Another DPMS G2 in .260 Rem with minor short stroking problems!

[Defcon-One]

Guys, I am new to this site!  Got here by searching on my problem.  I have read what I could find and think I get it, but would not mind some advice and mentoring.

I just put together a DPMS G2 with 20” rifle length gas, DPMS .260 Rem Hunter barrel in a YHM free float handguard.

SPECIFICATIONS:

G2 lower with A2 tube (9-11/16” inside) and stock.

Standard AR10 rifle buffer (5-5/16” long, weighs 5.4 ounces)

Standard AR10 rifle spring (12-3/4” long, 38-39 coils)

Upper is all DPMS G2 parts except the barrel nut and free float rail.

This blew my mind, but the gas port is 0.0625” possibly a bit larger as a 1/16th bit fits with a touch of wiggle, but not much.  (That is the best I can measure with a digital caliper.)  No pin gauges!

Rifle shoots good, feeds good, but does not lock the bolt back.  It is NOT a magazine issue, as bolt locks back fine when manually charged.  Also, fired brass and primers look fine.  It drops empty rounds at 4:00-4:30.  Pretty obvious from what I read above, this is a classic short stroking problem and the port is likely too small!

I have number drills on order!

Any comments would be appreciated.  Unless I get advice to the contrary, I intend to open the port up.  Probably go slowly until I get it to lock back reliably.

Ammo is Federal Fusion .260 Rem 120 gr. Factory loads.  About 50 rounds down the tube so far!

Thanks,

DC-1

Edited May 3, 2020 by Defcon-One

[DNP]

And that is how to share a bunch of details. How lubed up are you?  How does the gas tube land in the upper - does it fall right in the middle of the cutout?  I’m guessing they’ll suggest to drill up a hair, but I’d make sure the tube meets up at the right spot with the key and that you’re running her wetter than Pelosi fantasizing about Trump having Covid  

 

 

[Defcon-One]

Gas tube is dead center, maybe a tiny bit longer, probably about perfect!

The gun is well lubed, but maybe not Soaking wet.  I doubt life is the issue.

I figure I’ll start with a #50 (0.070”) and work The hole gradually up to a #46, which I think is around 0.081”.  Don’t want to go much over that and hopefully won’t need to go that far.

 

Edit to add: The only other thing I noticed is the buffer, a DPMS factory AR10 rifle buffer, is about 1/32” to 1/16” over the 5.25” standard.

Edited May 3, 2020 by Defcon-One

[98Z5V]

2 hours ago, Defcon-One said:

Guys, I am new to this site!  Got here by searching on my problem.  I have read what I could find and think I get it, but would not mind some advice and mentoring.

I just put together a DPMS G2 with 20” rifle length gas, DPMS .260 Rem Hunter barrel in a YHM free float handguard.

SPECIFICATIONS:

G2 lower with A2 tube (9-11/16” inside) and stock.

Standard AR10 rifle buffer (5-5/16” long, weighs 5.4 ounces)

Standard AR10 rifle spring (12-3/4” long, 38-39 coils)

Upper is all DPMS G2 parts except the barrel nut and free float rail.

This blew my mind, but the gas port is 0.0625” possibly a bit larger as a 1/16th bit fits with a touch of wiggle, but not much.  (That is the best I can measure with a digital caliper.)  No pin gauges!

 

What's the gas block diameter, on the barrel?...   I'll definitely agree that the gas port is too small, at 0.062" - it needs to be at 0.080" on a 20" rifle gas .260 Rem barrel.  BUT!...   fix your recoil system first. 

That's not a "standard AR-10 rifle buffer" - Armalite AR-10 rifle buffers are 5.200" long, period.  5 5/16" = 5.313".  I guarantee you that if you pick up a real Armalite AR-10 rifle buffer and measure it, it's 5.200" long.  I just posted about this last week, with pictures to back it up. You can probably get away with that buffer, but that spring...  

