Headspace Length Variation on Sig 716 DMR

[MMH]

Just measured the headspace length on cases fired by my Sig 716 DMR. The brass is R-P four time fired brass. The headspace length varied from 1.627" to 1,631" for brass coming out of the same rifle. Why is there so much variation?

 

[918v]

One factor is the brass is out of round after being ejected.

 

[MMH]

One factor is the brass is out of round after being ejected.

Is this because of the barrel chamber or a function of the brass and how it cools? Is this normal? Have not noticed this before in my rifles.

 

[tinker]

I would try good brass first to see if there is really an issue. If it was off the shelf ammo fired once I would for sure say it was the brass. But since you said it's on it's 4th firing.....I would try other good brass like Lapua or Alpha and see if the same thing happens.

 

[918v]

Is this because of the barrel chamber or a function of the brass and how it cools? Is this normal? Have not noticed this before in my rifles.

When a case is ejected it bangs against stuff. This will dent the brass and depending on where the case hits will make the case diameter oval. If that happens then your comparator will read it as longer. Your comparator needs a round surface to bear up against. If one is round and another is oval, the oval one will read longer.

 

[MMH]

When a case is ejected it bangs against stuff. This will dent the brass and depending on where the case hits will make the case diameter oval. If that happens then your comparator will read it as longer. Your comparator needs a round surface to bear up against. If one is round and another is oval, the oval one will read longer.

Thanks for the explanation. The case is definitely oval at the mouth as it bangs the barrel extension while it is ejected. Can also see two axial scrapes from the bolt teeth. Nothing severely distorted, but nonetheless, distorted. I assumed that the accurate reading was the shortest one (1.627) and sized it 0.003 under that.

Was running the rifle with a suppressor & will turn the gas flow down next time - hopefully still functions w/ less case distortion.