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Discussion Starter #1 (Edited)
Finally got my 308AR mostly finished.



Still need the vendor specific bolt catch hardware to mount my bolt catch, but it's shootable as-is.

Built it to be the "biggie-me" companion to my go-to AR15 build, on bottom.


Muzzle Device is the Gamma 762 by VG6. Not sure when I'll have a chance to get to the range for a shake-down, but it looks the part.


Build list:
Zombie Defense 308AR Lower [local maker, good guy owner]
CMMG LPK (bolt catch pivot doesn't fit, wrong thread size)
Aero DPMS High pattern upper receiver
CMMG heavy profile [0.936" gas journal] 18" 1:10 Stainless barrel
Midwest Industries 308 DPMS 15" M-Lok Free Float Handguard
Stainless low profile set-screw gas block
Aero BCG
Magpul UBR stock
Heavy Buffers 308AR carbine buffer & spring
Geisselle SD-E trigger
BCM Gunfighter charging handle
Magpul MIAD 1.2 Grip
Phase 5 extended bolt catch [to be installed when I get the correct pivot hardware from the lower manufacturer]

The scope is a POS 3-9 x42mm NcStar that's only on the rifle for function testing (generally in a repeatable hold) while I decide on the final glass. It was cheaper than the removable irons I was going to get for this stage, and this rifle isn't going to live permanently with irons installed, so I didn't bother. Around here, the longest range this rifle will see is about 300yards, so I don't feel too guilty about the super cheapy glass at this point. As long as rounds hit on paper while I ensure feeding and reliability, it'll be fine.

On another note, I built this one because the lower was gifted to me [in a legally documented transfer] a few years ago. I likely WILL NOT build another. 308ARs take a lot of planning to ensure part compatibility (for example, there's 3 different barrel nut thread profiles available depending on brand), parts are expensive, and the cycle of parts in-stock is aggravating to plan a build, guaranteed when you're ready to order a certain batch of parts, they'll be out of stock for several months.

AR15's, even in obscure calibers, are way easier to piece together. Anyone looking to build one, you've been warned.

Might be a while, but I'll report back when I can get it on the range.
 

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The Phase 5 takes up a bit more real estate than say the Magpul or Troy but being integrated with the bolt catch makes it easier to manipulate IMO.
 

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Finally got my 308AR mostly finished.



Still need the vendor specific bolt catch hardware to mount my bolt catch, but it's shootable as-is.

Built it to be the "biggie-me" companion to my go-to AR15 build, on bottom.


Muzzle Device is the Gamma 762 by VG6. Not sure when I'll have a chance to get to the range for a shake-down, but it looks the part.


Build list:
Zombie Defense 308AR Lower [local maker, good guy owner]
CMMG LPK (bolt catch pivot doesn't fit, wrong thread size)
Aero DPMS High pattern upper receiver
CMMG heavy profile [0.936" gas journal] 18" 1:10 Stainless barrel
Midwest Industries 308 DPMS 15" M-Lok Free Float Handguard
Stainless low profile set-screw gas block
Aero BCG
Magpul UBR stock
Heavy Buffers 308AR carbine buffer & spring
Geisselle SD-E trigger
BCM Gunfighter charging handle
Magpul MIAD 1.2 Grip
Phase 5 extended bolt catch [to be installed when I get the correct pivot hardware from the lower manufacturer]

The scope is a POS 3-9 x42mm NcStar that's only on the rifle for function testing (generally in a repeatable hold) while I decide on the final glass. It was cheaper than the removable irons I was going to get for this stage, and this rifle isn't going to live permanently with irons installed, so I didn't bother. Around here, the longest range this rifle will see is about 300yards, so I don't feel too guilty about the super cheapy glass at this point. As long as rounds hit on paper while I ensure feeding and reliability, it'll be fine.

On another note, I built this one because the lower was gifted to me [in a legally documented transfer] a few years ago. I likely WILL NOT build another. 308ARs take a lot of planning to ensure part compatibility (for example, there's 3 different barrel nut thread profiles available depending on brand), parts are expensive, and the cycle of parts in-stock is aggravating to plan a build, guaranteed when you're ready to order a certain batch of parts, they'll be out of stock for several months.

AR15's, even in obscure calibers, are way easier to piece together. Anyone looking to build one, you've been warned.

Might be a while, but I'll report back when I can get it on the range.
 

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Bigboom, I'm doing my first AR308 and wondering how you like the UBR stock? Also what/who's Buffer and Spring did you use in the UBR stock?
 

