New Build Appears “over-gassed”

[MaineMan2]

Took a new 308 AR build for its first range visit today. Ejection is very far forward - about 1:30 - and the cases are flying 10+ feet. Winchester white box M80 ammunition. The bolt is locking back correctly. I only fired single shots as my goal was to sight the rifle in.  I’d brought 2 rifles to sight in so I switched to the other one rather than shoot this 308 AR more. 

My first thought is that I need an adjustable gas block, but let me spell out as many details as I can and get your input. 

Aero Precision M5 upper and lower receivers, BCG, rifle length buffer tube, and M5 308 Rifle buffer kit. The buffer is 5.3” long and weighs 5.6 ounces. I used the exact same parts with a Fulton Armory 18-1/2” mid-weight barrel.  Fulton Armory specs an Armalite carbine AR-10 buffer tube for that barrel, so that is what I used. SLR adjustable gas block in the first build; however, I didn’t need to adjust anything.  That rifle ejects about 3:30 and runs like a top with the SLR adjustable gas block wide open. (Using the exact same ammo.)

I used a Faxon Big Gunner profile 16” barrel for this new build. Stock Midwest Industries gas block. A normal “mid-length” gas tube didn’t come close to the middle of the cam pin cutout in the upper receiver, so I got the longer Armalite AR-10 “carbine” gas tube. I don’t have the pin gauges I would need to measure the gas port diameter but am guessing that Faxon has over-sized it.  

So is an adjustable gas port the first step?

 I’m reluctant to mess with success, but swapping gas blocks between build # 1 and build # 2 might save me a little money. 

[shooterrex]

Switch upper and lower and see how the upper in question runs on the known lower.

That will show you if it's the upper or lower that is the problem.

If the gun is functioning right I don't care where my gun throws the brass.

Edited January 31, 2023 by shooterrex

[98Z5V]

4 hours ago, MaineMan2 said:

The buffer is 5.3” long and weighs 5.6 ounces.

Is this their specs, or have you measure this out?  Buffer should be 5.200" long, so that buffer bumper on the end needs sanded down 0.100" to get the buffer right.  5.6oz if tough to do in a standard .308 AR rifle buffer, most come in at 5.4oz.  That's just what you can work with, in the space that you have to work with it. Get rid of the Aero spring that came with that kit, and put in a known, verified, good spring.  Choices are Armalite EA-1095 spring, or the Sprinco Red spring.  Those two.  Or nothing. 

Rifle receiver extension should be exactly 9 11/16" internal depth.  Not close - right on. 

^^^Check that stuff first.  Verify it. Those simple changes will bullet-proof your Recoil System. 

4 hours ago, MaineMan2 said:

I used a Faxon Big Gunner profile 16” barrel for this new build. Stock Midwest Industries gas block. A normal “mid-length” gas tube didn’t come close to the middle of the cam pin cutout in the upper receiver, so I got the longer Armalite AR-10 “carbine” gas tube. I don’t have the pin gauges I would need to measure the gas port diameter but am guessing that Faxon has over-sized it.

^^^  Next up is this stuff.  Gas system.  Gonna need to know that barrel gas port diameter.  A cheap set of numbered drill bits (get the whole indexed set at Harbor Freight, if need be) will have that you need to measure that gas port diameter.  You'll know what clears, and the next one up that doesn't, and that will give you a pretty accurate gas port diameter.  Good job changing out that gas tube right away - that's an important step that many think isn't important. 

Post the info up on confirmation of this stuff, and we'll figure it all out. 

[MaineMan2]

OK inside of buffer tube is exactly 9-11/16” deep. Buffer measures 5.28” long. 5.6 oz. weight came off Aero Precision site - I don’t have a scale in that range. 
 

Checking with every drill bit, etc. I can find, the gas port is between .086” and .093”. Probably close to half way in between.  Based on everything I can find here, that is too big for a 16” barrel:

I will order a new spring when I order the adjustable gas block. I’m assuming dialing back the gas is in order so correct me if I’m wrong!  Need to know if it’s worth it to buy a set of numbered drill bits.

May be able to weigh the buffer later. The BCG doesn’t come close to the back of the receiver where the buffer sits at rest (don’t know what that’s called).  I know for an AR-15 you need to check and make sure BCG doesn’t hit that bridge before it bottoms out in buffer tube.  Clearance for AR-15 should be 2 quarters thick.  This was much more. 

[MaineMan2]

Turns out a #43 drill bit is in between the 2 data points I already have so I ordered just that bit. 

[dpete]

59 minutes ago, MaineMan2 said:

Turns out a #43 drill bit is in between the 2 data points I already have so I ordered just that bit. 

Any Ace hardware store has them too.

[MaineMan2]

45 minutes ago, dpete said:

Any Ace hardware store has them too.

Not my Ace Hardware 😜. I called them and #43 was “the only one we don’t have in stock”

[shooterrex]

The gun was working right?

