Cw308,
What velocity do you get and what is your barrel length? I pulled down some GMM308 168 back around 1998-99 and got 43.5 grains of 4064 and a rating of 2650 fps (24" test barrel). I put the same load into QuickLOAD with 57 grains for Federal case water capacity and got 2651 fps predicted, so I called that good validation of my surmise that the powder was 4064. My pull-downs also were very slightly compressed, and QuickLOAD gave 100.8% case fill, so that matched what I saw, too.
Later, someone told me Federal, after the ATK buyout, had switched to Reloader 15, but I didn't buy any to verify it. Subsequently, in developing Mk 316 Mod 0 for sniper systems to replace M118LR, which does use Reloader 15 (according to the brag at the Alliant site), they went back to 4064 because the RL15 had proven too peaky in desert heat. That makes me think they might have gone back to it for GMM168, but I haven't pulled down any recent production, so I don't know.
Even though bulk powder varies more in burn rate over time than canister grade, I'm wondering if yours is actually 4064 or not? The simple way to tell is to chronograph some of the GMM168's alongside your loads in the same brass to see if the velocity is the same. They still claim 2650 fps on their web site. Are you getting that (after adjusting for any barrel length difference)?
rebs said:
The brass I am talking about is all FC and the same lot as far as I can tell by the head stamp.
Unfortunately, the headstamp doesn't guarantee they are the same-lot. They can produce quite a number of lots in the same year. I remember A.A. Abbatiello's test of .30-06 National Match ammunition in the 60's and he reported having samples from 47 different lots. I don't know that it was all from the same year, but was likely not from too many years, since it was early 60's and none had been produced in the years from 1941 to 1956. So, lots of lots.
Also unfortunately, sorting by weight after firing in your chamber (to get outside case body dimensions the same), and then trimming to the same length before sizing (not normal procedure, but this is for sorting), and then by water capacity within that weight and length is the only way to have a reasonable assurance your cases even came off the same tool set within a lot. It's doesn't normally pay to put that much time into it unless you are shooting benchrest, and then you'd only do it after finding the cases with the lowest neck wall rounout, which will cull a lot with most regular brands.
Regarding the shooting: If a scope's parallax is set correctly for the range at which you are shooting and the scope is of good quality, the crosshairs should not move on the target when you move your eye but not the gun. That's the whole point of having a parallax adjustment available on the scopes that do. Those that do not have it will have the parallax fixed to at one range. Usually that number is 100 yards, but it's not costly just to ask the manufacturer to be sure. If I made a fixed parallax hunting scope I'd probably set it at 200 yards where Cooper put universal zero, and allow that suffering a little parallax error at 100 will amount to a smaller number of inches on a woodchuck than a 100 yard parallax scope could produce on him at 200.
Note that to eliminate parallax, the focus of the scope has to be correct. Assuming you have a scope intended to be free of parallax at your target range, you can try moving your eye left and right and adjusting the focus to make the crosshairs and target move toghether when you shift your eye position. This should be a small change in focus and not enough to make the image blurry if your vision is corrected. If you normally wear focus corrected lenses that you take off and allow the scope focus to compensate, tat will mess parallax compensation up by moving it to a different range. It only takes a few thousandths of an inch of adjustment of the lens positions to change parallax.
Note, too, that a cheap scope may have some parallax no matter how you set it up because, for example, chromatic aberrations in cheap lenses occur because different colors of light don't have the same focus point. Other cheap designs that have any looseness in the focus adjustment or main objective position can constantly shift the distance at which parallax is compensated out by changing the focal plane of either the main objective or the eyepiece on the crosshairs. But to start with, try tweaking focus to minimize parallax error. Also, check that moving your eye position up and down produces the same amount of parallax error (or non-error after focusing) in the crosshairs. If they are different, something is out of whack, like the lenses not sharing the exact same axis. A bent scope tube can do that.
Finally, since you are using a bipod, be aware the technique is a bit different than shooting off bags. It often helps to lean into the bipod slightly to preload the support a little. I'd recommend reading
Bugholes from a Bipod. Note the dry fire practice and loading techniques described. The Froggy's Lube mentioned (not the same as Frog Lube) is no longer commercially available. I was told it was simply powdered graphite suspended in alcohol.
A pause for the COZ said:
…Thats the cone, and its not random.
Actually it is random. It's just center-weighted randomness and not uniformly distributed randomness. Assuming a gun without tendency to string, mounted in a rest and fired in zero wind, it generally is a bivariate normal distribution made up of two perpendicular bell curve distributions. What this means is there is no such thing as
the cone. There will, instead, be a different cone diameter at any given range based on the confidence you want that a shot will stay within it. So, the cone that you can expect to have 50% of your shots within (aka 50% confidence your next shot land inside it), will be smaller than one you expect to have 90% of your shots within (90% confidence your next shot will land within it), which will be smaller than one you can expect 99% confidence your next shot will land within it, and etcetera. The cone will just keep getting bigger as your confidence requirement increases, becoming infinitely wide at 100%, which is of no practical use, of course. So you have to say what percent of your future shots you expect to keep inside the cone before you can assign a size to it based on the radial standard deviation of your shot history.
The other thing about the cone is it isn't actually a cone. It looks more like a herald's brass horn, with the sides curving gradually outward with a diminishing radius. This is due to the fact the bullet is slowing down as it flies, so that every hundred yards takes longer for it to traverse. Drift components that widen a group thus have longer to move the bullet away from the mean trajectory path with each successive 100 yards. Thus they increase the moa of error at each 100 yards a little.
Example: You fire a bullet with a G1 ballistic coefficient of 0.5 at 2600 fps and you get a 0.8 moa group at 200 yards. The time of flight (TOF) will be about 0.248 seconds. At 1000 yards the TOF will be about 1.735 s. So, assuming constant drift per second you had:
200 yds / 1000 yds × 1.735 s / 0.248 s = 1.399
0.8 moa × 1.399 = 1.12 moa expected at 1000 yards
Additionally, the horn is not quite straight. Spin drift bends it in the direction of the rifling twist (bends right horizontally for right hand twist, and left horizontally for left hand twist).
