6mm vs 7mm in the wind - Am I missing something?

[lewis reynolds]

Have you ever missed a change and dropped a point in the 5 ring and said aww that was close. That's where the extra inch of the 7mm comes in handy. It's not the 1mph changes that count its the half mph change no one seen. Regard lewis

[Chopper]

Maybe a 6mm Vs a 7mm team shoot is on the cards ??, Chop

[Quick]

Mike, I know what your saying. Im at work and had a brain wave and wanted to know what people think. I dont get to shoot again until next weekend and propbably wont shoot against a 7mm until the tassie teams next year. Its all theory in the end until its proven. Im just posing a question is all for some healthy debate. I know the 7mm is where its at for now in F-Open and I'm happy to jump on the bandwagon, if for me it makes the difference. I think I'll need to jump behind one for a few club shoots and see what the difference.

As it happens, Ive made my decision at this stage, 6x47L will be it for me until next year. My wind reading skills arnt good enough that a 7mm will make much off a difference I think.

Chop, distance wise I'm talking 1000yds. I didnt have the time to run the numbers for the short ranges but would be intesting to see what that is for the shorts aswell as the rest of the longs.

Anyway, gotta goto work now. Have fun all.

[bruce moulds]

shaun,
6mm - 7mm, what a choice.
a politician understands the art of compromise.
6.5 super lr.
keep safe,
bruce.

[IanP]

Yes BC does rule on paper. . . All things being equal. BUT generally the 6mm closes the gap on a lot of .284's because they are not equal in velocity.

Its practical to launch a 108 gr 6mm at 3100fps but there are a lot less people shooting 7mm's that are able to push a 180gr at 3100fps.

If you compare it the 6mm to say 7saum or a 7wsm doing 3050fps you might see the benefit of the bigger (read more ballistically efficient) bullet.

What does it all mean?! Flip a coin and shoot whatever blows your hair back! For what its worth I've shot more possibly with my 6.5x47 than my 7saum . . . .

Adam, its not just on paper that BC rules! Why is the 180gr Berger hybrid the most popular choice for 7mm being used as a long range target gun. There is just one answer, it has the highest BC for the calibre and the highest BC over most every calibre available for F-Class shooting.

Its popular because on paper it rules the roost and at Raton in the World Championships it ruled the roost!

I'm going to shoot my 300WM pushing 30 cal, 230gr hybrids out the end of the barrel at a slow 2880fps at the upcoming SA Queens. It takes a 7mm 180gr hybrid a velocity of 3250fps just to equal the 10mph cross wind drift at a 1000 yards. I doubt a Saum could launch a 180 grainer at that speed and if it could how reliable would it be.

I'm doing it because its a challenge and it looks good on paper. Its got a lot more recoil than the 7mm, 180gr bullets but if the usual crappy weather at Lower Light comes into play, the advantages may just out weigh the disadvantages.

Chances are that if a bullet has a high BC and can be launched at competitive speeds its probably a better option than a lower BC bullet. Usually that means heavy for calibre bullets are best and the shooter should weigh up the best options before choosing calibre.

If the competition is short range like 100m bechrest, etc, then BC is no where near as important for obvious reasons.

Ian

[AlanF]

Gents,

Just did some maths and worked out something regarding 6mm vs 7mm. Using my rifle and loads ballistics data, it drifts 77in in a constant 10mph wind at 1000yds, if there is a push by 1mph to 11mph, it drifts 84in, an increase of 7in of drift. So if you miss that, with the target dimensions they was they are, that will push you from centre of x ring to the 5 ring as the distance from centre of X to edge of 6ring is 5in. With a 7mm 180gr hybrid at 2850fps, it drifts 57in which is 20in less initial drift then the 6mm but when you add the push of 1mph to 11mph, it still increases 6in to 63in which still will push you to a 5 ring just 1" closer to the 6 ring. So by looking at that, you gain nothing in windy conditions shooting a 7mm vs a 6mm it seems. With both cals, a push of 1mph will still push you from centre of x ring to 5 ring. What do you reckon? It also works out the same for a 1mph drop with both cals. Got me thinking a fair bit.

I worked out the same data with a 2mph push and the 6mm increases or decreases by 14in approx and the 7mm by 11in but, when I worked out the drift with the target dimensions, you still get pushed from the X ring to 4 ring with both cals.

Gents, am I missing something?

