Build a Light Weight 308 AR

[igofast]

Thought I would add some data.  Relatively new to the LR-308 platform.  I got a 3rd hand custom DPMS with a SASS barrel and CF forearm that shoots old mil surp ammo way better than anything should, but that SOB is heavy with no provision for a front sight that's needed for 3-gun Heavy Metal(or He Man) division.

 

Here's what I used for my build.  Didn't come in quite a svelte as the previous one mentioned:

Part          Weight(oz)	Description
Upper	      14.2	DPMS .308 A3 Receiver (Stripped), Part Number 308-FTT-EA
BCG	      17.55	APF Armory Bolt Carrier LR-308 Nickel Boron, APF Armory Bolt Assembly LR-308 Nickel Boron
- comp kit     0.8	DPMS - 308AR BOLT COMPLETION PACK(Firing Pin, Cam pin, Retaining Pin) 
CH	       1.3	Bravo Company / Vltor 7.62mm 308 Mod 4 Charging Handle
Barrel	      38.3	Rainier Arms Match™ .308 Barrel - 16 (ODS); Stainless Steel Match Grade Blank
Gas Block      1.55	Low Profile Adjustable Gas Block Set Screw Mount .750 Mil-Spec 556 nitrided
Gas Tube       0.7	Spikes Tactical - Melonite-Coated Gas Tube, Mid-Length
Handguard     13.7	Samson Evolution Series Customizable Free Float Handguard DPMS LR-308 with Low Profile Upper Receiver Aluminum Black
Sight - F      1.3	SAMSON MANUFACTURING CORP - AR-15/M16 FIXED FRONT SIGHT
Sight - R      1.55	SAMSON MANUFACTURING CORP - AR-15/M16 FIXED REAR SIGHT
Comp	       3.85	Fortis Compensator RED™ 308 w/ crush washer

Even though this adds up to 94.8 oz, the completed build comes up as a weighted 95.1(5.9 lbs) - so obviously some rounding errors.  That's a little over 2 lbs lighter than the 18" bull barreled SASS scoped upper.  

 

With the lower I'm at 8.7 lbs.  Looks like I can trim a little weight if I go from my full A2 stock to a collapsible - which I may do.

 

 

 

 

 

 

No comments about how the comp is installed upside down.  I realized it just as I was tightening it down and then broke my fixture as I was attempting to take it back off.

[AdamO]

Hey um... you know you put your compensator on upside down!   <lmao> <laughs>

 

Seriously though, I'm down a very similar path as yourself building it from scratch and I'm on track for about 8.2-8.4lbs all said and done.  You can check it out here: 

 

You've got a nice selection of parts there.  I especially like that rail.  Does 13.7oz account for the barrel nut? 

 

The three main places you can save on weight are the barrel, the handguard, and the stock.  After researching lots of different options I've decided that a collapsible carbine style stock was the way to go for me.  If you want a full rifle stock that's light check out the Ace ARFX skeleton stock. 

 

Also the JP low mass bolt carrier will save you a few more ounces over the full size slug and since you have adjustable gas already it shouldn't be a problem. 

 

You could save about 2oz off the nose switching to an A2 flash hider (you're gonna be taking apart anyway!! :D)  Or you could even go with a simple thread protector for maximum savings.   

 

Even a shorter rail would save you some weight and cover your mid length gas system on that 16"er but I totally understand wanting the longer setup for your front sight... 

 

I know re-investing in parts you already have doesn't sit well for anyone, but if every oz. counts, you might want to look into it.

 

Oh, and welcome to the site from AZ! 8)

Edited September 11, 2014 by AdamO

[unforgiven]

Welcome from Indiana brother,looks like a good plan for a very nice rifle.

[igofast]

 

I especially like that rail.  Does 13.7oz account for the barrel nut? 

 

 

It does. It comes with a proprietary steel nut that just covers the threads(no timing of the nut needed for the gas tube).  Lots of negative reviews on it from Midways site mainly due to it clamping on instead of threading.  I won't say it's rock solid but after tightening it I feel it's solid enough to not move under normal conditions(I haven't encountered a 12+" rail that doesn't move if force is applied).  It fit like a glove with the DPMS specific lower and as you can see is same height(can't say I wasn't a little worried).

