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Is my twist rate OK for my 7mm??

 
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  #1  
Old 01-27-2013, 02:49 AM
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Is my twist rate OK for my 7mm??

I have been obsessing about this all weekend now so I am hoping for your imput!

I am shooting 160gr Accubonds from my 7mm Rem Mag 26" barrel with 1:10 twist. Origionally I tried 160gr Trophy Bonded Tipped Bullets (longer than the Accubonds) however I found that they shot 2" -3" groups at 100yards and the holes in the paper were oval shapped.

I switched over to the 160gr Accubonds and found that my groups were .254 and .356 for 3 shots groups....can't do much better than that...I can't anyway!

So now to my question. I have been reading that the 160 Accubond overall length of 1.410" makes it "marginal" on the Twist Chart for a 1:10 barrel. What I am concerned about it that even though the Accubonds are shooting incredable groups at 100 yards, will the bullets become unstable at longer ranges because the twist is too slow....IS it too slow??

Maybe I am having a bad night and reading too much into this so I am hoping to get your opinions on my findings.

Thanks!

FRank
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Old 01-27-2013, 05:14 AM
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Re: Is my twist rate OK for my 7mm??

Years back I spent a month taking some NRA gunsmithing classes at Lassen college.
I was in a pinch for time to get a couple of barrel blanks, so I ended up with a 10 twist blank for the 280 AI build I was doing. I haven't ever really wrung that rifle out, as I built it for my wife, and she prefers her 7-08. But I did play with the 160 NAB, and it seemed to shoot fine out at 700 yards.
I did most of my shooting at about 1000 feet elevation and in temps around 50 F.
With that said I would say the only way to know for sure what your rifle will do is to shoot it.
One thing to keep in mind, if you are on the edge of stabitity, a change in conditions could push you over the edge. One of my friends had problem shooting a match at lower elevation than he had tested his rifle at when shooting at home. His problem was more dropping through the sonic barrier, than a twist issue. But it shows that elevation changes, or temp changes can be an issue when you are on the edge.

Would you be running the same handle over on the Campfire?
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Old 01-27-2013, 10:19 AM
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Join Date: Apr 2011
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Re: Is my twist rate OK for my 7mm??

As advanced AGE forced me to quit hunting and go into long range target challenges the one consistent factor that I discovered was " Bullet Seating Depth "

Each bullet has a different ballistic co-efficient that is ONLY PUBLISHED for the BARREL USED in the TEST....that means THAT barrel used in the TEST wasn't yours,,,those numbers are ONLY a GUIDE and not a FACT for you.

What you must now do is start at least 5 different bullet seating depths for your barrel , bullet - charge weight , temp , and humidity...keep a written journal and then try to duplicate your loads to the same conditions of the day that produced the best groups for you, consistently.

Next, now repeat those results between 300 and 400 yds to establish bullet stability. Now you've got your accurate load for your barrel.

At 100 yds you're only chunking Rocks at a target...no accuracy at that range.
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Old 01-27-2013, 11:18 AM
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Re: Is my twist rate OK for my 7mm??

Punch your digits into the Berger twist rate calculator and see what your working with. http://www.bergerbullets.com/litz/TwistRuleAlt.php It looks like you should be OK to me, as the 168 Berger is stable in a 1-10 twist but your elevation will make a big difference.
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Old 01-27-2013, 12:19 PM
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Re: Is my twist rate OK for my 7mm??

Thanks Fellas.

I did plug the 160gr Accubond into Berger's twist chart and got a reading of 1.39 at sea level (New York State) which is "marginal" catagory...they reccommend a score of 1.5.

I also entered Berger's own 168gr VLD Match Hunter and it showed 1.36 which also falls into the marginal catagory...why then does Berger reccommend a 1:10 twist then....confusing.

JC,
Great advice but I just don't have access to a shooting range past 100 yards here in New York. When you say 100yards is "Chunking Rocks" do you mean that 100 isn't far enough to have the groups open up if the twist rate isn't enough??

I looked at my 100yard groups again last night. My 3 shot groups put 2 rounds into the same hole with the 3rd just off...again my groups at 100 yards are .254 and .356.

Wapitihunter...that's me.

What do you all think?
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Old 01-27-2013, 04:03 PM
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Re: Is my twist rate OK for my 7mm??

I have a 7mm rem mag with 26" barrel 1-10 twist shootin 168 berger vld's this is a 400 yd group Is my twist rate OK for my 7mm??-rifle-129.jpg 2932fps. I think your fine.
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  #7  
Old 01-27-2013, 07:39 PM
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Re: Is my twist rate OK for my 7mm??

That looks like allot of people's 100 yard targets!

I am also contemplating Berger's 168gr VLD Classic hunter...pretty much the same OAL as the 160gr Accubond but with much higher BC. I just wonder how the Berger will do at close range on a big (Elk, Zebra) animal when the velocity is still high. Also since the Berger isn't bonded, I wonder how it will do on a quartering on/away shot or when it hits a large bone. Just about any bullet will kill a deer or kill, or even an Elk with a broadside rib cage shot.

I wonder how the Berger's will do on that??
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