7mm or .308?

[nralifer]

question for you hardcore guys, if its all about the BC why isnt everyone shooting the same caliber and load?

That really is a good question. I guess the answer is that it so much fun to play with all the other calibers and loads. For long range shooting, though, it is mostly about BC. Out to 400 yds or so, BCs in the 0.4 range and MVs in the 2700 -3000 fps range work well. Beyond that is where bullets really start to drop and lose their energy. It is us hunters and amateur shooters creating a demand for better bullets and rifles that has proven to be a remarkable defense asset for the country. Consider that the two best snipers our military has had, Carlos Hathcock and Chris Kyle, both started out as hunters. Also consider a little fact that I heard Chuck Yager point out, and that is that something like 90% of the air to air kills in WWII were done by pilots that bird hunted.

[rancilio]

This PrecisionRifleBlog.com article suggests that the long range match folks are overwhelmingly using one bullet, the 6mm 105gr Berger Hybrid.

http://precisionrifleblog.com/2014/12/11/reloading-components/

[STOMP442]

The reason we all use something different depends on our rifles as much as the shooters. Rifle barrels have very unique personalities, what my rifle likes another may hate. For example 9 out of 10 of my rifles shoot berger bullets. Why because I feel they maximize the potential of each cartridge by providing better down range performance due to their higher BCs. On that rare occasion I have a rifle that just won't quite shoot as good as I would like it too with a berger I move on too the next best thing which in my opinion is an Amax or match king. It all depends on what the rifle likes most while still providing me with the best down range performance possible. The reason we don't all shoot the same bullet is the same reason we all drive different vehicles. Different strokes for different folks.

[nralifer]

Rancilio, thanks for the article. Very illuminating. Confirms my our thinking about our bullets. very important to get the right ration of BC to bullet weight

[308Nut]

 

Consider this. What has a higher specific gravity and sectional density, a 500 gr lead musket ball or a 500 gr Barnes 458 brass solid? I don't know off hand and just thought of the question, but clearly the shape would play a big role in determining the BC of each.

The lead musket ball will have the higher specific gravity. Which has the higher sectional density? The one with the smaller diameter. Which has the higher BC? My guess (since I'm not a round ball expert) is the one with the more aerodynamic shape. Why do I say that despite what I have been expressing? Because the balance between SD and form factor is extreme in this case. If both had equal dimensions regardless of whether or not they were round balls or modern boat-tail bullets, the lead version would hands down have the higher BC.

 

I agree 100% that shape has much to do with BC. That's why the rule of thumb equation for BC is SD/i where SD is sectional density and i is form factor. Both are critical to the equation.

 

When I'm referring to the importance if SD in BCs, in the context of this discussion, I'm comparing bullets with equal dimensions or equal form factors to illustrate SDs importance. Bullets of dimensions that have differing weights do not have the same BC. The heavier of the 2 will always have a higher BC.

 

Comparing the 170 solid to a 168 Hybrid, the percentage of difference in form factor seems large on paper but in reality, when it comes to drag while it is in flight the change between the 2 is very small. regardless of what any calculators will tell you. Is it possible for your 170 to be higher than the 168 hybrid? It's absolutely possible (banding aside).

 

It would be possible to improve the BC of the 168 hybrid. But not by much. It would take the same form factor as the 215 hybrid to get it up to .544g1. Adding the 2 grains and subsequent increase in SD, the 170 would reach the .550g1 mark if it had the same form factor as the 215.

 

To illustrate my point, Berger's 300 grain hybrid 338 bullet and the 250 grain hybrid 338 bullet have the same form factor. The big difference between the two since the caliber is the same is weight and subsequent SD yet the 250 has a lower BC. In fact, it is directly proportionate. (.818/300) * 250 = .682. .818 is the BCg1 of the 300 grain and .682 is the BCg1 of the 250. 83.4% is the difference in not only the BC but 83.4% is also the difference between their SDs as well. This should illustrate the importance of raw sectional density in regards to BC.

[pwrguy]

Mike that gave me a headache

[firstcoueswas110]

i shoot them with my 7mm and they die. i cant tell you much else.

[nralifer]

 

 

Consider this. What has a higher specific gravity and sectional density, a 500 gr lead musket ball or a 500 gr Barnes 458 brass solid? I don't know off hand and just thought of the question, but clearly the shape would play a big role in determining the BC of each.

The lead musket ball will have the higher specific gravity. Which has the higher sectional density? The one with the smaller diameter. Which has the higher BC? My guess (since I'm not a round ball expert) is the one with the more aerodynamic shape. Why do I say that despite what I have been expressing? Because the balance between SD and form factor is extreme in this case. If both had equal dimensions regardless of whether or not they were round balls or modern boat-tail bullets, the lead version would hands down have the higher BC.

 

I agree 100% that shape has much to do with BC. That's why the rule of thumb equation for BC is SD/i where SD is sectional density and i is form factor. Both are critical to the equation.

 

When I'm referring to the importance if SD in BCs, in the context of this discussion, I'm comparing bullets with equal dimensions or equal form factors to illustrate SDs importance. Bullets of dimensions that have differing weights do not have the same BC. The heavier of the 2 will always have a higher BC.

 

Comparing the 170 solid to a 168 Hybrid, the percentage of difference in form factor seems large on paper but in reality, when it comes to drag while it is in flight the change between the 2 is very small. regardless of what any calculators will tell you. Is it possible for your 170 to be higher than the 168 hybrid? It's absolutely possible (banding aside).