Second, that's not a "standard AR-10 rifle spring" either.  The Armalite EA1095 recoil spring is 13.750" relaxed length, 34 coils, and 0.072" wire diameter.  That's not an AR-10 spring you have, and I'm not sure what it is, really.

Again, fix your recoil system first - but that port diameter will need to go up.

Edited May 3, 2020 by 98Z5V

[Defcon-One]

98Z5V:

Thanks for the reply!  That is why I am here.

Buffer is a DPMS, so must be made slightly wrong.  It measures 5.303”!  Wondering if I can remove enough from the plastic bumper pad to get to 5.20?  It would only be about 1/10th  of an inch.  Weight is right on and that little shouldn’t change it.

I bought two of those springs, supposed to be AR10 Rifle, so somebody stuck me.  The one in my .308 seems to work fine, though.

I will get two new springs that meet the spec and try it out.  I might get the buffers too.

You really have to watch what you are getting with these AR10 parts!

Thanks for the help,

DC-1

 

[Defcon-One]

Sorry, I missed the first question that you asked me.

Here you go:  The gas block diameter is 0.750”, standard on the DPMS guns, except the bull barrel version.

I checked further and the buffer and spring that I purchased and am using are both stock DPMS G2/AR10 parts, made for this rifle.  The buffer is in their spec at 5.30” long and 5.4 ounces.  The buffer spring is in their spec at 12.75” long and 38 coils.

Based on that and the fact that my DPMS G2 Recon in 7.62mm x 51 with a 16” barrel (Mid-length gas system) has the same spring and buffer and it runs like a champ with everything I feed it.  I have to assume that the real issue here is the gas port size.

Also, I could easily trim the buffer down to 5.20” long if need be and the weight is already right!

Everybody seems to be out of the Armalite springs (EA1095) right now, so I am gonna have to wait on them.

Would you agree that opening up the gas port to at least 0.070” (#50 bit) would be a good idea, or am I getting ahead of myself?

Thanks,

DC-1

[98Z5V]

5 hours ago, Defcon-One said:

Sorry, I missed the first question that you asked me.

Here you go:  The gas block diameter is 0.750”, standard on the DPMS guns, except the bull barrel version.

Based on that and the fact that my DPMS G2 Recon in 7.62mm x 51 with a 16” barrel (Mid-length gas system) has the same spring and buffer and it runs like a champ with everything I feed it.  I have to assume that the real issue here is the gas port size.

Also, I could easily trim the buffer down to 5.20” long if need be and the weight is already right!

Everybody seems to be out of the Armalite springs (EA1095) right now, so I am gonna have to wait on them.

Would you agree that opening up the gas port to at least 0.070” (#50 bit) would be a good idea, or am I getting ahead of myself?

Thanks,

DC-1

First, keep in mind that the DPMS GII is a weird little bybrid bastard of the Big AR World.  It's not normal.  Don't try to compare that gun or it's parts to the logic that you should apply to diagnosing all these other "normal" ones... 

Next, I already told you what you need, port-size wise, to run a 20" rifle gas .260 Rem AR.

9 hours ago, 98Z5V said:

What's the gas block diameter, on the barrel?...   I'll definitely agree that the gas port is too small, at 0.062" - it needs to be at 0.080" on a 20" rifle gas .260 Rem barrel.  BUT!...   fix your recoil system first. 

That's not a "standard AR-10 rifle buffer" - Armalite AR-10 rifle buffers are 5.200" long, period.  5 5/16" = 5.313".  I guarantee you that if you pick up a real Armalite AR-10 rifle buffer and measure it, it's 5.200" long.  I just posted about this last week, with pictures to back it up. You can probably get away with that buffer, but that spring...  

Second, that's not a "standard AR-10 rifle spring" either.  The Armalite EA1095 recoil spring is 13.750" relaxed length, 34 coils, and 0.072" wire diameter.  That's not an AR-10 spring you have, and I'm not sure what it is, really.