 

 

The three main places you can save on weight are the barrel, the handguard, and the stock.  After researching lots of different options I've decided that a collapsible carbine style stock was the way to go for me.  If you want a full rifle stock that's light check out the Ace ARFX skeleton stock. 

 

Considering going with a Mission First Tactical carbine stock.  Have it on my AR15 and love it.  However it's low on my list.  For me the weight closer to the body doesn't matter as much as the weight further out.

 

 

 

 

Also the JP low mass bolt carrier will save you a few more ounces over the full size slug and since you have adjustable gas already it shouldn't be a problem. 

 

You could save about 2oz off the nose switching to an A2 flash hider (you're gonna be taking apart anyway!!  :D)  Or you could even go with a simple thread protector for maximum savings.   

 

 

Waiting on TacCom to release their UL recoil system for the .308.  Again, I run it on my AR15 and it works great.  Much nicer to spend $25 on an upgrade than a few hundred.  Of course bigger benefits would be realized by doing both.

The comp does wonders on a .223 platform, I'm hoping it will make the .308 stay as flat.  If it does I'm willing to pay the few oz for the performance.  Though I do not envy anyone standing next to me.  That effer is going to be loud.

 

 

Even a shorter rail would save you some weight and cover your mid length gas system on that 16"er but I totally understand wanting the longer setup for your front sight... 

 

Exactly.  This is another place that I don't mind a few oz's as not only do I get the longer sight radius, but also can hold my hand out further.

 

 

Thanks all for the welcomes.  Now I'm off to say how awesome my F class skilz are and you you are all idiots for choosing anything less than a 32" barrel.   :))

Edited September 12, 2014 by igofast

[AdamO]

Thanks all for the welcomes.  Now I'm off to say how awesome my F class skilz are and you you are all idiots for choosing anything less than a 32" barrel.   :))

 

HAHAHAA!!!  Let me know how that works out for you!   There's a certain gorilla lookin dude around here that I'm sure would LOVE to meet you!  <lmao> <laughs>

Edited September 12, 2014 by AdamO

[Robocop1051]

He's in the fridge right now, waiting....

[BLKSHEEP]

He's in the fridge right now, waiting....

 

LIKE THIS!!

 

"I'll be hiding in his fridge, and snatch out his soul when he goes for his midnight snack..."  :eek:   <laughs> Shits I've lost 10 lbs. just thinking about that. <lmao>

[Rsquared]

igofast might have just found an easy way to slide into this crowd.

laughing

[breastroker]

My Gen 1 DPMS lower SSA upper with 18 inch Faxon mid-weight barrel weighs 5.73 pounds without scope and base. With heavy Vortex 6-24x50 scope it weighs 7.23 pounds.

 

The upper weighs almost 3 ounces too much. A pencil profile 18 inch barrel could weigh less than 32 ounces, maybe as low as 28 ounces. Titanium carriers are coming soon, another 3-4 ounces less.

 

So a hunting MSR in 308 could be as light as 5 pounds.

[FaxonNathan]

My Gen 1 DPMS lower SSA upper with 18 inch Faxon mid-weight barrel weighs 5.73 pounds without scope and base. With heavy Vortex 6-24x50 scope it weighs 7.23 pounds.

 

The upper weighs almost 3 ounces too much. A pencil profile 18 inch barrel could weigh less than 32 ounces, maybe as low as 28 ounces. Titanium carriers are coming soon, another 3-4 ounces less.

 

So a hunting MSR in 308 could be as light as 5 pounds.

 

We'll accept that .308 pencil challenge. 

 

The issue is you have to use a certain amount of material to maintain safety factor on the barrel. 

[breastroker]

Nathan,

 

I really want a sub 32 ounce barrel, with rifle length gas system. I believe the area next to the barrel nut holds a lot of extra mass that doesn't do anything. A Savage bolt gun is 1.06 inches while the AR 308 is 1.2 inches. That has to be close to 2 ounces there alone.

 

So maybe one inch length at 1.2" OD, then immediately down to 1.0". The step down could start immediately after your extension.

