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I have two 556 ERs, one with an Eotech and one with a scope/bipod for shooting longer distances. I really like the new DMR with the rails, but wonder how the 1 in 10 twist rate 21" barrel would fare against the 1 in 7 twist rate 16" barrel I already have when using 75gr ammo. For a medium range shooter (2-300 yards), which would be more accurate with those heavier grain bullets? What I have read suggests that the 1 in 10 twist will not sufficiently stabilize a 75gr bullet. I would like to get others' thoughts/experiences with this issue before I decide whether or not to trade in on a DMR (if I can find one ...).
 

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...i don't own a DMR...but i've fired rifles chambered for the 223 / 5.56 in various barrel lengths and twist rates...including 1/7-1/8-1/9-1/10-1/12- and 1/14...

...based on my experience and considering the DMR has a 21 inch barrel...my "best guess" would be that it won't do as well as the 556 with a 16 inch barrel and a 1/7 twist rate when firing the longer 75/77 grain projectiles under most atmospheric conditions...especially at low altitudes in cold...dense air...

...i would again "guess" that the DMR...given the velosity achieved in the longer barrel...should shoot acceptably well with 68/69 grain lead projectiles under most atmospheric conditions...it may do relatively well with the heavier projectiles at higher altitudes in hot...thin air...

...the 68/69 grain projectiles are good for well past 300 yards...and much further out too...
 

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Discussion Starter #4
Thanks Ullie. I am not adamant about shooting 75gr, but I do like them. If the 68/69gr are questionable with that barrel, I may shy away from the DMR. Sure is a nice gun, though. I wonder why Sig would make it with that twist rate.

Perhaps someone will make a good aftermarket 20+ inch barrel with a better twist rate for heavier grains at some point.
 

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...years ago i built an AR using a free floated stainless steel broach cut barrel chambered in 223 Remington...it would consistantly shoot less than 1/2 MOA out past 300 yards using projectiles in the 68/69 length range...and it had a 1/10 twist 16 inch barrel...

...the DMR with the longer barrel...and the greater velosity developed...should do relatively well with these length projectiles...however...keep in mind it's not a competition or target rifle...nor is it marketed as one...
 

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jcmd2003 said:
Sure is a nice gun, though. I wonder why Sig would make it with that twist rate.
...i'm "guessing" again that Sig Sauer simply followed the Swiss barrel specifications regarding twist rate for the SG 550 and SG 550 sniper version as used by the Swiss military...both of which have 1/10 twist rates as specified by the Swiss military for use with the GP 90 cartrage...

...a lot has changed since these rifles were designed in the late seventies and early eighties and put into service in ~ 1990...Sig Sauer should have better thought out their offering...but they didn't...along with the short "carbine length" gas systen used on the DMR...i suppose it was easier to use what they already had at hand...not good...IMO...

...another "guess" is that somewhere down the road Sig Sauer may start offering the DMR with a rifle length gas system and a faster barrel twist...something they should have done from the get go !
 

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Hi

The original Sig 550 has a 21" barrel and a twist 1;10. We use gp90 thats a 63 grain FMJ bullet out to 600 meters. Do your math. :wink:
 
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