 under the hand guard on there Grendel between the upper receiver and the gas block so who really knows. They do use there thermal dissipation to stiffen the barrel as they note on the link below.
http://www.jprifles.com/buy.php?item=JPSM6.5G-22M8
http://www.jprifles.com/1.4.5_hs.php

As far as contouring, I talked to my machinist and he has blanks that are 1.5" from one end of the blank to 1.25 at the other end. All contouring and cambering is done after rifling and he doesn't do any stress relief. According to him its not needed and he has done lots of barrels in all kinds of lengths. He makes Silencers and all kinds of custom SBR rifles so a 5lb barrel blank turns into a 1.5lb or less barrels sometimes.

[maverick5582]

Well like I just read from LRRPF52, every manufacture does things different.

JP Rifles uses .650 under the hand guard on there Grendel between the upper receiver and the gas block so who really knows. They do use there thermal dissipation to stiffen the barrel as they note on the link below.
http://www.jprifles.com/buy.php?item=JPSM6.5G-22M8
http://www.jprifles.com/1.4.5_hs.php

As far as contouring, I talked to my machinist and he has blanks that are 1.5" from one end of the blank to 1.25 at the other end. All contouring and cambering is done after rifling and he doesn't do any stress relief. According to him its not needed and he has done lots of barrels in all kinds of lengths. He makes Silencers and all kinds of custom SBR rifles so a 5lb barrel blank turns into a 1.5lb or less barrels sometimes.

Tedward:

I take NRA Gunsmith courses at Montgomery Community College in Troy, NC. I turned this blank in the Alternate Caliber AR-15 course last March. There were 7 other guys taking the course and we all had problems finding enough parts to build our AR 15. If it was not for the instructor I would have never found a bolt for my barrel. Some built 300 Blackouts and the rest 6.5 Grendel. The course lasted 5 days.

I wish I knew then what I know now. I would have made some changes then. But, it was a good experience and I enjoyed it very much. They offer these courses at various community colleges across the nation. All that is required is a little knowledge in how to read blueprints, use a micrometer, and follow instructions. I am fortunate in that I have been doing this kind of work since high school. I have a lathe, milling machine, and TIG welder in my shop that allows me to work on most of my projects.

I would recommend these courses to anyone who has the time to do them as some of them are offered on the weekends as well. I wish I lived close enough to a range that offered marksmanship courses. That would be a big plus I am sure.