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Discussion Starter #10
Bigboom, I'm doing my first AR308 and wondering how you like the UBR stock? Also what/who's Buffer and Spring did you use in the UBR stock?
I love my UBR stock, this is my second. It's heavy, but for heavy barrels, it balances out nicely off-hand. Some guys complain about the weight, and compared to an "average" AR build, the UBR does bulk it up a bit, but my other favorite rifles to shoot are all WWII Mil-surps, so I don't notice the weight honestly.

The Buffer and Spring was a gunshow purchase, adverstised as heavy buffer's brand, but without any packaging, and at a price that said definitely not, but worth experimenting with.

Here's the 308 Carbine Buffer next to a "normal" Carbine Buffer, in this case a Spikes ST-T2.


It is a 308 carbine buffer for sure though on length, comes in a little light, at 3.7 oz, to be a real heavy buffer's brand though. This was my first time weighing it, just a few minutes ago. It's still in the anticipated weight range though, so it'll probably work fine.


Compare that to the Spikes ST-T2, and it's not way off in left field, but I'll probably end up sourcing a heavier buffer, as most 308AR barrels are known to be over gassed. I'll check it first, but that's my plan.


For the Spring, seems a standard carbine spring, from left to right: BCM Carbine Spring, No-name gun-show carbine spring, no-name gun-show 308AR Carbine spring.


It might look like the 308 spring is longer, it just hasn't been fired yet to settle into length, it's the same number of coils and construction as the other no-name spring, and has the same dimensions and outward appearance as the BCM sourced spring. Could be this spring doesn't work correctly, but until I get it to the range, I'm not sure. Hand cycling, the bolt is moving the correct distance and everything, but it might still need to be shortened by a few coils, hard to say because there doesn't seem to be a defined "spec" for 308AR carbine springs.

Hope that clears up your questions, this is an area where 308AR builds differ and require a bit more tweaking than an AR15 build though.
 

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Discussion Starter #11
So, since the question was asked, I did some more digging, and discovered that the carbine spring I was using, wasn't going to work correctly. For one, the bolt was only allowed to travel *just* to the bolt catch groove, and was stopped by coil bind, rather than the buffer bumper.



I'm actually glad I hadn't had a chance to test fire this rifle yet, as it would certainly have been beating parts up more than it should.

So, to rectify, I took a trip to a local AR shop to see what they had, and what they had was this:


Which, according to that part number, is an Aero precision 308 Carbine specific action spring.

Comparing to the spring I had installed, the differences are pretty apparent.

Same length as the AR-15 carbine spring, but much fewer coils. That part is crucial, because the wire diameters are the same.


Curiously, the Aero spring is wound in reverse-direction to the no-name spring, which is reverse to even the known BCM spring. It won't make a difference in function, just something I noticed. The Aero spring also grips the buffer body quite a bit better, which I like.

While I was at the shop, I was able to weigh two other brand 308 carbine buffers they had for sale, one was 3.7oz, one was 3.75oz. So, for the time being, I'll see what my buffer does.

It would seem everything will have a much better chance of living happy now, as the bolt can retract clear of the ejection port, giving the [still not installed] bolt catch plenty of room/time to do it's job as well.



I knew a 308AR build would be a bit of a mix-n-match adventure, and that's certainly the case. Pleasantly, this fix was $5.35 out the door for a "correct" spring from a brand I can name.
 

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I assume most AR10 recievers use a standard mil spec tube then?

I might give the build a go in the future. Or just build a 6.5 Grendel and anything else for long range will be handled by a light weight bolt gun.
 

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Discussion Starter #13
I assume most AR10 recievers use a standard mil spec tube then?

I might give the build a go in the future. Or just build a 6.5 Grendel and anything else for long range will be handled by a light weight bolt gun.
The receiver extension (buffer tube) is the same between the 308AR/AR10 and the AR15. The difference is in the buffer length, and apparently the spring coil count in the carbine variant.

I've heard, but not confirmed, that the spring is the same specs between a rifle buffered AR15 and a rifle buffered 308AR, but since you see different part numbers for the two uses from pretty much every maker, I'm going to say that at a minimum, the spring rates are different.

I've also not had a 308 rifle buffer next to a AR 15 buffer at the same time, though my pops has both those configurations, so I may take some photos, measurements, etc, to document those differences in this thread while I'm at it.

I should note that I could have made the other spring work by cutting 4-5 coils off, but that doesn't keep the free length in the range it should be, so $5 is a worthy trade-off to avoid any hassles due to bubba'ing.
 

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Discussion Starter #14
Finally got my bolt catch pin parts from the lower manufacturer:
It's the stainless pin and set screws along with the bolt catch spring and plunger.


That let me finish test fitting my phase 5 tactical extended catch, and I had to take it to the flapper disc on my grinder to clearance for function, since the channel on this lower is a bit tight:





The shiny spots are where I had to take off a bit of material, final picture shows the worst-case fit left-right on the lower. I didn't get a before picture, but I just took off a little at a time until I was happy with the pivot action. I touched up the shiny sections with cold-blue before assembly.