Again, fix your recoil system first - but that port diameter will need to go up.

 

[Defcon-One]

OK, I’ll find the parts that you recommend, install them, then gradually open up the gas port until it runs like it should!  And yes, 0.080” will be my target.

That is a pretty simple project, once I find the parts.

Thanks for the advice!

DC-1

[Defcon-One]

OK, my number drills arrived yesterday.  Please note; I still have the stock DPMS spring and follower in the rifle for the simple reason that I could not purchase Armalite replacement parts anywhere.  All sold out and on back order.  Also, this is a DPMS G2, not an Armalite AR10, and I have to assume that DPMS would know what works best in their gun design.

I followed my plan and went up from 0.062” to 0.070 (#50 bit) and fired a few test rounds out back.  Still no lock back consistency though, one did lock the bolt back.  So, I figured that I was getting very close.

I went ahead and used the 0.073” (#49 bit) and opened it up a bit more.  I then fired five more rounds out back using the “one in the mag at a time” routine and all five shots now locked the bolt back.

It looks like I have this fixed (tuned) but I need to test some more.  I may need to go up to 0.080” like suggested above, but I am gonna hold that option on reserve for later.  It is easy to go up, but hard to go back down.

I intend to go out to the range this weekend and give the rifle a good workout.  I will report back here once I know the final results.

Thanks to 98Z5V for getting me on track!

[98Z5V]

Think about the ammo that you'll be using.  Do you load your own/handload?  If not, and you're going to use commercial ammo...  some will be stout, some will be weak.  That's when you need more gas port diameter, the weaker ones.  Good job on bringing the port up, little at a time, and testing - that's really the best way, once you find that ammo that your gun likes.

I have a badass handload for my .260 that's running the 147gr Hornady ELD-M projectile.  Perfect function with my (then) 0.070" gas port diameter.  Some of by other handloads wouldn't even cycle the action or move the BCG.  By several load manual refences, those loads should have been pretty powerful loads, too.  Nope.  My moment of rational clarity, even though I'd been denying it for awhile, was when I shot some NEXUS ammo loaded with 139gr Lapua Scenar projectiles...   Wouldn't cycle the gun.  I punched it up to 0.080" that next weekend.  Runs fine now, with anything that I throw at it.

Wilson Combat even increased their gas port diameter from 0.070" (what mine was when I bought it) to the current offering of 0.080".   This was long after I found out what I did, though.

Edited May 9, 2020 by 98Z5V

[Radioactive]

I had to open my 260 barrel gas port up too. I went up .086 I believe. I’m running a 22 inch barrel with a plus 2 gas system. That fusion ammo is pretty stout I think. I read somewhere it’s not recommended for gas guns. Shot a couple boxes through mine with no probs after opening port up before that it wouldn’t cycle

[Defcon-One]

Finally got out to the rage yesterday.  Rifle shoots well, but it still misses a few times regarding the bolt hold open.  Maybe 1 or 2 out of 10 do not lock the bolt back.  This confirms what I have been told here.

I am gonna go up a drill size and try it again.

I love these .308 caliber ARs and I am getting closer.  Everything else works perfect!

 

[98Z5V]

Good work - we'll get it running. Punch it up one more time, and you're there.  Make sure you let us know what the final port diameter is.  I keep saying 0.080" for a .260 Rem with a 20" barrel and rifle gas, 0.750 journal diameter - but mine is actually 0.081".  Minor detail, but it all comes for to the drill bits used.  I measure every one that I use, before I use it - some are a thou larger or smaller than adverstised, but I measure them just to know what's what.

[392heminut]

On 5/23/2020 at 7:20 PM, 98Z5V said:

some are a thou larger or smaller than adverstised, but I measure them just to know what's what.

Sometimes OCD can be a good thing! We've all benefited from your knowledge at one time or another!