 

About 0.725" before the gas block and 0.625 after gas block. Maybe even tapering to 0.60" at the muzzle brake area.

 

The gas block area does not need to be over 2 inches long. Most light weight gas blocks are barely over 1 inch long.

 

My 18 inch 6.8 SPC started out at 28 ounces, cut it to 16.1 inches and re-profiled. Now 21.9 ounces and shoots o.5 MOA for the first 3 shots. Under I inch after that.

 

So a true light profile barrel AR 308 could weigh 30-31 ounces.

 

That would be the start of many 6 pounds AR 308s.

 

And get my AR 308 closer to 5.0 pounds

[blue109]

If you Havnt seen it, criterion posted pics of the prototype 16" rifle gas barrel in their forum here. Profile looks awesome. It will be my next barrel for sure.

[Matt.Cross]

Nathan,

 

I really want a sub 32 ounce barrel, with rifle length gas system. I believe the area next to the barrel nut holds a lot of extra mass that doesn't do anything. A Savage bolt gun is 1.06 inches while the AR 308 is 1.2 inches. That has to be close to 2 ounces there alone.

 

So maybe one inch length at 1.2" OD, then immediately down to 1.0". The step down could start immediately after your extension.

 

About 0.725" before the gas block and 0.625 after gas block. Maybe even tapering to 0.60" at the muzzle brake area.

 

The gas block area does not need to be over 2 inches long. Most light weight gas blocks are barely over 1 inch long.

 

My 18 inch 6.8 SPC started out at 28 ounces, cut it to 16.1 inches and re-profiled. Now 21.9 ounces and shoots o.5 MOA for the first 3 shots. Under I inch after that.

 

So a true light profile barrel AR 308 could weigh 30-31 ounces.

 

That would be the start of many 6 pounds AR 308s.

 

And get my AR 308 closer to 5.0 pounds

 

 

The chamber area is directly in front of the barrel extension. Good luck getting companies to take on the liability of getting that area too thin. "Here's your custom-ordered pipe bomb sir, enjoy in good health."

[darb]

The chamber area is directly in front of the barrel extension. Good luck getting companies to take on the liability of getting that area too thin. "Here's your custom-ordered pipe bomb sir, enjoy in good health."

Does anybody know if the extra mass on the AR chamber area needs to be more than in a bolt action? I would get some professional verification before removing an material.IMO  Keep posting and Good Luck

[jtallen83]

I can't give you any fancy calculations or formulas to back it up but don't mess with the chamber area, hell don't mess with what a manufacturer will give you anywhere on a barrel. The extra mass for a semi vs. bolt may relate to the ability of the mass to absorb and disperse the extra heat involved in rapid fire (mag dumps), heat weakens the metal causing failure. Why either risk a failure or limit yourself to what a bolt action could do for a rate of fire.

One of the main reasons the original version of the AR-10 failed the military trials was a lightweight barrel that burst.

[FaxonNathan]

I can't give you any fancy calculations or formulas to back it up but don't mess with the chamber area, hell don't mess with what a manufacturer will give you anywhere on a barrel. The extra mass for a semi vs. bolt may relate to the ability of the mass to absorb and disperse the extra heat involved in rapid fire (mag dumps), heat weakens the metal causing failure. Why either risk a failure or limit yourself to what a bolt action could do for a rate of fire.

One of the main reasons the original version of the AR-10 failed the military trials was a lightweight barrel that burst.

 

That is correct. We build our barrels to handle standard MIL-SPEC firing sequences, which include full-auto fire. 

 

There are "fancy calculations" which we undertake and will get close on when we go down the pencil .308, but for now, the Medium-Tapered is a great place to start. We created that one to balance accuracy and maneuverability, much like the GUNNER series of barrels in 5.56. 

[FaxonNathan]

Nathan,

 

I really want a sub 32 ounce barrel, with rifle length gas system. I believe the area next to the barrel nut holds a lot of extra mass that doesn't do anything. A Savage bolt gun is 1.06 inches while the AR 308 is 1.2 inches. That has to be close to 2 ounces there alone.

 

So maybe one inch length at 1.2" OD, then immediately down to 1.0". The step down could start immediately after your extension.

