7mm at 300

[macguru]

"holding the x ring at 300", given that the x ring is 35mm, means about 1/3 moa. There are plenty of good 6mms that will hold 1/4 to 1/3 moa... (and 223s for that matter)

I have not seen many 7mms or 308s that hold 1/3 moa but they are usually the guys winning comps so of course they are around. There are quite a few 308s that will hold 1/3 until the wind picks up.

[AlanF]

In my opinion 300 is a different beast. You can have quite bad velocity variation without vertical. At 500 you start to get affected by it, and at 1000 it will really hurt you.

[DenisA]

Sorry guys, I don't mean to sound like a smart arse, but I'm not understanding the discussion.

If a .284 is a top contendor for holding the X ring (approx .5 moa) at 500y or 600 yards and further where its subject to longer time of flight and atmospheric affects, why would it be any less accurate trying to hold an X ring (approx .5 moa) at 300 with less time of flight.

The 6mm has the advantage of less recoil, however the recoil of a .284 is a non issue anyway.

In my experience and understanding, a load that shoots great at the longs, shoots great at the shorts. A load that shoots great at the shorts doesn't necessarily shoot great at the longs.

32mm X ring or not, its still .5 moa............. except 800 and 900.

What am I missing?

[DaveMc]

Chop, I have the luxury of many 7mm barrels and an environmental condition that turns the practice pad into a shooting gallery every afternoon. During the day it is a different story. During the day I am lucky to hold half a minute (x ring or so and can be much worse) with all barrels but given the right conditions about 80% of the barrels I have will hold better than quarter moa elevation at 300 (some significantly better) and that is for 10-20 shots. BUT only a very small percentage of them will hold superv elevation at 1000.
This statement holds for those that come out here to load develop as well.

It is the same for my 6mm and 6.5mm barrels - I see no practical difference in "accuracy". The only difference I really see is the 6BRs can be shot by almost anyone on almost any platform and shoot at least reasonably well (I don't mean that as an insult to 6mm shooters - it is certainly not a reflection on their shooting or wind reading skills) . The extra recoil and torque make the 7mm an artform. If you have the right platform it makes it a lot easier. It is why I don't like shooting the 6BR much anymore (I actually - LOVE shooting the 6BR but it makes it hard coming back) and use either the 284SH or RSAUM from 300 through now - even at club level and most of my club members do too now. Scores in our club from several shooters are regularly 60.8-60.9 at 300-600. If you don't get a high x count on any half decent day you aren't in the running.

[AlanF]

Sorry guys, I don't mean to sound like a smart arse, but I'm not understanding the discussion.

If a .284 is a top contendor for holding the X ring (approx .5 moa) at 500y or 600 yards and further where its subject to longer time of flight and atmospheric affects, why would it be any less accurate trying to hold an X ring (approx .5 moa) at 300 with less time of flight.

The 6mm has the advantage of less recoil, however the recoil of a .284 is a non issue anyway.

In my experience and understanding, a load that shoots great at the longs, shoots great at the shorts. A load that shoots great at the shorts doesn't necessarily shoot great at the longs.

32mm X ring or not, its still .5 moa............. except 800 and 900.

What am I missing?

Denis,

The MOA differences, slight though they may seem, are enough for me to say that I am more likely to get a 60.10 at 800yd than 300yd. In terms of subtended angle, the easiest ICFRA target of all is the 700m, and the hardest is the 400m. See the figures below. Note that if you allow for 8mm gauging, then the MOA figures would all be larger, but would retain the same order of difficulty.

[SENDIT]

Hey chop, I tried a light load in my saum, all I got was a few hang fires. The case is to big you have to fill them up. So I binned it and went back to 6mm for shorts.

[DenisA]

Denis,

The MOA differences, slight though they may seem, are enough for me to say that I am more likely to get a 60.10 at 800yd than 300yd. In terms of subtended angle, the easiest ICFRA target of all is the 700m, and the hardest is the 400m. See the figures below. Note that if you allow for 8mm gauging, then the MOA figures would all be larger, but would retain the same order of difficulty.

I agree Alan. I believe the 800 and 900 use the same dimension target as 1000 yards.

My way of thinking says that if a 7mm can hold the x ring at 600 yards for example (.451 moa) including all error attributed to the extra time of flight, why would it not hold .401 at 300 yards not being subject to the extra parabolic error of 600.

Edit: Thinking on it, I'm probably being a little presumptious. Though I think theres some logic to it.

