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Ballistic Coefficient Question

1

1simplemann

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Joined
Nov 8, 2013
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Location
Great Falls, Montana
Guys, I'm thinking about building a gun. Because who doesn't want a nice gun in their safe? Anyway, I've been researching, talking to a bunch of my customers that shoot. I had a conversation w/ my taxidermist. I mentioned that I was considering the 300 Win Mag. He mentioned the term BC and how I should take that into consideration before I choose a caliber. I understand the concept. Basically, Some bullets fly better than others. Makes perfect sense to me. Well he's a big fan of his Rem 7MM. He has a custom gun and the gun(his whole family uses it) has over 20 elk kills w/10 or 12 of them being over 500 yds. He explained to me that he shoots his w/ 180 gr Bergers at about 3020fps. His bullet has a higher BC than a simular 180 in 300 Win Mag. Even though the 300 win starts out faster than his 7MM, his berger's actually catch up to the 300 Win and at 500 yds his bullet has more energy left because of it's BC. Basically he said his setup shoots flatter w/ the same size bullet. This sounded plausible to me. I asked him where he got this info from. He said the guys at Gunwerks gave him the details. I even called his gunsmith and he said the same thing although he also said that I couldn't go wrong w/ either caliber. Is this the real deal? Hard to argue w/ success. If so, I may have to look at the 7mm's. I'm not a huge fan of recoil so a gun that's a killer AND enjoyable to shoot would be a plus in my book.
 
It's simple really. Equal form factors and equal weights, the smaller caliber wins in the BC department. That said, the larger caliber will always have higher BC potential with heavier weight capabilities. Imagine trying to get a 22 cal bullet to equal that of a 50 bmg. The same principal applies to every caliber between them.

You will always give up something to gain another. The 300 WM will always have the potential to hit harder than a version necked down to 7mm. The downside is more recoil. The benefits are greater down range energy AND barrel longevity. Bigger bores also drive equal bullet weights faster than their smaller competitors, case dimensions and barrel lengths being equal.

Each has their own strengths and weaknesses but pure ballistic performance alone, the 300 WM will eat the 7mm RM.

Compare the 180 vld 7mm bullet at 3000'sec versus the 215 hybrid 30 cal bullet at 2900'sec. energies at the muzzle and at 1300 yards and everywhere between as well as wind drift.

Trajectory means virtually nothing these days. Laser rangefinders and adjustable turrets or holdover reticles solve the range and drop problem. It's the wind you need to beat in this game. It's not as predictable as drops. At least the wind conditions themselves that is. Higher BC bullets drift less period. Compare a 30 cal 168 smk at 3400'sec versus a 208 amax at 2650'sec at 1000 yards and 10 mph. Energy too for that matter.

Hard to go wrong with either but I give the nod to the 300 WM between the two. I don't like recoil either but when I'm looking at a trophy bull moose or elk at beyond 1/2 mile, I want the mass and higher energy. I shoot 308 winnies and 6.5x284s for everything else. If you can stomach the short life of the 6.5x284, it's a great low recoiling performer equal to the 300 WM minus the energy. Trajectory and windage are the same and also with high sectional density. The performance to recoil ratio is near unbeatable with the 6.5x284
 
The previous post pretty much put everything out there. All the info you would need is right there. Basically, look at shooting the heavier bullets in whatever caliber you are looking at. 180gr, 7mm wins over the 308 cal equivalent in most cases, start getting to 200 grains, the 308 cal starts to take over.

Of a little side not, Berger has a 195 gr 7mm bullet in works that they are speaking of being around a .8 BC if I remember correctly. If you run there through a fast 7mm of some sort, it will then get to a point of some serious performance. Problem is, it has been put on the back burner with the shortage of their other bullets at the time. If you are gonna build a 7mm, just put a barrel with an 8.5 twist, then you can shoot the 180's right now, and the 195's when they come out. Or shoot a 300WM or Ultra mag with the heavier bullets.
 
If you run the 7mm 180 VLD against the 308 185 VLD, the 7mm will eventually catch up to the 308 185, but it will be a lot farther down range than 500 yds and it is basically comparing apples to oranges. You are comparing the highest 7mm BC bullet to a med 308 BC bullet. The 300 WM will shoot the 185 200 fps faster or more, than the 7 RM will shoot the 180.