 

About 0.725" before the gas block and 0.625 after gas block. Maybe even tapering to 0.60" at the muzzle brake area.

 

The gas block area does not need to be over 2 inches long. Most light weight gas blocks are barely over 1 inch long.

 

My 18 inch 6.8 SPC started out at 28 ounces, cut it to 16.1 inches and re-profiled. Now 21.9 ounces and shoots o.5 MOA for the first 3 shots. Under I inch after that.

 

So a true light profile barrel AR 308 could weigh 30-31 ounces.

 

That would be the start of many 6 pounds AR 308s.

 

And get my AR 308 closer to 5.0 pounds

 

We hear you and thanks for the suggestion on the profile. Getting there will be a mathematical challenge, as we will not exceed a certain "safety factor" (the formal mathematical rating for a barrel's ability to handle pressure as it gets hot.) 

 

For us, it will be dictated by that safety factor, we'll get as thin as it allows. 

[darb]

We hear you and thanks for the suggestion on the profile. Getting there will be a mathematical challenge, as we will not exceed a certain "safety factor" (the formal mathematical rating for a barrel's ability to handle pressure as it gets hot.) 

 

For us, it will be dictated by that safety factor, we'll get as thin as it allows. 

Just out of curiosity: By getting the barrel down as,  "thin as it allows". By how much would that effect round count in the life of the barrel, or would it?

[FaxonNathan]

Just out of curiosity: By getting the barrel down as,  "thin as it allows". By how much would that effect round count in the life of the barrel, or would it?

 

Assuming general shooting or even 3-gun, very little. Full-Auto fire (getting it really hot), would have a noticeable decrease. 

 

However, it would effect groupings when the barrel is hot (which it gets hot faster). How much is yet to be seen, but the thinner you go, the less metal to support itself. 

[darb]

I have seen people shave stock off the barrle of an AR , it always ends with a gun that shoots poor groups.

 if you want a lighter gun go to a 5.56

Weight and speed seem to be an attraction like bees to flowers, but the yield may differ. I enjoy watching as I can learn from the failures and successes.I say strive for limits never accomplished, as long as safety requirements are met. Happy Shooting.

[blue109]

So everyone should have pig 12lb 308s? This guy shoots just fine, and is much nicer to carry than the HB it started as.

[jtallen83]

So everyone should have pig 12lb 308s? This guy shoots just fine, and is much nicer to carry than the HB it started as.

Curious how you decided how much to shave? I'm afraid I would develop a flinch from not knowing how much was too much.

[breastroker]

I cut 4 inches off my Douglas Savage barrel, groups went from 0.60 to 0.18 inches.

 

My 3.5 pound AR 15 in 6.8 only shoots 0.5 inch 3 shot groups. Opens up to a huge 1 inch when I fire rapid. The 18  inch barrel was cut to 16.1 inches and re-profiled much thinner.

 

My 5.7 pound AR 308 shoots sub 0.5 inch groups with the light weight Faxon barrel. This one also opens up to huge one inch groups.

 

There is a reason AR 308s generation 1 did not take off, and it is the weight. But at 6 pounds it would sell great.

 

I am just seeing how far it can go.

 

I have lined up a 12 ounce lower (minus 3-4 ounces), a 11 ounce upper (minus 3 ounces), and a 32 ounce fluted 18 inch 308 barrel (cuts 4 ounces). Combined with a titanium carrier (minus 3 ounces) that would easily be UNDER 5 pounds.

[breastroker]

We are talking about 5 to 10 round magazines, not the 60-100 round mags.

 

My 6.8 magazine weighs 2.0 ounces and with 5 rounds it weighs 7 ounces. 120 SST Hornady

 

1911, my rifles weigh LESS than AR 15s in 556. Shoot softer and shoot with better accuracy.

[98Z5V]

There are "fancy calculations" which we undertake and will get close on when we go down the pencil .308, but for now, the Medium-Tapered is a great place to start. We created that one to balance accuracy and maneuverability, much like the GUNNER series of barrels in 5.56. 

 

Yes, yes it is - I love my 16" medium taper i got from you - damn, that is a nice barrel.  <thumbsup>