Assembly went ok, aside from the fact that I broke off the tip of my precision allen driver trying to get it out of the set screw. I'll deal with that if I ever need to take out the bolt catch, which isn't very likely. If I'm very lucky, the broken tip will shake loose one day at the range, never to be seen again.

Anyway, I still haven't had the time to get to the range with this rifle yet, but it is now officially 100% complete.
 

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Nice write up i have the factory dpms. In looking to change out some parts. To get the most out of it.
I was looking at silent capture buffer. Your feedback on this please.
My trigger on this is fine so far, any thoughts on what can be improved please share thanks
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this is as she sits now i am going to freefloat the bbl , soon the glass primary arms 14x40 . She has 400 in the pipe. No ftf,fte nada
 

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Discussion Starter #16
Don't know the distance, but that's not a bad grouping.

On the trigger, it's all about the break. Precision triggers should hit a "wall" where you continually add pressure and the hammer falls without a discernible movement. Doesn't really matter if it's single stage or two stage, that's what I look for in a trigger for precision work. I really like the SD series from Geissele, I have the SD-E in my 308 build, which was previously in my AR-15, I now have an SD-C in that one to be NRA highpower legal for pull-weight. I've also used other triggers, and there's some nice ones available at good prices, but finding the good ones while having a flat bow is more difficult, which is why I keep coming back to Geissele, that's just personal preference though.

A really good budget trigger is the ALG-QMS, which I have in the wife's build.

Free float will help settle things down as well, just double check your tang height before you order a rail. There's 2 different heights DPMS has made, depending on how old the rifle is.

The silent capture buffer system is generally really good, I just couldn't justify the price up front. If I end up having some buffer/spring related issues, it would be on my short list as a remedy.
 

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Discussion Starter #17
Finally got this build on the range today for a few minutes.

Zero problems, only had time for 20 rounds before I lost the light, and was only able to shake down at 50 yards.

But again, locks back on empty, feeds every time.

Tested with some 2015 GGG Lithuanian 7.62x51 (which is actually some of the best NATO ammo you can get), last three shots after walking the scope in to POA/POI were a single clovered hole, so I'm really excited to see what it does with handloads when I can stretch the legs on it.

Brass shows some extractor marks, but not anything I can call deformation on the rim, just evidence of a sharp extractor claw. Zero ejector swipe seen so far, I'll inspect closer tomorrow.

Rifle does seem a tick overgassed, but I only say that because ejection grouping was a bit spread out.

The real story is the muzzle brake though, not especially loud from behind the trigger, but the side blast is not likely to earn friends on a crowded firing line. Extremely effective though, felt recoil is on par or a little lighter than my 5C1 flash hider equipped 5.56 AR.

Even more happy with the rifle now after getting a glimpse of its performance potential, but highly unlikely I'll build another large frame AR.
 

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The lack of proper mil-spec interchangeability turn you off on the large frame?

I keep thinking I want an AR10, but I think I'd be better served dropping the money in a Savage BA Stealth instead.
 

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Finally got this build on the range today for a few minutes.

Zero problems, only had time for 20 rounds before I lost the light, and was only able to shake down at 50 yards.

But again, locks back on empty, feeds every time.

Tested with some 2015 GGG Lithuanian 7.62x51 (which is actually some of the best NATO ammo you can get), last three shots after walking the scope in to POA/POI were a single clovered hole, so I'm really excited to see what it does with handloads when I can stretch the legs on it.

Brass shows some extractor marks, but not anything I can call deformation on the rim, just evidence of a sharp extractor claw. Zero ejector swipe seen so far, I'll inspect closer tomorrow.

Rifle does seem a tick overgassed, but I only say that because ejection grouping was a bit spread out.

The real story is the muzzle brake though, not especially loud from behind the trigger, but the side blast is not likely to earn friends on a crowded firing line. Extremely effective though, felt recoil is on par or a little lighter than my 5C1 flash hider equipped 5.56 AR.

Even more happy with the rifle now after getting a glimpse of its performance potential, but highly unlikely I'll build another large frame AR.
So what bolt do you have?

I like Tactical Springs for most things.

Heavy Buffers are pretty popular with 308. My build was a real eye opener. It's fun though.
 

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The lack of proper mil-spec interchangeability turn you off on the large frame?

I keep thinking I want an AR10, but I think I'd be better served dropping the money in a Savage BA Stealth instead.
The overwhelming majority of parts... For DIYers... Are all DPMS. You really have to go out of your way to do Armalite, or others. So as far as "interchangeability" problems... There really isn't any. It's all DPMS pattern.
 
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