You just didn't like where it was throwing the empties.

I would replace the spring w/the Armalite spring.

How many rounds through the gun?

It was locking back on an empty mag. Unless it's not feeding the next round what is the problem?

[dpete]

2 hours ago, MaineMan2 said:

Not my Ace Hardware 😜. I called them and #43 was “the only one we don’t have in stock”

Thats inconvenient!

[98Z5V]

On 1/30/2023 at 5:43 PM, MaineMan2 said:

Faxon Big Gunner profile 16” barrel for this new build. Stock Midwest Industries gas block. A normal “mid-length” gas tube didn’t come close to the middle of the cam pin cutout in the upper receiver, so I got the longer Armalite AR-10 “carbine” gas tube.

So, looking at this from a known barrel configuration, and changing for this one.  18" midlength gas with a 0.750" journal, .308 Win, that config needs a gas port that's 0.080"~0.085".  I know for a fact that 0.085" works better for any off the shelf ammo that you might pick up somewhere.  Confirmed.  Now, your barrel, 16" midlength gas, 0.750" journal.  You're 2" less in dwell time, so go up 0.005" on port diameter.  You'll need 0.085" minimum, and maybe as high as 0.090" for a gas port.  Sounds like you're close.  When that #43 drill bit comes in, if it doesn't fit - drill the gas port out with it.  That's gonna put you at 0.089", and that's good enough. 

Sounds like you're close - change that recoil spring like Rex said.  I bet that spring you received is too weak, can't control the ass and mass of that BCG properly, and is letting the speed get too high.  That's why your cases are flinging out there at 1:30...   Just the spring change will fix that.  I don't think you need an adjustable gas block - unless you just want one. 

[MaineMan2]

11 hours ago, 98Z5V said:

So, looking at this from a known barrel configuration, and changing for this one.  18" midlength gas with a 0.750" journal, .308 Win, that config needs a gas port that's 0.080"~0.085".  I know for a fact that 0.085" works better for any off the shelf ammo that you might pick up somewhere.  Confirmed.  Now, your barrel, 16" midlength gas, 0.750" journal.  You're 2" less in dwell time, so go up 0.005" on port diameter.  You'll need 0.085" minimum, and maybe as high as 0.090" for a gas port.  Sounds like you're close.  When that #43 drill bit comes in, if it doesn't fit - drill the gas port out with it.  That's gonna put you at 0.089", and that's good enough. 

Sounds like you're close - change that recoil spring like Rex said.  I bet that spring you received is too weak, can't control the ass and mass of that BCG properly, and is letting the speed get too high.  That's why your cases are flinging out there at 1:30...   Just the spring change will fix that.  I don't think you need an adjustable gas block - unless you just want one. 

If the gas port is the correct diameter, then the problem must be mass of the buffer + BCG -OR- spring. Armalite EA-1095 spring is inbound. So is a scale that should be “good enough”. 

[blue109]

On 1/30/2023 at 6:41 PM, shooterrex said:

 

If the gun is functioning right I don't care where my gun throws the brass.

I just want to second this statement. I want all my guns a little over gassed. 

[MaineMan2]

Scale and #43 drill bit arrived. Please note - I’m basically keeping notes here at this point. Waiting on Armalite spring and email response from Springco. 
 

I am curious though about what potential problems a slightly long buffer might cause. 
 

I measured something wrong the other day. The Faxon Big Gunner 16” barrel gas port is between 0.0807” and 0.086”.  That should be small, if anything, based on info above. 

#1 AP M5 enhanced 18” barrel, mid-length gas, rifle receiver extension - works fine

BCG 18.5 oz.

Buffer 5.3” 5.4 oz.

#2 AP M516” barrel, Armalite mid-length gas, rifle receiver extension - ejects 1:30 10-12 feet

WW 16” barrel, mid-length gas tube, carbine receiver extension

BCG 18.6 oz.

Buffer 5.28” 5.4 oz. 
 

I do need to put upper #2 on lower #1 to see if the problem is in the lower as we suspect. Finally, I’m not compulsive about where the brass ejects.  However, when it’s flying out almost straight ahead and going over 10’ something isn’t right. 

[shooterrex]

8 to 10 feet is about right.

[shooterrex]

I've got in AK on 223 that throws brass 25 to 30 feet.

Edited February 2, 2023 by shooterrex

[MaineMan2]

New Armalite EA-1095 spring arrived after sunset. Here’s a pic comparing it two Aero Precision “rifle extension” springs:

AP spring from “over-gassed” gun on top measures 12” long with 39 coils. 
Middle spring is another AP rifle spring from a build that works fine, has been assembled for months and shot a lot more. It measures 12-3/4” long, also 39 coils. 
Bottom is the EA-1095 measuring 13-1/2” long with 34 coils. 
 

I need to go to the range tomorrow and see how the rifles run with different springs, but I’m betting the spring was the problem. 

