7mm-300WM. Barrel Twist

[RJ]

I'm ordering components to put together a 7mm-300wm. I'll be shooting Hornady 175gr.ELD-X and the 180gr.ELD-M. I haven't decided on the twist rate of 1-8 or 1-7.5 Do any of you guys have experience with which twist works better and still gives good accuracy at extended ranges with those bullets? I'm thinking the velocity I'll be shooting will be around 3,000-3,100 fps. The Twist Rate Stability programs I've used tell me the 7.5 twist is more stable with these two bullets. But... Everything looks good on paper until you try it.

 

[Orr89rocz]

8 should be more than plenty with those. Guys have blown up the eldm’s so watch out

 

[L.Sherm]

I would be looking at a 8.5 or 9

 

[FeMan]

The twist required will depend on your elevation. When I tried to shoot 180 ELDMs at sea level in a 9 twist they key-holed. An 8.5 twist stabilized them fine. Even in an 8.5 twist at 2865 fps, I started blowing the 180s up when the barrel got hot and had some alligator hide to it. My recommendation is to not over-twist them.

 

[Bill K]

The maker of the reamer you will be ordering should be able to give you the information on what would be the best twist for what you want.

 

[whatwind?]

Winter 8.7, summer 9

 

[J_idaho]

Why not make a 7mm 300 prc. 300 wm brass has the shortest life span.

 

[L.Sherm]

Why not make a 7mm 300 prc. 300 wm brass has the shortest life span.

Why?

 

[L.Sherm]

When you run the 180ELDM in JBM with the numbers with a 9 twist its 1.65 S.G which is plenty, that is with 1.538 bullet length and .140 tip length

 

[J_idaho]

Why?

I’ve been shooting 300wm along time and the brass doesn’t last long because of the belt. 7-8 firings is max I’ve ever got. I have switched to petersons long brass but have not fired it enough to know yet. There doesn’t seem to be a thin spot above the belt so I’m hopeful. I guess if you’re using a custom reamer you won’t have any problems. I’m just still skeptical that all 300wm brass is weak above the belt. I really don’t know anything about 300 prc other than a little more case capacity without the belt.

 

[L.Sherm]

If you size the cases just like a non- belted case like .003 bump the belt becomes a non issue. I read a guys test were he took a mini lathe and turned off the belt and sized them like any other case and it shot great.
People have troubles with case separation because they set up there dies wrong belted or non belted

 

[FeMan]

When you run the 180ELDM in JBM with the numbers with a 9 twist its 1.65 S.G which is plenty, that is with 1.538 bullet length and .140 tip length

That is what it gives you. However, I think the JBM calculator is incorrect. A plastic tip on a bullet reduces stability by moving the center of pressure forward and farther away from the center of mass. The JBM calculator actually adds stability for a plastic tip rather than reducing it. Try running it with everything the same and taking the tip out but leaving the OAL the same. Its hard to believe but I think the program is written incorrectly. .

I have tried that bullet and the 175 eldx in 9 twists and booth went keyhole at 100 yards in two rifles.

 

[L.Sherm]

Theres people that say JBM is better than Berger's or others. Anyway it comes out to 1.37 when you take out the tip, which will still shoot. Berger has the 55 20 cal all the way down to around 1.2 with a 9tw and it shoots to 1,000.

 

[L.Sherm]

If you go in to bergers twist calculator and change there 180 to 1.538 instead of 1.51 that it lists the 180 Berger there calculator says 1.45 at sea level.
I'm not saying you did not keyhole the Horny, I just know I've shot several 9 tw with the 180 Berger and they all shot great out to 700 yards

 

[L.Sherm]

I have tried that bullet and the 175 eldx in 9 twists and booth went keyhole at 100 yards in two rifles

The 175 eldx is longer than the 180 eldm maybe that little bit was the tipping point, just like some 12 twist in a 20 cal wont shoot a 40 horny bullet but will a 39 Sierra BK.
That all being said I dont trust any numbers to me real world stuff is what I go by, in my opinion I still wouldn't go faster than a 8.5.
I believe the O.P needs to take into consideration everything i ran through the calculators was at sea level just 2,000' elevation jumps the S.G up from 1.4 to 1.65 now go on a hunting trip at 7,000 and see where that puts you.

 

[RJ]

The reason I was asking about the 1- 7.5 twist real world results is when I calculated the bullet rpm at 3,100 fps its 297,600 rpm. I've spoken with Hornady in the past and they indicated it's not advisable to spin these bullets above 300,000 rpm that's when you'll get the lead core separation from the jacket.
My thoughts were that if I went with the 7.5 twist it would remain stable longer when velocity started falling off at extended ranges. 297,000 is pretty close to the point of bullet seperation.

I live on the Coast of N.C., I'm at 13 ft above sea level. I hunt large agriculture fields. Some are open as far as you can see. I took in consideration if I would take this rifle to different elevations. Most likely not. This Rifle is going to be purpose built to Long Range whitetail hunt and maybe shoot ground hogs in the summer when I'm able to make time to go.

 

[rgc54]

I have a 280 AI with a 9 twist Kreiger and I shoot 180 ELDM’S at see level without problems

 

[El Matador]

The 7mm PRC was pretty much designed to shoot the 2 bullets you mentioned. They build those with a 1:8" twist.

 

[ebb]

Just curious, why would you go to the trouble when there is a 7mm rem magnum already? Is there enough difference to warrant the effort?

 

[RJ]

Ebb,
Good question, Well... I've been a fulltime Riflesmith for about 30 years. I had a client and friend that passed just as I had components for his build coming to my shop.

I was building a 6.5-284 Norma Long Range Target Rifle for him, Doan Trevor was making the Rifle stock with the wood my client sent him. Doan never had the chance to complete the stock. The Action is a Stolle / Kelbly Long F-class Panda RB LP. I have two bolts for the action 308 and magnum bolt face. And a Jewell trigger. Since I have the action and some other components around the shop I decided to build myself a switch barrel rifle in my spare time. I did sell the 1.250 Brux 6.5 mm barrel from the build on this site.

The reason I chose the 7mm- 300 is I have a bunch of ADG 300WM brass and 300wm bushing dies. So, Its an easy build for that cartridge. With the long throated reamer I'm going to use I can push those bullets pretty hard if I have to. If I don't like it I can just chamber another barrel for the rifle. I already have a 7 RM

This is why I was asking for real world experience for the 7.5 twist barrel at extended ranges and how far . I wanted to see how far I could push those bullets before they lost stability with a 7.5 twist. I was interested if others found any down sides of running that twist. Maybe save me some time determining what the limits are.