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Old 06-14-2021, 07:51 PM
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Nomercy448
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Default LR Fundamentals: Matching Cartridge Trajectory with Optic Capacity

As CalHunter and I spoke about this new forum, we agreed that building a few reference topics on fundamentals of long range shooting might bring value for some folks. So here we go:

While this may be second nature among experienced long range shooters, I’m not sure I’ve ever seen an article or blog entry specifically describing the need to select an optic with internal adjustment which matches your cartridge trajectory, so if the next line needs no explication for you, then save yourself the time reading, or offer your own advice here as well, but if anyone is needing help working through this topic, I hope this info helps save time and headache…

The principle is simple: Just as our cartridge needs to be capable of reaching our desired distance, our optic has to have enough internal adjustment (with or without angled bases or shimmed rings) to accommodate the trajectory of the cartridge at that range.

Most shooters start contemplating their long range journey from one of two perspectives: either with a particular range in mind, for which they then divine a cartridge to be used, or with a particular cartridge in mind and the intent to stretch it as far as they can. In either of these cases, the next step is often missed and I’ve seen many new long range shooters get frustrated by wasted energy and money on a simple oversight. Optics must have sufficient internal adjustment to enable the cartridge to be used for the task.

Transonic Boundary Limitation

I’ll interrupt myself here to introduce another relatively common trap which prevents many new shooters from reaching their max range goals with a chosen cartridge: Transonic Boundary. Without excessive detail, the Transonic Boundary is the transitionary span where the bullet starts slipping out of supersonic velocity flight into sub-sonic, and the flow regime of air around the bullet changes dramatically, which can (at least temporarily) destabilize the bullet and open groups considerably. For example, a 50grn V-max from a 223rem will slip transonic around 600yrds, and be fully subsonic by 700yrds – so reaching 1,000yrds with a 50 Vmax can be… challenging! Comparatively, a 73 ELD in 223rem may transition around 900yrds, a 308win with a 168 BTHP around 950yrds, and a blazing 6mm Creed with a 105 at 3145fps will make it almost to 1400yrds before falling transonic. Some big ELR cartridges can make it past a mile before transition! So, a shooter has to understand the capabilities of their cartridge and calibrate expectations for maximum range potential before buying or building a rifle – we CAN shoot past our transonic boundary, but it’s much more difficult than staying supersonic. (Image borrowed from Accurateshooter.com)



Choosing an Optic which is well suited for your trajectory needs

Again, the principle is simple: Whether a shooter starts with a desired range in mind, or with a specific cartridge, the trajectory compensation required to reach this range with this cartridge must be met by the internal adjustment of the optic - possibly extended by an angled base or specialty rings to increase the usable portion of the internal adjustment.

Interrupting myself briefly here again to define “Optical Center” of a scope. When a scope is optically centered, the erector tube is centered within the scope body. This can be accomplished by rolling the scope in front of a mirror until only one reticle can be seen, OR by counting clicks from top to bottom and side to side, and dividing each in half (acknowledging elevation Zero Stops can complicate this).

We can use this theory in practice; an Optically Centered scope mounted in standard rings, on a level base, will be roughly parallel to the bore axis, so we can optically center a scope, mount it on the rifle, dial “up” the optic height expressed in MOA at 100yrds plus the absolute gravitational drop occurring over 100yrds (as found by our ballistic calculator), and be very close to zeroed at 100yrds. The graphic below depicts the principle – the scope on the top rifle is Optically Centered, so the line of sight (Green) is parallel to the bore axis (Red), and the bullet trajectory (blue) falls even LOWER than both. We dial UP the optic height (distance between red and green lines), plus dial UP the absolute drop of the bullet at 100yrds (distance between red and blue lines), and we have our rifle very close to a 100yrd zero without ever firing a shot.

Optical Center + optic height (as MOA) + Absolute Gravitational drop at 100yrds = rough 100yrd zero



The above method allows us to estimate where our 100yrd zero might be relative to optical center, then we can go back to the ballistic calculator and determine how much elevation correction we need to reach our desired maximum range or our transonic boundary – then we’ll know how far below optical center we’ll need to be able to dial – we double that number and to identify how much total elevation adjustment range we need in whatever scope we buy.

For example: a 308win firing a 168 BTHP at 2700 with 50mm objective scope sitting 2.1” above the bore will be roughly zeroed at 100yrds by optically centering the scope then dialing UP 2.0MOA for the optic height, plus UP another 2.5MOA for the gravitational drop to 100yrds.

So, now we’re zeroed at 100yrds. Our erector tube is very nearly centered in the scope body tube, leaving us less than HALF of our adjustment range left to compensate for shooting at longer ranges – “half,” less the 4.5MOA UP we dialed to zero. To reach 1,000yrds, this load will need another 37.0MOA “UP” over a 100yrd zero, meaning from optical center, a flat mounted scope would need 41.5MOA of “Up” with this load – and since optical center is only HALF of the adjustment span, we’d need ~83moa total adjustment, which most scopes simply do not have. The diagram below shows an optically centered scope on top, with the erector tube effectively zeroed for 100yrds in a flat base. The bottom scope reflects the erector tube dialed as far “up” as can be, and the divergence between the red and green lines shows the maximum correction we can make.



