Barrel Extension Doesn’t Go Far Enough into Upper Receiver

[MaineMan2]

First time 308 AR builder so please bear with me. If this is in the wrong forum section I apologize. 
5
Aero Precision M5E1 “enhanced” upper receiver. “Accepts standard DPMS 308 components.”

Fulton Armory 308 AR 18.5” barrel made by Criterion for FA. “For use on DPMS/Remington/FA pattern rifles…”

The “M4 feed ramps” do not match up flush with the barrel extension feed ramps. It looks like the barrel extension needs to go 1/16” or more further back into the receiver for the receiver feed ramps to flow into the barrel extension feed ramps. However, that is not physically possible. Yes, I am positive that the barrel extension alignment pin is all the way into the groove in the upper receiver. 
 

Obviously I’ve got a compatibility problem, but I don’t know which manufacturer to contact first in order to resolve the problem.  I foolishly thought that if both parts are “DPMS” pattern, then they should fit together. The barrel is the first part I spec’d so I’m not keen on sending it back. Unfortunately, there are several things I really like about Aero Precision’s “enhanced” upper receiver and I already picked up their matching M5 lower. I do NOT want to return an FFL item…

On a side note, just before I hit Checkout on the Fulton Armory site, I thought I’d look at their rifle length gas tubes. 15.5” long “for use on FA-308 AR barrels… will also work on “Armalite AR-10”. The barrel description simply says “rifle length gas system” with no mention of Armalite pattern. Based on discussions here, I ordered the FA “rifle length” gas tube. It lines up correctly with the cam pin cutout in the upper when I dry fit everything - adding to the feed ramp mystery. 
 

Thanks in advance for suggestions on how to approach this problem. 

[98Z5V]

You tighten the barrel nut yet?

[MaineMan2]

18 minutes ago, 98Z5V said:

You tighten the barrel nut yet?

No - just dry fitting parts to check compatibility. Honestly, I’m waiting on the correct barrel wrench to arrive along with headspace gauges. 

[98Z5V]

8 minutes ago, MaineMan2 said:

No - just dry fitting parts to check compatibility. Honestly, I’m waiting on the correct barrel wrench to arrive along with headspace gauges. 

Is the barrel flange flush with the face of the receiver?

[MaineMan2]

2 hours ago, 98Z5V said:

Is the barrel flange flush with the face of the receiver?

Good question. I think so; however, the Aero Precision enhanced upper is different so you can’t see directly.  The barrel nut is threaded on the outside and it goes into a housing also used to mount the handguard later. The locator/alignment pin on the barrel extension seems to be as far back as possible but I can’t see it. Need to figure out a way to measure and confirm that. 

I expect that I’ll be spending time in the phone with Aero Precision tomorrow morning. I will start by asking dimensions from barrel flange to end of extension and where the alignment pin should be. 

[98Z5V]

13 minutes ago, MaineMan2 said:

Good question. I think so; however, the Aero Precision enhanced upper is different so you can’t see directly.  The barrel nut is threaded on the outside and it goes into a housing also used to mount the handguard later. The locator/alignment pin on the barrel extension seems to be as far back as possible but I can’t see it. Need to figure out a way to measure and confirm that. 

I expect that I’ll be spending time in the phone with Aero Precision tomorrow morning. I will start by asking dimensions from barrel flange to end of extension and where the alignment pin should be. 

I understand the M4E1 Enhanced, and what it is.  The only way to know is to measure.  Measure the seating surface on the upper - measure it's depth from the outside of the threading,  Measure the thickness of the barrel flange.  Insert the barrel, measure the outer of the seating flange.  That's the only way to know it if went flush to the upper...

[MaineMan2]

9 hours ago, 98Z5V said:

I understand the M4E1 Enhanced, and what it is.  The only way to know is to measure.  Measure the seating surface on the upper - measure it's depth from the outside of the threading,  Measure the thickness of the barrel flange.  Insert the barrel, measure the outer of the seating flange.  That's the only way to know it if went flush to the upper...

MY APOLOGIES 🥴 TL:DR fit is ok when barrel nut is torqued down guttentite.  

Measurements checked out in terms of barrel extension shoulder to back edge (feed ramps) being almost equal to distance from forward side of receiver shoulder to back side of opening inside receiver. Second measurement was difficult to get accurately. 
 

I can’t use torque wrench on my old barrel wrench. That’s why I didn’t just install the barrel before. German torque setting did the job just fine, although I obviously need the barrel wrench my torque wrench will fit into to finish the job. Barrel fit into receiver is pretty loose until the last 1/8” or so which is very tight. Like too tight to press barrel in by hand and a bit of a challenge to pull barrel out again after tightening barrel nut. 

[shooterrex]

If you heat the receiver and freeze the barrel it should slide right in.

[98Z5V]

11 hours ago, MaineMan2 said:

MY APOLOGIES 🥴 TL:DR fit is ok when barrel nut is torqued down guttentite.  

Measurements checked out in terms of barrel extension shoulder to back edge (feed ramps) being almost equal to distance from forward side of receiver shoulder to back side of opening inside receiver. Second measurement was difficult to get accurately. 
 

I can’t use torque wrench on my old barrel wrench. That’s why I didn’t just install the barrel before. German torque setting did the job just fine, although I obviously need the barrel wrench my torque wrench will fit into to finish the job. Barrel fit into receiver is pretty loose until the last 1/8” or so which is very tight. Like too tight to press barrel in by hand and a bit of a challenge to pull barrel out again after tightening barrel nut. 

Good news!   Right on, man!