Your precision requirements will determine your approach. If your expectation is that the rifle and load need to be capable of hitting very small targets at your self-imposed maximum shooting range of 320m, then you will likely want to either know the drops from your own measurements or be satisfied with predicting these from an external ballistics calculator where a chronographed muzzle velocity will be a required input. In answer to your question I think either approach will be fine. Measure the muzzle velocity and use a trajectory calculator if access to a 320m range is not convenient, otherwise go ahead and record the actual drops. Both together are not needed, though the one can be used to verify the other. The deviation from the actual drops by instead using a calculator in combination with an accurate MV as an input to predict these will be insignificant out to 320m. Best to also note here that having gone and found your trajectory by either method, if shooting out to only 320 metres, you can then pretty much neglect any day-to-day or situational variables that contribute to changes in the point-of-impact other than if shooting steeply up- or down-hill, and of course movement from wind.
You will still need to be able to accurately zero your rifle — and I would recommend doing this on an ongoing basis to get a handle on how well your equipment holds its setting. If you can dial your scope – or have and will use a tree-style scope reticle – choose either a 100m or 200m zero; it doesn’t really matter much which. If you can’t dial your scope and have a plain duplex reticle then use a 200m zero as it will be easier to raise the point of aim for the smaller elevations required at maximum range that result from the more distant zero. So access to a 200m range for your zeroing checks may determine your approach.
If on the other hand the rifle and load are to be used where the allowable deviation in the points of impact away from the point of aim can be that much larger for the shooting to still be considered successful (such as perhaps for hunting deer) then I would say neither the measuring of drops or the muzzle velocity is necessary; enjoyable to carry out of course as with all shooting, but not necessary. MD’s earlier advice is then applicable.
As with many of the questions asked here, we are often short of a few pieces of the puzzle to start with, and this can result in the replies from members being quite varied and often seeming contradictory. This may be a result more of the assumptions being made to fill in the information gaps as much as differences in approach. Knowing your "short" barrel length would have been helpful for example. It also then becomes difficult for yourself to filter the quality of advice unless the replies you receive have laid out the assumptions being made to cover off the missing detail.
In line with this, I’m also going to also then make an assumption, based on the advice being requested, that there may not be an expectation in the equipment and yourself of always being able to place your shots within say 40mm of the point of aim at 320m. This is a 1MOA grouping and difficult enough to achieve off a bench let alone in field conditions at that range. If this is true, then as above you can dispense with the tall-target drop assessment and/or the chronographed MV as long as you choose an appropriate zeroing distance.
The reason for this, and the point MD made, is well demonstrated by plotting the trajectories for the likely minimum and maximum muzzle velocity from your rifle rather than for a single MV. With the bullet and load you have given, the MV will be close to 2500ft/second from a 15” barrel and 2700ft/second from a 20” barrel, so let’s have a look at the difference this makes, while knowing that your load will be somewhere within this range, but without knowing the exact MV?
Using in this case the JBM Ballistics online calculator (I’ve also taken the liberty of increasing the scope height to 45mm and filled in the other variables with typical entries that to 320m make very little difference to the outputs); if you decided to take shots believing that the actual MV was midway between these two extremes at 2600 ft/sec, giving a point-of-impact predicted at 16.5” below the line of sight at the max range of 320m and with a 200m zero, then you would never be more than 35mm away if the velocity was by chance either the full 100 foot-per-second faster or slower. Your actual look-up corrections, should you get to measure them, will be found within the bounds of the 2500 ft/second trajectory shown in blue and the 2700 ft/sec in red graphed below.
Attachment 214888
You’ll need to decide if you can adjust your point of aim upwards at your maximum range to accommodate this amount of drop without a dial-up scope. It can be read off the graph that limiting yourself to 300m instead brings the correction needed back to about 13 inches, an amount that might be more manageable?
Similarly you may not be particularly happy about the bullet rising to around 3” above the line of sight at around the 120 metre mark. This can be addressed by pulling the zeroing distance back to 100 metres. I’ve graphed what the expected trajectories will then look like, again the 2500 ft/sec MV in blue and the 2700 ft/sec MV in red.
Attachment 214889
While the bullet rise issue is now fixed, the problem for a non-dial-up scope is then transferred out to your maximum range where – if you assume the predicted 25.9” drop below the sight-line for a 2600 ft/sec muzzle velocity – you may be as much as 70mm off from the true drop if by chance the true MV was at either end of the velocity extremes.
Even if you knew the actual muzzle velocity, or had recorded the actual drops, so were certain of where the true drop lay out at 320 metres, I’m not sure how one best deals with this amount of fall below the line of sight without a dial-up scope, so I would propose that a 200m zero is your best bet with a set-and-forget style reticle adjustment.
All interesting information on which to ponder? As usual there is a lot of mixing of metric and imperial dimensions in the above, but we are all used to this now.