If we add a 20moa base to the rifle, angling the tube body down slightly towards the barrel (exaggerated in the diagram below), we gain range from our scope. Using this 308win example, we have to dial DOWN 15.5MOA from Optical Center when using an 20moa base to zero at 100yrds, and when we dial UP 37.0MOA from our 100yrd zero to reach 1000yrds, we only dip 21.5MOA below optical center (+15.5 – 37 = -21.5). That means we only need need a scope with ~43MOA of total elevation adjustment to reach 1,000yrds with this load when using a 20MOA base – which is possible with many common rifle scopes on the market. In the diagram below, the angled base tips the front of the scope down slightly and we can see that we’ve had to dial “down” into the top half of the scope to keep the green line (roughly) horizontal for our 100yrd zero. Comparing the top and bottom scopes in this example to the two above, we see we have more room left below our 100yrd zero to dial for longer range before the erector tube bottoms out - about twice as much as the flat base case above.



As mentioned above, cartridge choice matters a lot. Let’s say instead of that 308win firing 168’s, we choose a 6mm creedmoor pushing a 105 Hybrid at 3145fps - the same scope sitting 2.1” over bore will only need to compensate for 1.8moa of gravitational drop for a 100yrd zero because the 6 creed is so much faster, totaling 3.9moa UP (instead of 4.5) to zero at 100, and then reaching 1,000yrds only requires 22.2moa correction above a 100yrd zero (instead of 37.0). With a flat base, we’d only need 26.1MOA “up” to reach 1,000yrds – meaning we need a scope with only ~53MOA total adjustment. With a 20MOA base buying back some of the top half of our adjustment, we’d only need 33MOA total elevation adjustment – which is possible even with inexpensive 1” tube scopes! In short, 6 creedmoor is considerably less demanding than a 308win, so a shooter can buy a scope with much less adjustment, or avoid the expense of an angled base.

So, we finally get down to optic choice: since erector assemblies are roughly the same size within different scope tube diameters, increasing tube size (typically) adds adjustment capacity. The erector tube has more room to move, which means more adjustment capacity. Below is a list of a few examples of limited capacity and large capacity scopes, and their relative tube diameters:

· Nikon Monarch (discontinued) 6-24x50mm 1” tube with only 30moa internal adjustment

· Bushnell Engage 6-24x50mm 30mm tube 50moa internal adjustment

· SigSauer Tango4 6-24x50mm 30mm tube 60MOA internal adjustment

· Vortex Razor AMG 6-24x50mm 30mm tube 71moa internal adjustment

· Burris XTR II 5-25x50mm 34mm tube 90 moa internal adjustment

· Bushnell DRM II 3.5-21x50mm 34mm tube 103moa

These differences can and do matter when we start stretching out long. Using the same 168grn BTHP in 308win at 2700fps as an example – the Nikon Monarch 6-24x would run out of adjustment around 615yrds, and can only be stretched at most to 900yrds by adding 15MOA of additional correction with Burris Signature rings (more than 15MOA cannot be used, else the rifle cannot be zeroed at 100yrds). Alternatively, a Bushnell DMR II could take that same load out to 1200yrds in a flat base and rings, or out to ~1650yrds with 50moa of correction made in the base and rings (20MOA base + Burris rings using 30moa of shims). Again, acknowledging this exceeds the transonic boundary for the cartridge, AND acknowledging dialing to the maximum limit AND holding over in the reticle can increase the shooter’s reach even further.

In Practice:

I’ve been working with my boy on his long range marksmanship skills the last few years, and one of the tools we’re using is a Ruger 10/22 Charger pistol. Originally, I placed a (terrible choice) Nikon Buckmaster SF 6-18x40mm, with 1” tube, 50MOA internal adjustment, mil-dot reticle, and 1/8 IPHY adjustable turrets. Using an EGW 20MOA rail, and Remington Golden Bullet 36grn hollow points (1.8” optic height + 13.5MOA gravitational drop compensation in 100yrd zero), my 100yrd zero was ~4.5MOA above optical center, leaving me a total of 30MOA of usable adjustment… Which only allowed me to reach about 275 yards by dialing, and reaching 325yrds on our home range meant I was stuck holding over 3mils in the reticle on top of all of the dialing. Adding 15MOA of shims to Burris Signature Zee rings, I was able to reach out to the end of my home range without holding over, but couldn’t reach any further – AND of course, mixing and matching 1/8 IPHY turrets with the mil-dot reticle, and especially living on a second focal plane reticle was… sub-optimal…





In redeploying this pistol as a tool for training my son – the smarter way, instead of the harder way – I replaced the little Nikon Buckmaster with a Bushnell DMR II 3.5-21x50mm, with 34mm tube, 30mils (103MOA) of internal adjustment, and G3 milling reticle – and of course, 0.1milliradian click adjustments which match the milliradian based reticle. Even with a slower load, CCI Standard Velocity 40grn ammo at 1070fps (17MOA of gravitational drop built into the 100yrd zero + 2.0MOA of optic height = 19.0MOA), the generous internal adjustment of the Bushnell DMR II (still using the 20MOA EGW rail) allows us to reach 340yrds without ring shims, or holding in the reticle. On the menu for this little pistol is a set of Burris XTR Signature rings, which will let us add another 30MOA/8.7mils in the rings, reaching a hair past 500yrds with this pistol before holding over in the reticle – and using all but ~1/2mil of the full adjustment range. Here, my boy is warming up with transitions on the 50yrd falling plate rack with the Charger and one of his DMR II’s:



For this pistol, the difference between two scopes meant a max range difference of over 50%! So having the right scope to match our expectations makes a pretty significant difference in what we can do with the pistol.

Hopefully, all of that makes sense, if not, questions are welcome!
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