Quick,

I think your analysis above tends to over-simplify the situation. With the examples you use, it just so happens that the resulting scores are unchanged, despite the fact that the higher BC 7mm is significantly less affected by the change. This has more to do with the width of the scoring rings than any similarity of performance.

Dave McNamara wrote an Excel application which simulates many 1000s of shots at an ICFRA target, where you can change a multitude of parameters such as velocity and BC, and you can fiddle with errors with among other things, wind speed estimation. Looking at your figures for your 6mm, I've deduced that you might be using 107gn SMKs at about 3200fps? So I compared that with 180gn hybrids at 2850fps in the simulator. Using a wind speed estimation error SD of 0.5mph, average scores at 1000yd in a 10mph crosswind for your 6mm and 7mm will be 58.5 and 59.6 respectively, which I think you'll agree is a significant difference.

You asked for the opinions of members of the Raton Team. Mine is that high BC, regardless of velocity, calibre, bullet weight etc., is the single most important factor in reducing wind drift. Its no coincidence that nearly all F-Open shooters at Raton used 7mms or bigger. And I'm of the opinion that because my 7mm is accurate enough to get 60.10 at 300, there is no need to complicate things by using more than one calibre. On the other hand if you also do other disciplines such as BR, then lighter calibres may be appropriate.

I hope this helps to convince you not to abandon your 7mm project.

Alan

[Quick]

Alan,

Thanks. I have no intentions of abandoning my 7mm project (just have to decide on cartridge). Thats very interesting. When I posed the question, I knew I had to be missing something but was unsure what so I decided to ask. I know the performence between cal is very different, I was taking into account the width of the scoring rings and that is indeed one aspect that got me thinking. I didnt do a calc based on 0.5mph change as I think that would give the 7mm a score advantage. I was chatting to Brett B a few months ago about all this and he said its the push or drop off that you have to take into account and I thought I had when I did these calculations. I probably did miss something as it was 5am and I had been awake since 3pm the previous day.

Thanks for all the imput guys. It got me thinking is all and as your probably know, I ask alot of questions!!!

Cheers guys.

[DaveMc]

Hi Alan - If you have the space/allocation you can post the excel spreadsheet here if you want. - Might help not only Quick but a few others. Cheers,
Dave

[RAVEN]

Alan you are correct the hybrid is popular for two reasons
1: High BC
2: easy to tune jump or jam

I would have been just as comfortable using VLD's or Hybrids as I'm a jammer.

Long range grouping accuracy is KING
Followed by BC
Then velocity IMO
As 1 of the coaches we couldn't tell the difference between 180H @ 2800 and a SAUM or WSM @ 2950 - 3000
The team success was due to other factors and not belting out some 7mm or 30cal at light speed
Use a 6 /6.5/7 or 30cal develop an accurate load then learn to read the wind simply really


RB

[Quick]

Dave,

Thanks mate. That would be awesome. I'd like to have a look.

[macguru]

Yesterday we shot at 500 m in variable winds that settled down a little before the second stage. I had a 6BR 105gr at almost 2900, another shooter had a 7mm with 180gr VLDs.

The vertical from both was really tight. The 6br horiz spread was 4-5in, the 7mm managed about 3in. Both had about 1.5 in vertical, the 6br maybe a bit better.

The 7mm held the 6 ring for all but one shot and the 6mm dropped a couple of extra ones. I think thats what you can expect if its moderately windy ...

[RAVEN]

macguru isn’t it all relative to the calibre you shoot?
RB

[macguru]

Its relative to the BC (6mm=0.5, 7mm=.65), not the calibre, or we would all be shooting 8mm

[Matt P]

IMO, it's about a balanced package,accuracy, BC,recoil,stock design. Looking at BC and wind drift figures don't really tell the whole story, for example it's based on all shots going through the one hole (nice theory but doesn't happen in the real world), being able to fire 30-40 good shots in a day, now some out there maybe able to handle the recoil of a big 30 shooting heavy bullets, but most can't, I'm not the worlds smallest guy but there's NO WAY I could !!!!
IMO the "best package" at the moment is the 7mm bullet in the 162-180 grain mark with a MV from 2800-3000, stock design needs to allow quick follow up shots when conditions allow and be comfortable to shoot over 10 stages over 3 days. The 6mm's probably have greater accuracy, but with the current scoring zones it's advantage gets eaten by the 7mm BC, that's what I mean when I say a "balanced package".

Matt P

[macguru]

Ditto to all that
and the results in Raton back it up