.308 V 7mm SAUM & Wind

[Stuck]

Last Saturday I was shooting @ Diggle in less than ideal conditions (unusual for Diggle I know!) :roll:

The weather was bad so the comp was moved from 900 to 1000 yards so we could shoot from the undercover firing point, it was raining hard & the wind was gusting between 10 & 30 MPH.

At the end of the comp I was talking to another shooter that was using a 7mm SAUM from a 31" barrel (168gn bullet I think) & I was using a .308 with a 30" barrel & 155gn bullet.

We shot the same detail so the weather was exactly the same & I'm assuming we both had muzzle velocities of around 3000 fps but I had to dial in 14 mins of wind, he had to use 6!

Now I know the .308 is on the limit in these conditions & I'm shooting F/TR whilst he's in F Open but would you have expected such a difference?

[Laurie]

Short answer: No.

Most 7mm short magnum equipped F/O competitors shoot 180s at long range and the SAUM will give such ~3,000 fps comfortably from a 31-inch barrel, more if 'pushed'.

Let's assume your acquaintance was using a 180, not a 168, and that it was the most efficient model readily available on the UK market, the new Berger Hybrid. It has an average G7 BC of 0.345. With a 3,000 fps MV, Bryan Litz's own PM Ballistics Solver v2 says it'll be travelling at 1,704 fps and would move 5.01-MOA in a 10 mph true crosswind at 1,000 yards in my take on Diggle's conditions on Saturday (50F, low pressure + 900 ft ASL and 90% humidity).

You don't say what 155 you used in your .308, but let's input a G7 BC of 0.237 which is that for the Berger 155.5gn BT Fullbore, somewhat more efficient than the Sierras, not a lot more than the Scenar or Foxy's HBCs. At 1,000 it's computed to be travelling at 1,334 fps and would see 8.41-MOA drift in the 10 mph wind.

That makes the SAUM 40% more efficient in coping with the wind compared to a 308/155 right in line with what I'd expect. 14-MOA reduced to 6 is a near 60% reduction, a lot more. The actual wind speed and direction isn't an issue in itself as it's the relative efficiency that matters, a 15 mph crosswind simply increasing the actual values by 50% but not changing the relationship.

So, one or both of you had an inaccurate wind zero, none too accurate windage adjustment values provided by your scopes, or produced an off the top windage figure from memory, more likely a combination of these factors.

I would normally have been with you on Saturday, but as I drove down from North Yorks to the British Shooting Show at Stoneleigh leaving at around 06.30 Saturday and was buffeted and travelled through heavy rain and sheets of spray even on the sheltered eastern side of the Pennines, I did wonder just what sort of conditions people would see in the F match a little later - now I know!

[Stuck]

Thanks very much Laurie.

I appreciate your reply.

[Maggot]

We usually see about 40% between the Open effers and F/TR Stuck.

As Laurie says though, there are a lot of variables. BC and balistics aside, Not all scopes are true MOA scope to scope.

Not all wind zeros are spot on (I know mine needs looking at) and then you should have about 1moa spin drift at that range (if you were smack on niil wind at 200, you would still be about 1 moa right at 1000 identical conditions).

Add to that I guess you were not firing through identical bits of air, at the same instant, different lanes, different trajectories (the wind gradient in a gust can show massive differences in velocity over quite small changes in distance from the ground, the harder the wind the greater the gradient/difference, so he may have had an MOA or 2 on you through this alone....just go mountain flying...brown trouser stuff!!!).

Wind reading is a black art mick.

Just ask those who shot in the last detail of the euros individual....some went on with 10 and came off with 17 on the rifle.

Tim Stewart summed it up quite nicely to a punter at 1000 on the open day. "If you could mark the trace of each bullet as it flew it would blow your mind. few if any would be the same or where you would expect them.

[ovenpaa]

I spent years with an Accuracy International zeroed at 100m on a still day, then I shot it in an underground range as was surprised to find it was 0,2 mils out (nearly 3/4 minute) on wind zero, it had never occurred to me it could be wrong. It made no difference to the rifle accuracy or my wind calls once it was on true zero and I use the wind zero on the turret as a reference point, the rifle is never shot with the turret set to zero because that 'perfect' day will never exist. Closest I got was at Ulfborg 600m shooting a 7mm SAUM Wildcat with 1/4 left wind on a Nightforce 42X BR

[meles meles]

Do you actually adjust your scope settings for wind, or just aim off ? We find it easier to zero to a true zero on an underground range, then aim off. Dragunov scopes have a reticle that makes aiming off relatively easy - more so we think than changing the turret settings.

[Maggot]

Hi Brock

I shoot F/TR and regularly get mixed with F open.

I used to get a mean wind correction them aim off and my shooting suffered with verticals and all sorts.

It is quite awkward to plot (but not impossible) and it can lead you to forcing the rifle onto aim.

The open guys have usually to do less compensation and can easilly move the front end with a joystick, I tend to let the bipod bed in and move the rear.

It is also easier to get lost without good wind data recorded relavent to the actual point of aim, which is on the plot sheet. We can be well over 10moa which is a full rotation or more on some scopes.

I guess it depends on you tactics, mean wind and bookend it, or correct for every shot. I do a bit of everything (in so far as I am capable of, its an uphill learning curve).

If I was shooting tac and had a scope with Mills or MOA hashes I would use that, but for F/TR I am using a LRTD ret which really works well being dialled.

Having the time and the luxury to plot, I use it. Proper TAC shooting, I would aim off but I think it needs a decent ret to do this properly as you can repeat the POA IAW your wind data.

Do what works I guess

[ovenpaa]

Do you actually adjust your scope settings for wind, or just aim off ? We find it easier to zero to a true zero on an underground range, then aim off. Dragunov scopes have a reticle that makes aiming off relatively easy - more so we think than changing the turret settings.

A combination of both. If say 7 minutes is required I will make the major correction and then work with a mix of minor corrections and aiming for safe shots. By safe I mean if I estimate my correction to be 1 minute however it could be slightly less I aim slightly into the wind on the V so the fall of shot should remain in the V however if the wind has picked up marginally I will be blown across the V and absolute worst case fall into the 5.

Hope that makes sense and it is fair to say I mostly work with Mils however they tend to confuse people.

[Stuck]

Thanks,

All interesting stuff.

:goodjob:

[Maggot]

Do you actually adjust your scope settings for wind, or just aim off ? We find it easier to zero to a true zero on an underground range, then aim off. Dragunov scopes have a reticle that makes aiming off relatively easy - more so we think than changing the turret settings.

A combination of both. If say 7 minutes is required I will make the major correction and then work with a mix of minor corrections and aiming for safe shots. By safe I mean if I estimate my correction to be 1 minute however it could be slightly less I aim slightly into the wind on the V so the fall of shot should remain in the V however if the wind has picked up marginally I will be blown across the V and absolute worst case fall into the 5.

Hope that makes sense and it is fair to say I mostly work with Mils however they tend to confuse people.

MONG :55: