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So I am in no way new to shooting so please let’s keep this all based on the firearm and components please. That said this is my first and old Sig556 swat. And I have mounted a Vortex 1x8 strike eagle with a Burris pepr Mount. Well within sub-moa at 25yrds with 55gr federals and lake city’s. But at 100yards there is no pattern other than 12” right with about a 15” pattern???
Shot from bags, lead sled, and had another long range precision shooter try. Same results.

My question, is there an adjustment to the top rail that may need to be checked? Could this gun just want more grain? I’ve heard it prefers 62gr and up?

Thank you very much gentleman,

Charlie Alpha
 

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I can say that my old Sig556 Swat (2006 if I remember the date code right) absolutely loves the heavy stuff. 75gr. It will shoot the 62gr acceptable, but nowhere near the accuracy of the 75gr. Granted the 75gr is match while the 62gr is mil surplus. I haven't even bothered trying the 55gr mil surp. Grab a box or two of PPU .223 75 gr match. They also make a 69gr match. runs about .50 cents a round but that Sig 556 barrel really loves it.
 

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Two things have always plagued this rifle.
Canned rails and inaccuracy when warmed up.
The combination of the pencil barrel and a gas block driving a long stroke piston generally will pull the barrel odd directions after warm up. Not unusual for piston guns in general.
For zeroing in for hunting, I would do the Hathcock method; one shot per day, dialing in on the cold shot.

For the rail, take a carpenters straight edge, strap it to the rail extended to the front sight. if there is cant, you should be able to see it.

But do try the heavier balls to see what you get.
 

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So I am in no way new to shooting so please let’s keep this all based on the firearm and components please. That said this is my first and old Sig556 swat. And I have mounted a Vortex 1x8 strike eagle with a Burris pepr Mount. Well within sub-moa at 25yrds with 55gr federals and lake city’s. But at 100yards there is no pattern other than 12” right with about a 15” pattern???
Shot from bags, lead sled, and had another long range precision shooter try. Same results.

My question, is there an adjustment to the top rail that may need to be checked? Could this gun just want more grain? I’ve heard it prefers 62gr and up?

Thank you very much gentleman,

Charlie Alpha
I suspect grains is a big part of your problem. My SIGs and Colt M4's do not shoot 55 gr (193's) well at all, but my Bushmasters (1:9 twist) do. Bad occasional flyers is the clue to a grain problem. My Thompson Contender pistol w/.223 barrel will not shoot or even close chamber with 5.56 62 gr Federal green tips (XM855). It shoots .223 55 gr well, HOWEVER with 50 gr. high-quality ammo, it thread needles better than the rifles! One wouldn't think 5 grains would matter, but in some weapons it does.
 
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