A better comparison is the 7mm 180 vs the 308 215. The 300 WM will shot the 215 just as fast as the 7 RM will shoot the 180. Advantage goes to the 300 WM, big time.

The 3020 fps velocity you are quoting is very high for a 7 RM shooting a 180. Not impossible but definitely on the upper end. My max for a 160 bullet using Retumbo out of a 24" barrel was 3100. A 180 bullet would have been more than 200 fps slower. 215's are usually in the 2950-3050 range out of a 300 WM, depending on barrel length. So you see, when you compare apples to apples the larger cartridge will win every time. There is no magic or voodoo that overcomes the physical laws of nature.
 
I'm also of the opinion that .8 is a bit optimistic for the 195. Possible? Maybe, but it will take a pretty radical nose and boat tail design (more than what they're already doing) to get there. Otherwise it's in the low .7s. I could be wrong. We'll know when (if) they arrive. I remember the 338 300 grain was suppose to be just under .9 when they turned out to be in the low .8s

I realize they didn't have any other previous 338 offering with established data to compare them against but we do have 7mm bergers with known proven data to compare them against. With that, I think .714 is it for a 195 VLD and .730 is it for a hybrid.
 
MontanaRifleman said:
If and when the 7 195 comes out it will be in the class of the 308 230. Higher BC's but lower MV's. Bottom line, whatever you shoot in the 7 RM, the 300 WM will always be able to shoot a larger bullet at the same speed and BC.
Click to expand...

Yup. That's my point! I love it when I'm not the only one who understands this.
 
Couple things I will offer here from my data.

Yes Berger missed on their first attempt with the 300 gr 338. I was one of the first to report this or at least bring it to their attention. But they nailed the 7mm 180's , 30 cal 210's, 215's and 230's and the revised .388 300's. As well as almost every bullet I have used of theirs. Much better than some other manufacturers have done by a hell of a long ways. So I too may be a little in doubt of the initial proposed .795 BC of the 195 gr. But lets be fair about it, their track record is second to none.

The addition of 5 gr of weight with the 215 Hybrid over the 210 VLD resulted in the 215 being a faster bullet with a lighter powder charge. However the 230 Hybrid is substantially slower than the 210 or 215 from a 300 win. So 15 gr made a huge difference here between the 215 and 230 as far as velocity from a 300 win. I suspect this is related to bearing surface. Can we assume the added 15 gr of the 7mm 195 over the 180 will result in the same ?? maybe, or maybe not quite as much as we think. I have the measurements for the 195 but would need to revisit them to compare bearing surface lengths. But again off memory I believe I have a 7mm-300 win reamer right here that will be perfect. I am just as anxious as the next guy to test the 195's. It just could be what moves me from the 300 win I love to a 7mm-300 win. But first I want my own tests of the 195's and a confirmed BC over 750 +. .795 would be pretty awesome at 3150~3200 fps. I know for a fact, I have a 338 that is in this class now.


Jeff
 
If/when the big Berger 7mm shows up there is no guarantee of the BC or if your barrel will like it....

If it does work, how long would it be until they take the same design brief & apply it to a 240-250gr 30 cal, or 315-330gr .338 or even the .375 when that finally shows up ?
 
Jeff, I hope you know I wasn't knocking Berger for their mistake. I agree they do a better job than most. That's why I also stated that they had no previous 338 pills to reference data against. With the 7mm, there are references. If you cross reference form factors against the sectional densities of each (or using the proportion method), you'll find that to get the hybrid above .730 will take serious form factor changes.
 
All good points Jeff... I'll add that when they stretched the 215 hybrid to the 230, all they did was to lengthen the bearing surface. The nose and tail of the 2 bullets is the same. This might be why the 230 take a hit in velocity in the WM? Another reason might be that the capacity just does not produce enough horsepower to efficeintly push large pills. In the later case I think the same is true of the 7 RM and the 195. That's an awful lot of bullet to be pushed by that cartridge.

Last, we all know that 7mm barrels are usually a tighter twist and therefore the caliber lends itself to better (longer) form factors than the typical 308's. The 195 is going to require a good bit more twist than the 230.

With the development of new powders like RL33 the heavier bullets will do well in the more moderate cartridge like the WM and RM.

Speaking of RL33, I'm doing some temp regression testing as I type, actually between typing :)
 
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