[98Z5V]

I'm gonna love the results of tomorrow's test session.  

[MaineMan2]

On 2/7/2023 at 9:35 PM, 98Z5V said:

I'm gonna love the results of tomorrow's test session.  

Yesterday didn’t go as planned. I was almost packed up to head to the range when unrelated fecal material hit the impellers.  I did go this morning. 
 

The Armalite EA1095 buffer spring made a small difference in the ejection pattern, moving it to about 1:30 from 1:00, and the brass was not being thrown as far. 
 

I also put a second EA1095 in my good 308 AR build that had the longer Aero Precision spring in it (see above). The ejection pattern didn’t change from 4:00 but the brass was closer together. 
 

I understand the guys saying “if it cycles ok and the bolt locks open, why worry?”  However, if my buffer weight and spring are both good, then the brass should not be flying forward so much. That tells me the BCG is moving too fast (correct me if I’m wrong about that). One thought that occurred to me is that maybe the gas port isn’t perfectly round. IOW tightest inside diameter looks OK but it lets too much gas through. I’m not sure how to check for that. 

[MaineMan2]

Had a great conversation with Alan at Springco. Short version:  he doesn’t make rifle length springs because there isn’t much demand. That makes sense. He offered to help me solve this problem but agreed that an adjustable gas block was the best solution. I found it very interesting that he says “each rifle is an example of one” and says that they after need to be tuned individually to get them running right. Based on our conversation, I will not hesitate to use Springco in the future for any build with a carbine buffer tube. 
 

We did briefly discuss the difference in length of the two Aero Precision springs. I will not be buying one of those again. 

[Matt.Cross]

On 2/9/2023 at 3:35 PM, MaineMan2 said:

However, if my buffer weight and spring are both good, then the brass should not be flying forward so much.

Quantify for me where it should be going and why. Bear in mind, this is not the standard AR platform, so I'm not interested in the old wives' tales from the standard AR side of things.... Where does the brass need to go, and more importantly, why?

[98Z5V]

5 hours ago, MaineMan2 said:

Had a great conversation with Alan at Springco. Short version:  he doesn’t make rifle length springs because there isn’t much demand. That makes sense. He offered to help me solve this problem but agreed that an adjustable gas block was the best solution. I found it very interesting that he says “each rifle is an example of one” and says that they after need to be tuned individually to get them running right. Based on our conversation, I will not hesitate to use Springco in the future for any build with a carbine buffer tube. 
 

We did briefly discuss the difference in length of the two Aero Precision springs. I will not be buying one of those again. 

I agree with this, from Alan at Sprinco.  You've done everything you can to the recoil system, so it's bullet-proof.  If ejection pattern is the thing here, and it's flying forward, and you want it like AR15 charts, and somewhere around 4 o'clock...   then you need to cut the gas down to accomplish that.  Nothing but an adjustable gas block, dialed down, will do that for you.

[98Z5V]

5 hours ago, MaineMan2 said:

he doesn’t make rifle length springs because there isn’t much demand.

Oh, he does make rifle length springs - he just doesn't realize it.  The Armalite EA-1095 is the only spring Armalite uses, and it works perfectly in either carbine recoil systems or rifle recoil systems.  That's due to the brilliance of Eugene Stoner, right there.

The Sprinco Red spring is a direct equivalent of the EA-1095.  So, he makes rifle springs...  the Red spring works perfectly in rifle or carbine .308 recoil systems, provided the Carbine Receiver extension you're using has an internal depth of 7 5/8". 

Sprinco also makes a variance of 5.56 rifle springs - listed directly on their website. 

Edited February 10, 2023 by 98Z5V

[MaineMan2]

Many thanks to all of you who helped me get this squared away. I thought I’d come back for a wrap up on this build. Total weight is 8 pounds 4 ounces. 

With an SLR adjustable gas block dialed back, brass goes into a nice circle about 8 feet away at roughly 3:00. To me, consistent ejection and not throwing brass too far tells me the rifle is running well.

Based on my experiences here Aero Precision is not precise. You can see the spring inconsistencies above. I had to juggle AP bolts between rifles to get bolt/barrel combinations that passed GO/NO GO tests. Clearly Faxon isn’t any better since they made the barrels; however, variances in the bolts were the only reason I didn’t have to send something back. Then there’s the Aero Precision handguard. I got this model because of weight and no need to mount anything forward other than the front sight. After tearing my hair out and bottoming put the front sight, I figured out that the small forward rail was not coplanar with the receiver rail (using precision straight edge). I really like Aero Precision’s “enhanced” handguard attachment system but this was aggravating. 
 

[MtnMike]

Glad to hear you got it running like you wanted.  That's a nice lightweight rig. Sometimes it can be a long journey getting our toys to play as nice as we like ... as I dig out the lapping kit for the latest.

[98Z5V]

Glad to see this one is running, and solved, brother - nice work.