Accuracy Problems with 69 SMK's in my SIG 556

[samnev]

Is any one esle having accuracy problems with the SIG 556 shooting 69 grain SMK's. I have tried 748, 4895 & 3031 in various loads and 5 shoot 100 yard groups are all 3+"
Yet the 556 will shoot 1.25-1.5" 5 shot 100 yard groups. with 748. It's an early 556 with swiss internals and barrel with 1:7 twist, yet behave like a 1 in 12 twist. I have varifed the 1 in 7 twist.
Strange.

 

[DALLAS ROOTS]

Re: Accuracy Problems with 69 SMK's in my SIG 556

Try some varget, about 25gr

How does it do with 55gr pills?

 

[ORD]

Re: Accuracy Problems with 69 SMK's in my SIG 556

I know you said you verified the twist is 1:7, but are you seeing any signs of bullet tumble/instability on your targets?

Also, what OAL are you running on your loads?

You might try loading them single-feed and see if the accuracy improves as it could be a mag/feeding issue causing damage to the bullets upon loading/chambering.

Finally, Dallas' suggestion on the Varget is a good one. Accuracy load of 25.3gr of Varget with an OAL of 2.250" is a great load and runs sub-MOA out of all my AR-15's with 1:7 twist barrels!

 

[Bongos]

Re: Accuracy Problems with 69 SMK's in my SIG 556

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Santo</div><div class="ubbcode-body">
Yet the 556 will shoot 1.25-1.5" 5 shot 100 yard groups. with 748. It's an early 556 with swiss internals and barrel with 1:7 twist, yet behave like a 1 in 12 twist. I have varifed the 1 in 7 twist.
Strange.

</div></div>

How much powder? and what is your OAL? I can't get my Sig556 to like 55, 60, or 69... What I do notice is the first three shots out of the gun is best with cold bore (actually got MOA with 55gr 25.3gr H335 but it only happened once and the rest were 3-4". I think due to the thin barrel, you really need to have it cooled a bit.

 

[samnev]

Re: Accuracy Problems with 69 SMK's in my SIG 556

My 52 SMK's load is 26.1 grains of 748. No signs of bullets tumbling with any bullets. COAL is 2.26" with both 69 SMK's and 52 SMK's. 55's shoot about 3" 60 Hornady Spire points about 1.5 to 1.75" with 24 grains of 4895. I don't shoot more than 10 shoots and slowly fire those in strings of 5 then switch to an Colt 6600 HBAR for comparison. It will shoot 1-1.25" groups with the same 69 SMK loads that shoot 3+" out of the SIG 556.

 

[fusiachi]

Re: Accuracy Problems with 69 SMK's in my SIG 556

Without knowing more about how you're developing your loads it's hard to say.

 

[sobrbiker883]

Re: Accuracy Problems with 69 SMK's in my SIG 556

Just how accurate do you expect the Sig to be?

 

[Bongos]

Re: Accuracy Problems with 69 SMK's in my SIG 556

Expect 1.5", get 3" to 4"

 

[sobrbiker883]

Re: Accuracy Problems with 69 SMK's in my SIG 556

How does it run with baseline quality ammo (FGMM,BH,etc)?

 

[Bongos]

Re: Accuracy Problems with 69 SMK's in my SIG 556

mine does the same thing with the 69gr SMK 3"-4" on the Sig556 but using the same ammo in my LWRC M6A2, I get 1.5"-2"; I suspect it's the barrel diameter, both are 1:7 but the LWRC's barrel is 1/3 thicker than the Sig.

All of my test are reloads, I do not have any match factory on hand to test as of yet.

 

[samnev]

Re: Accuracy Problems with 69 SMK's in my SIG 556

Same as above all my ammo is reloads. All trimed to spec, loaded with a RCBS charge master 1500 combo checked against an Ohous 10 10 scale and seated with a forster Ultra Match seating die to Coal of 2.26". Loads were laddered first by 0.5 grains, then 0.2 grains after chosing the best loads from the powders I used. I don't expect 1 MOA from the 556 but would expect it to shoot as well as the 52 grain SMK's. It's the only rifle I own that won't shoot the 69 grain SMK's decently. The 1:7 twist is supposed to shoot the heavier bullets better, but not in my 556.

 

[myerfire]

Re: Accuracy Problems with 69 SMK's in my SIG 556

I don't think this is your problem, but do you have enough neck tension? Measure a couple rounds, COAL, load them into the magazine, let the bolt slam the round into the chamber, eject it and remeasure the rounds.
myerfire

 

[Rovieairto]

Re: Accuracy Problems with 69 SMK's in my SIG 556

I never met a rifle with a twist rate of 1:9 or tighter that didn't love the 69 SMK.
I've met a lot of rifles in that category too.

Don't expect too much from a piston gun, 1.5 moa would be doing good.
Varget and RL15 are known accuracy propellants for that bullet.
</= 26.0 grains of either and test it again.

Also:
Check your throat for fouling.

 

[steve123]

Re: Accuracy Problems with 69 SMK's in my SIG 556

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Santo</div><div class="ubbcode-body">Same as above all my ammo is reloads. All trimed to spec, loaded with a RCBS charge master 1500 combo checked against an Ohous 10 10 scale and seated with a forster Ultra Match seating die to Coal of 2.26". Loads were laddered first by 0.5 grains, then 0.2 grains after chosing the best loads from the powders I used. I don't expect 1 MOA from the 556 but would expect it to shoot as well as the 52 grain SMK's. It's the only rifle I own that won't shoot the 69 grain SMK's decently. The 1:7 twist is supposed to shoot the heavier bullets better, but not in my 556. </div></div>

My Sig556PT shot 1.5 MOA with the Hornady 75HPBT and 23.6gr H4895.It shot some of my friends 69"s about 2 MOA.

Steve

 

[samnev]

Re: Accuracy Problems with 69 SMK's in my SIG 556

I been told by quite a few people including the moderator of sigarms556.com that the 556 will shoot the 69 grain smk's accurately. Mine will not. I checked the neck tension and it's ok. I'll have to try Varget. Maybe my gun is the exception to the rule.