7mm bullets and 1:10 twist

[blksno]

I realize 180gr Berger VLD bullets <span style="text-decoration: underline">recommend</span> a 1:9 twist but I also know that a 1:10 will most likely work, with velocity and range being a bit more touchy if it did.

I'm planning a 7mm SAUM build with a 24" barrel in a semi-auto platform. (most likely a MEGA or POF build)

With mag restrictions being the way they are, would I gain anything by going with a 1:9 twist and HOPING to find a 175-180 vld bullet that will feed and shoot accurately?

OR would I be better off going with a 1:10 twist and accepting the 160-170gr bullets? (there are some real peaches in that weight range as well, no doubt)

 

[Longshot38]

Re: 7mm bullets and 1:10 twist

If you want to shoot the heavies your best best is to stick with a faster twist. And while 180s are king of the hill in 284 caliber rifles the 168 Berger is not a bad option. My 7mm RM ran a 10 twist barrel thus I ran with the 168 Berger and found them to be a very good bullet. For a gas gun you would probably also be better off with the 168s. Due to short mag length those 180s will have to be seated deep and thus eat into your powder capacity.

Truth of the matter is that VLD bullets don't get a chance to really shine until you have a chamber that is setup properly for them. If you can get the throat of your chamber cut properly then VLD style bullets are hard to beat. But if not them are stuck with some limitations that can suck.

 

[blksno]

Re: 7mm bullets and 1:10 twist

I hear you about the chamber being specifically cut.
I've been trying to run 95 and 105 VLD's in my .243WSSM AR and while the 95's show promise, my chamber is cut way to goofy for me to take advantage of them. In order to touch the lands, the 95 vlds have to be loaded out to something like 2.515" OAL. WAY to long to use an an AR.
I've backed them down to 2.385-2.400 and have been getting sub MOA but still not quite as good as I want.

I can have my smith cut my chamber however I want/need so what I had planned to do was figure out the twist and bullet, and then mathematically figure out a decent seating depth that would still keep my bullet of choice inside mag length with a manageable jump.

For the VLD's that would be either barely in, touching, or barely out.

For example, I was looking at 180 VLDs and came to this conclusion.

A SAUM case length of 2.035", and a 180vld length of 1.525". The supposed max oal for an AR10 mag is 2.8" (but I can improve that with the VLD mag mod that I have been doing for PMAGs)
SOOO,


.....(2.8")max mag length
...- (2.035")case length
...= 765" of the bullet can stick out of the case (the ogive is at .764)

With my VLD Mag mod, I should be able to get an extra .15" of OAL or better. Hopefully making it possible to load the 180 vld to an OAL of 2.95 (not quite ideal, but getting awful close)

If I were to cut the chamber specifically for a round that was loaded to 2.90"...then THEORETICALLY, I could effectively run the 180 VLD's at "mag" length in a SAUM AR10.

NOW, do I wan't to bother taking that chance, or stick with the 1:10 twist and 162 and 168 bullets?

Oh decisions decisions.

 

[blksno]

Re: 7mm bullets and 1:10 twist

Any more input fellas? I'm trying to get narrowed down the kind of barrel I need to get ordered.

Thanks.

 

[BobinNC]

Re: 7mm bullets and 1:10 twist

Now sure why the consternation? Go with a 1-9" twist regardless of your planned 7mm bullet weight. 162-168"s shoot well in a 1-9" twisty barrel, and if your shooting in SD in winter,you might just need that extra margin of stability.

BTW: Hornady on there boxes of 7mm 162 gr A-Max's say, 1-8" twist recomended, as does Sierra on their boxes of 7mm 175 SMK's.

Just saying.....

 

[Jerry m]

Re: 7mm bullets and 1:10 twist

175 grain 7mm SMKs shoot great in my 9.25 twist 7mm Rem Mag.

Good luck

Jerry

 

[RobG]

Re: 7mm bullets and 1:10 twist

9 twist. It will shoot any weight you want. I would guess that if you can fit the 180's in the mag, you would need the stabilization of the faster twist.

 

[blksno]

Re: 7mm bullets and 1:10 twist

Alrighty, 1:9 it is. Now, to narrow down whos barrel to get and what kind of land/grove option to pick.

 

[michael_aos]

Re: 7mm bullets and 1:10 twist

The new Sierra 180gr recommend 1:8" twist on the box as well.

I went with a 1:8.5".

 

[blksno]

Re: 7mm bullets and 1:10 twist

suggestions on AR10 barrels in a 1:8, 1:8.5 or 1:9?

 

[BobinNC]

Re: 7mm bullets and 1:10 twist

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: c_bass16</div><div class="ubbcode-body">suggestions on AR10 barrels in a 1:8, 1:8.5 or 1:9? </div></div>

Acording to Brian Litz the optimal 7mm twist for the 168 VLD is 1-8.9" For the 180 VLD it is 1-8.6" under worse case conditions, as defined as Worst case: 2600 fps, 0 degrees F, 29.85” Hg, and 0% humidity. But 1-9" will work in all but worse case.

You may find this article interesting:

Berger's VLD 7mm's Part 1

When I order a new barrel for my 7mm-08, I will ask for one with a 1-8.7" twist. I am currently running one of 1-9.25", and have no stability problems with 7mm 162 gr A-max's or 7mm 168 SMK's running at 2700 fps. But it is rather hot in NC, most of the year.

But when I opt for a new barrel, I want to be able to stabilize any of the better 7mm bullets in all conditions.

Bob

 

[RobG]

Re: 7mm bullets and 1:10 twist

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Michael Aos</div><div class="ubbcode-body">The new Sierra 180gr recommend 1:8" twist on the box as well.

I went with a 1:8.5". </div></div>

I believe I read Sierra gives that recommendation mostly for those that use it at 7mm-08 speeds. I run them in my 7RM with a 9 twist with outstanding results.

 

[blksno]

Re: 7mm bullets and 1:10 twist

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RobG</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Michael Aos</div><div class="ubbcode-body">The new Sierra 180gr recommend 1:8" twist on the box as well.

I went with a 1:8.5". </div></div>

I believe I read Sierra gives that recommendation mostly for those that use it at 7mm-08 speeds. I run them in my 7RM with a 9 twist with outstanding results. </div></div>i dont know


It's odd then that the Berger website calls 1:9 the "recommended" twist for the 180s.

This is going to be a 24" gas gun in a 7mm SAUM, but South Dakota has far from ideal conditions and so I do understand why that faster twist would be beneficial.

 

[blksno]

Re: 7mm bullets and 1:10 twist

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BobinNC</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: c_bass16</div><div class="ubbcode-body">suggestions on AR10 barrels in a 1:8, 1:8.5 or 1:9? </div></div>

Acording to Brian Litz the optimal 7mm twist for the 168 VLD is 1-8.9" For the 180 VLD it is 1-8.6" under worse case conditions, as defined as Worst case: 2600 fps, 0 degrees F, 29.85” Hg, and 0% humidity. But 1-9" will work in all but worse case.

You may find this article interesting:

Berger's VLD 7mm's Part 1

Bob </div></div>

Thanks for the link, I read his article about 308s and what is considered, light, medium and heavy bullets. A great read, but I don't think I ever saw the 7mm one.