Federal 308 Premium Gold Medal 168gr Sierra MK velocity out of my AR10

[Rick2001]

Good evening to all. I am a little confused, and I need your help if possible.
I am shooting the federal Premium Gold medal 168gr Sierra MK out of my 22" barrel AR 10 suppressed (rifle built by myself, suppressor Dead Air Nomad 30), and I am super pleased with the performances of both the rifle and the ammo, but I am struggling to grasp the difference between the manufacturer-declared speed and the one I get from my rifle (esteemed). Unfortunately, I do not have access to a Chronograph for the moment, but I am using a ballistic app (Chaingun Elite) that I find very accurate with all my other calibers, 6mmARC, 6.5CM, 6.5Grendel, 224 Valk, etc. Now my range extends to 675 yards so I can validate the trajectory in the app at decent distances, usually, I do it at 550yards, and while it is not uncommon to witness up to 75-100 yards discrepancies between what a manufacturer declares out of a 24" barrel and my rifles, the Federal 308 seems a little out of scale. Federal declares 2650 fps (from a 24" I assume) so I used 2550 fps as a starting point with my 22" barrel but at 550 Yards I was pretty low. I had to go up 2 MOA to get on target pretty close to the center, and when I validated the firing solution in the app, it returned me a velocity of 2259 fps, which is like 400 fps lower than Federal declared and 300 less than I was expecting. Can anyone help me to understand what is going on? Am I missing something? I have adjusted everything for the current weather conditions and zeroed at 100 yards before shooting at a distance. Thank you in advance.

 

[wvfarrier]

The only way to be sure will be a chrono but that seems pretty low

 

[isofahunter]

Believe the bullet. Bolt gun 2578fps, AR10 2255fps with FGMM.

 

[Shoots 700's]

Gas gun and bolt gun velocity with be different for sure

 

[Rick2001]

Sorry, I typed the wrong info. My Ar10 barrel is 20", not 22.

 

[sinister]

Believe the bullet. Bolt gun 2578fps, AR10 2255fps with FGMM.

Ain't no way 168 FGMM is doing 2255 out of a 20-inch, even if it was a smoothbore .315.

Garbage in, garbage out. You're guessing/interpolating, not reading off true velocity.

 

[BCP]

What is your data at 600ish 15min approx?

 

[Rick2001]

What is your data at 600ish 15min approx?

I don't have validated data yet.... my app says it should be 23.7 MOA up, assuming the 2259 velocity is right. Tomorrow, I will use the Chronograph and I hope to understand what is going on

 

[Shoots 700's]

What was you actual dope at 550

 

[Rick2001]

What was you actual dope at 550

20.3

 

[Shoots 700's]

I’ve never seen it that slow. Even out of my 18” gas gun it’s much faster

 

[BCP]

Maybe you goofed when you zeroed it somehow? Accidentally dialed it down when setting zero stop.

 

[Rick2001]

Maybe you goofed when you zeroed it somehow? Accidentally dialed it down when setting zero stop.

I think so

 

[isofahunter]

Ain't no way 168 FGMM is doing 2255 out of a 20-inch, even if it was a smoothbore .315.

Garbage in, garbage out. You're guessing/interpolating, not reading off true velocity.

I did not see you there when the shots were recorded, but thanks for the useless commentary.

 

[45-90]

Chronograph the ammo, you need a decent measurement of it's velocity, not an interpolated guess.
I doubt it's going that slow...
I shoot 208 eldm at 2442 ave fps out of a 16" AR 10, and 2756 ave fps with 168 eldm, with handloads.

 

[tucaz]

with fgmm 168’s i get 2496fps out of a sig cross 16” barrel

go buy a chrono and stop dicking around guessing.

 

[Jsp556]

I did not see you there when the shots were recorded, but thanks for the useless commentary.

@sinister is one of the most knowledgeable guys you can talk to about precision gas guns. The guy is a legit SME. Try not to run one of the best guys on the forum away.

 

[Evintos]

Did you check for a POI shift between suppressed and unsuppressed at zero distance?

 

[Conrad101st]

No way it’s that slow. My 16 inch LMT shoots FGMM @ 2512 FPS.

 

[Rick2001]

Did you check for a POI shift between suppressed and unsuppressed at zero distance?

Yes there is generally about one inch of difference in about all my rifles.

 

[FredHammer]

168gr FGMM 2525-2550fps is realistic from a 20" AR.

 

[357Max]

There is a big difference between lots of 168 FGMM, but no way it's that slow from a 20"!

My 16" AR with newer Brown box 168FGMM is 2430, older White box 168FGMM is 2510

In my 26" bolt gun same lots as above 2680/2788

 

[Rick2001]

Hi all. I think I have solved the mistery... I feel quite dumb right now, though. So my 20-round average speed is this:

Now dialing the suggested elevation from Chairgun elite

I hit exactly the rock that anchored my target hanging from a 2x4 at 550 yards.

Now what I didn't account for is that the center of the 15 inches gong is about 3 feet from the ground so I needed to go up quite a bit to hit the steel (that is where my 20.5 MOA came from) and when I input that elevation on the app, it gave me back that low velocity.
I probably never noticed so much difference with all my other cartridges because they all shoot flatter at that distance than the 308.
This is the target :

Sorry for having bothered you all.... I should have thought about it a little more.

 

[Shoots 700's]

Glad you got it figured out

 

[bfoosh006]

You didn't bother us all... just remember the internet can not convey comments as well as in person comments.

 

[BCP]

Um if you dial say 15moa and hit a rock on the ground at 550 then you should be able to hit the 550 gong with that same 15moa sort of thing. And that strike eagle 1-6 has a BDC reticule correct? I would shoot the reticule.

 

[Rick2001]

Um if you dial say 15moa and hit a rock on the ground at 550 then you should be able to hit the 550 gong with that same 15moa sort of thing. And that strike eagle 1-6 has a BDC reticule correct? I would shoot the reticule.

right it is a strike eagle 1-6 with a Trybe doubler that makes it 2-12. I am not using the BDC reticule because I think (but not sure about it) that with the doubler it screws up the BDC marks

 

[Rick2001]

Um if you dial say 15moa and hit a rock on the ground at 550 then you should be able to hit the 550 gong with that same 15moa sort of thing. And that strike eagle 1-6 has a BDC reticule correct? I would shoot the reticule.

If I should hit the gong with 15.5 I am lost because I definitely don't

 

[smoothy8500]

So how many MOA does it take to hit center of your target.....Dialed MOA, not combo of dialing and BDC/christmas tree/hash marks?

 

[Rick2001]

So how many MOA does it take to hit center of your target.....Dialed MOA, not combo of dialing and BDC/christmas tree/hash marks?

About 20.5 but I will confirm later when I can go back to the range

 

[357Max]

About 20.5 but I will confirm later when I can go back to the range

Personally I’d ditch the doubler. Your 1-6 probably has a 100y fixed parallax. So I think your also doubling any parallax error.

 

[BCP]

Get rid of the doubler, zero it with a six o clock hold at 100 and I bet the 550-650 marks will be just about dead on.