Lr-308 build. Precision questions

[Nikguy]

TLDR: I have a AR 308 build with parts from multiple places, and I'm getting 4-4.5 moa at best. (Best group with Hornady black 168 gr so far) Should I expect better/worse? I can shoot 1.25 moa on my most accurate ar15.

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So to start this out, if I was going to do it again, I'd 100% just buy a pre-built gun from a nice manufacturer (or maybe intentionally build one from the ground up). But anyway:

So some years ago I bought a used DPMS lr308 ap4 with an aftermarket bolt/grip. Today, the only thing left that was dpms, is the upper/lower receiver, takedown pins, and the bolt catch (I know, money waisted.... But I didn't know what I know back then).
The BCG is the only aftermarket part that the previous owner supplied and I can't identify it. My build is generalized as follows.

Faxon 16in "big Gunner" steel barrel.
Cmc flat trigger
Surefire break
Fixed a2 stock
Lancer carbon fiber free float hand guard
*Mystery nickel coated BCG (I added a high strength extractor)
Sig tango 4 3-12x44 FFP scope


I've tried a few types of rounds and I've reduced the groups to 4-4.5ish moa with Hornady black 168gr. I shoot tighter about 1.25ish with my best rifles.
Do you see anything based on my explanation that might tighten my group at all (maybe the mystery bolt??). I'm at the point that I've already thrown so much money at it that I might as well keep going.

What do you think?
Feel free to dis any of the stuff I bought. The rifle has slowly evolved over the years as i gained knowledge and money.... also open to ammo suggestions.

Thanks
-Nick

 

[MojoNixon]

What’s the twist rate of the Faxon barrel?

 

[Nikguy]

What’s the twist rate of the Faxon barrel?

1:10

 

[801tactical]

Man, that sounds like it could be a tough one. Barrel would be my first place to look, however, it's likely one of the most expensive to look. If you can find a loaner from someone, you may want to try a different bolt carrier (if you can ensure the headspace with the loaner).

The barrel may be picky. One of my Creedmoors likes heavier bullet than the others.

 

[Nikguy]

Man, that sounds like it could be a tough one. Barrel would be my first place to look, however, it's likely one of the most expensive to look. If you can find a loaner from someone, you may want to try a different bolt carrier (if you can ensure the headspace with the loaner).

The barrel may be picky. One of my Creedmoors likes heavier bullet than the others.

Yeah, the bolt is the elephant in the room. It is quite possible though that maybe I need to keep trying new ammo. After a while though, I'm gonna be burning a decent amount of cash on ammo that doesn't group. Also, the barrel is brand new as of a month or so ago (I thought the barrel would be my ace in the hole).

 

[MojoNixon]

Both of my .308 guns are 1:10 and thrive on bullet weights from 145 to 155 gains. Give the Hornady Black 155 a shot also try 150 gr. Federal Fusion.

 

[801tactical]

Yeah, the bolt is the elephant in the room. It is quite possible though that maybe I need to keep trying new ammo. After a while though, I'm gonna be burning a decent amount of cash on ammo that doesn't group. Also, the barrel is brand new as of a month or so ago (I thought the barrel would be my ace in the hole).

How many rounds through it after cleaning it? My Bergara needed over 50 before it started grouping. But your groups should be much better than what they are as a starting point.

 

[Nikguy]

How many rounds through it after cleaning it? My Bergara needed over 50 before it started grouping. But your groups should be much better than what they are as a starting point.

I did 60. But maybe I need more? I'm going to try to look into some different rounds and maybe see what I can do about the BCG.

 

[801tactical]

I did 60. But maybe I need more? I'm going to try to look into some different rounds and maybe see what I can do about the BCG.

Doubtful that will help on the round count. Mine went from 3/4 moa to less than 1/2 moa around 50 rounds. You're starting way too big. Twist may be a factor. One of the first and easiest things to check is the scope mounts. What are your mounts like and what did you torque them to as final?

 

[Nikguy]

Doubtful that will help on the round count. Mine went from 3/4 moa to less than 1/2 moa around 50 rounds. You're starting way too big. Twist may be a factor. One of the first and easiest things to check is the scope mounts. What are your mounts like and what did you torque them to as final?

I'm not sure about the torque honestly. It was a while ago. Just checked and everything is tight for sure. The mounts I have are the Burris "Xtreme Tactical" 6 bolt 30mm rings.

I get how changing the torque would change the POA, but would having the wrong torque (if it's tight) cause a random shift in point of aim?

 

[Nikguy]

I actually put a total of 120 rnds through the new barrel. I forgot about the previous weekend, shooting some crap ammo

 

[LukeNCMX]

4-4.5 MOA at best in a precision rifle is bad. We are shooting from prone right? Check all your torque values for everything. Use the ring torque specs from the scope manufacturer. Try a different scope. Shoot some Federal Gold Medal Match 168 and 175.