 

It would be possible to improve the BC of the 168 hybrid. But not by much. It would take the same form factor as the 215 hybrid to get it up to .544g1. Adding the 2 grains and subsequent increase in SD, the 170 would reach the .550g1 mark if it had the same form factor as the 215.

 

To illustrate my point, Berger's 300 grain hybrid 338 bullet and the 250 grain hybrid 338 bullet have the same form factor. The big difference between the two since the caliber is the same is weight and subsequent SD yet the 250 has a lower BC. In fact, it is directly proportionate. (.818/300) * 250 = .682. .818 is the BCg1 of the 300 grain and .682 is the BCg1 of the 250. 83.4% is the difference in not only the BC but 83.4% is also the difference between their SDs as well. This should illustrate the importance of raw sectional density in regards to BC.

 

308Nut, you seem very well versed in ballistics. I would like to share with you the saga my friend and I have had attempting to measure BCs. Lets say we learned a lot about chronographs, and the conclusion is that the Superchrono is not accurate, and could not be used for BC measurements. What we finally have settled on is using the Oehler 35 to measure the MV, and are using an acoustic wave form program that we found on the Audacity web site for free! My friend, Jason, who also is the bullet designer, found a paper by Michael Courtney on an acoustic method for measuring flight times using that program by placing your computer half way between the shooter and the target, and measuring the time difference as shown on the wave form of the shot between the muzzle blast and the impact of the bullet on a steel gong. We tried it over 400 yds and as long as the wind was not blowing we can get reasonably consistent data. The problem is that the wind is always blowing in SD. Instead I bought 2 Midway walkie talkies and placed one behind the gong, used a rubber band to keep it transmitting, and the second one on the computer so that the computer would record the noise from the impact of the bullet. The computer was behind me about 10 feet from the muzzle of the gun. This method seems to be largely wind proof, and you can get data on flight times that are consistent to +- a millisecond. You have to correct for the difference in distance the computer and the recording walkie talkie are from their respective sound sources, and figure out how many milliseconds it takes to travel that distance difference and add that to the flight time if the computer is further from the muzzle than the walkie talkie at the gong is from the gong. Turns out that you need to know the atmospheric pressure and temp to see what the speed of sound is at the time of your test. If you do all that and have loads that vary in velocity by not more than 1.5%, then you will get very consistent flight times. I have tried it at 200 yds, and next week will try at 400 yds. We got some very interesting and encouraging results, but would like someone else to try the method on whatever bullets they want to see what happens. To do this you have to be a little nuts, but it keeps one busy in the off season.

[Havasuhunter]

30-30 or 220 swift for armadillos?

[murfys69law]

Why not just shoot a .338. That's all a 7mickey mouse is. Just necked down. I agree it will shoot flater in 7mm than 338 but you can hunt grizz with a 338.

[nralifer]

Has been some time since I posted. Have been busy testing the 308 bullets we make. Redesigned the 170gr bullet a bit and it is now a 168 gr bullet. The BCs I quoted for the 170 were optimistic. We have a more accurate measurement method described in a previous post. Works well at 650 yds. The BC of the 168 bullet is 0.520, and that of the 180 gr bullet is 0.560. Both are more or less equivalent to the Bergers. The bearing surfaces allow the bullets to exit from a 308 Win 20" barrel at about 40-70 fps more than the Berger loaded to the same powder charge, and the bullet lengths and meplats are more uniform. I have been using the Leverevolution powder, for which there is no published data in the 308Win, and have been very happy. Can get less than 0.5 in. 3 shot groups with both bullets. Like Barnes bullets, they seem to like a little jump of about 0.035 in. off the lands. The max charge of LVR I have found to be 46.5 gr for the 168, and about a grain less for the 180 gr bullet. With the 168 my MV averages 2735 fps, and the 180 2650 from a 20 inch barrel. WLRM primers gave me the lowest spread in MV. Have tried 215M primers which do not seem to work as well, which is surprising. A friend of ours has bought a few of the 168s. From his 308 Win he has gotten sub 5 in. groups at 500 yds. He is going to use them in a 1000 yd match next month in Pierre, SD. I also have a 300 WSM with a 27 in. Benchmark barrel and these bullets shoot well also. The 168s MV using Superformance powder will easily give 3200+ MV and the 180 3150.

[308Nut]

Very good post and thank you for sharing.

 

You have no idea how much I respect you for and appreciate your honest here. Most new bullet makers and their reps swear until the bloody end that their BCs are correct. Thank you for admitting that they were initially optimistic.

 

.52 for a 168 is much more believable as is .56 for a 180. Still very much on the high side but those are mathematically possible values.

[roninflag]

wondering where the OP went ? i used a 30-06 to harvest a wolf. i use a 7mm often too. K. Adams chooses a 7mm for long range. according to the article he has 5 red ones . i think hs has shown the 7mm works well also. ... .. .. http://www.accurateshooter.com/competition/f-open-champ-kenny-adams-7mm-rsaum/

[nralifer]

Had a chance to shoot our 168 bullet using my 20 in. 308 at 1020 yds yesterday. Got 3 shot groups ranging between 0.5 to 1.6 MOA. Wind was variable. Used 46.5 gr Leverevolution and the MV was 2735. That load groups at 0.2 in at 100 yds.

[ngazhunter]

7mag out performes most