[AlanF]

...My way of thinking says that if a 7mm can hold the x ring at 600 yards for example (.451 moa) including all error attributed to the extra time of flight, why would it not hold .401 at 300 yards not being subject to the extra parabolic error of 600.

Yes to that too.

There is one other effect that can complicate things. I believe a barrel can be tuned for a particular range only. This could result in it having more vertical at a shorter range than the "tuned" range.

Alan

[DaveMc]

[See the figures below. Note that if you allow for 8mm gauging, then the MOA figures would all be larger, but would retain the same order of difficulty.

Not Quite - 300 becomes relatively easy (x wise) - around half a minute when gauged.. The 700 yard target becomes the hardest to get the illusive 60.10


yards x
actual gauged
mm moa mm moa
300 32 0.40 40 0.50
400 43 0.40 51 0.48
500 65 0.49 73 0.55
600 72 0.45 80 0.50
700 80 0.43 88 0.47
800 127 0.60 135 0.63
900 127 0.53 135 0.56
1000 127 0.48 135 0.51

[AlanF]

it was late....

Here's the list which allows for 8mm gauging for both yds and metric :

[bartman007]

Chopper, I have run my 284W at the shorts and now after cutting the Krieger 1" off the end, it actually shoots 60.5 59.6 instead of the 60.1 that it used to shoot. Fine tuning this weekend will tell if I can get a few more centers out of it.

My personal preference is to run the 6mm or 6.5x47L for the shorts, as the center count is much higher! Which says to me that these two cartridges are easy to fine tune. They are also easier to shoot, allowing for shots to be gotten off quickly.

My SAUM at 800 yards originally got 90.12 with 3 differing loads during that shoot. I have shot a couple of long range 180's with it. It is super accurate, but I have never shot it at 300 yards, as I consider it a waste.

[bsouthernau]

There is one other effect that can complicate things. I believe a barrel can be tuned for a particular range only. This could result in it having more vertical at a shorter range than the "tuned" range.

Alan

I'm not saying it ain't so but I'd be very interested in an explanation of how this comes about. Perhaps I'd better check that I've understood you correctly. Just plucking some figures out of the air - are you saying that if a rifle shot a vertical spread of .40moa at 500 the vertical might have been .45moa at 300? In this age of electronic targets we can conceivably conduct such trials.

For this to take place it means that the bullets which have been deviating from the centre of the group for the first 300 yards now reverse their errant behaviour and come back in. And they do this every time! As Prof Miller used to say "Why is it so?"

Barry

[RAVEN]

Barry the stability of these long bullets out of fast twist barrels can take some distance to fly straight meaning the gyroscopic is more pronounced at shorter ranges
This can be more evident at 100 yards
And doesn’t apply to all barrels I have experienced this personally with my old 7mm Rem Mag it would group better at 300yds than 100yds


http://bulletin.accurateshooter.com/2013/07/over-stabilization-of-bullets-why-is-too-much-spin-a-problem

[DaveMc]

Although I think this is a slightly different statement to the one Alan is making Richard it is an interesting experiment. For those that have not done it before - try doing a load development ladder at 25m (if your scope parallax will go down that far and you have to be very careful about setting it - not just on focus but so any eye movement does not change target image) then 100 then 300m. I know someone that very successfully tunes at 25m with a good chrony.

I have tried this many times and in my barrels (which admittedly aren't over twisted) it would appear that the epicyclic swerve is very small and only sometimes evident at 25m. If you can get a one hole group (looks like one bullet hole) at 25m then it generally shoots very well further out (as long as velocity spread is low).

I have occasionally seen groups that are great at 100 but slightly overlapping at 25m - but big question marks on this one and always hotly debated.

[AlanF]

There is one other effect that can complicate things. I believe a barrel can be tuned for a particular range only. This could result in it having more vertical at a shorter range than the "tuned" range.

Alan

..."Why is it so?"

Barry,

I may have used slightly the wrong words and implied that it can be done with any barrel.

I ladder test at 500yds, and get varying results with the shape of the graph, as tuners and bullets etc change in different barrels. Occasionally I see part of a graph where as velocity rises, the POI drops. This gives a very good tuning opportunity. As range increases, the barrel harmonic effect won't change in MOA terms, but the velocity variation effect will increase. What this means is that at some range further out, there will be a point where velocity variation is compensated very closely by the barrel harmonics and a very good tune is achieved i.e. the rifle will have less vertical (in MOA) than at 500yds.

Alan