 

[Nikguy]

4-4.5 MOA at best in a precision rifle is bad. We are shooting from prone right? Check all your torque values for everything. Use the ring torque specs from the scope manufacturer. Try a different scope. Shoot some Federal Gold Medal Match 168 and 175.

That's what I thought. I've used the bench at a range I use to sight in guns. I've tried both sand bags or my Harris bipod. Same results either way. I'll look up torque specs, but I'll either have to order a tiny torque wrench or try to catch up with a friend soon who owns one.

Do you think the bolt/carrier could affect accuracy some how? It's the only part I left up to the previous owner.

 

[801tactical]

I'm not sure about the torque honestly. It was a while ago. Just checked and everything is tight for sure. The mounts I have are the Burris "Xtreme Tactical" 6 bolt 30mm rings.

I get how changing the torque would change the POA, but would having the wrong torque (if it's tight) cause a random shift in point of aim?

My first though was that the scope was loose. You've confirmed that it is not. Those are good rings, I use them on some guns. Over torquing the rings to the scope can break the erector (ask how I know). But if the scope is zeroing fine that's not the problem. I'm looking for the simple and inexpensive things first. You may want to switch the scope out, I see them from time to time that are just bad and won't hold a zero, shifting around on recoil. That's a cheap thing to look at.

Next on my list would be to look at the bolt carrier since it's an unknown. Following that, you may have to look at the barrel.

 

[801tactical]

That's what I thought. I've used the bench at a range I use to sight in guns. I've tried both sand bags or my Harris bipod. Same results either way. I'll look up torque specs, but I'll either have to order a tiny torque wrench or try to catch up with a friend soon who owns one.

Do you think the bolt/carrier could affect accuracy some how? It's the only part I left up to the previous owner.

I think it could, but I really doubt that it's the culprit. My guess at this stage is that it's the barrel or a bad scope. Do you have backup sights or another scope to try?

 

[Nikguy]

I think it could, but I really doubt that it's the culprit. My guess at this stage is that it's the barrel or a bad scope. Do you have backup sights or another scope to try?

Ha well I suppose I do... I might have to take something off one of my other guns then re-sight it later.

But yeah didn't think about the scope. I'll try it next time.

 

[Yondering]

Do you think the bolt/carrier could affect accuracy some how? It's the only part I left up to the previous owner.

No, not at the level you're dealing with; the bolt and carrier have nothing to do with causing 4 moa shot patterns.

Barrel, bullets, and bedding (how securely the barrel is fixed in the receiver). Those three are the keys for accuracy. Also verify the scope and mounting; loose mounts or a bad scope are a common cause of what you're seeing.

You should make sure the rifle isn't severely overgassed, as that can cause some accuracy issues, but probably not as extreme as you've described.


One other thing to consider is that large frame ARs are more difficult to shoot accurately. If 1.25 moa is the best you're able to do with an AR15, you may be doing something not quite right during shooting that the large frame is a lot less tolerant of. Tuning the recoil system (either adjusting gas or heavy buffer) correctly can help a little with that, but shooting technique is critical with these.

 

[Nikguy]

No, not at the level you're dealing with; the bolt and carrier have nothing to do with causing 4 moa shot patterns.

Barrel, bullets, and bedding (how securely the barrel is fixed in the receiver). Those three are the keys for accuracy. Also verify the scope and mounting; loose mounts or a bad scope are a common cause of what you're seeing.

You should make sure the rifle isn't severely overgassed, as that can cause some accuracy issues, but probably not as extreme as you've described.


One other thing to consider is that large frame ARs are more difficult to shoot accurately. If 1.25 moa is the best you're able to do with an AR15, you may be doing something not quite right during shooting that the large frame is a lot less tolerant of. Tuning the recoil system (either adjusting gas or heavy buffer) correctly can help a little with that, but shooting technique is critical with these.

Ok thanks for the response (everyone). Here is what I will try. If a step doesnt help, I'll move down the list :

1. Have someone else more skilled shoot it (if I find someone)
2. Try new ammo again (and hopefully repeat step one)
3. Try my ar15s trijicon 1-8 to test the scope
4. Re-assemble the barrel. (Maybe something weird is going on?
5. New bolt
6. Adjustable gas block? (mid length gas on a 16in barrel is where I'm sitting now)
7. Buffer weight?
8. ? ...

 

[8RoundPing]

1. Take barrel off. Put barrel back on. Make sure to use appropriate barrel torque.
2. Take scope out of mount. Take mount off gun. Reassemble.
3. Try a different optic. If same problem persists then rule out scope mount etc.
4. Switch ammo to a known standard like Fed Gold Medal 168 or if you reload you have more options.

I went through some similar difficulties with my CMMG 308, 1in10 twist, but not such bad groups. I disassembled and reassemble entire gun, retorqued barrel, replaced handguard, and groups went down to 2" from 3 or 3.5". Eventually I settled on Sierra 155 Palma bullets with Varget and I'm down to about 1" now.

 

[Nikguy]

Ok. Thanks for the help.