Brugha84 wrote: »

Hi folks. Looking for some advice from ye! Just bought a Tikka T3X CTR and waiting on a vortex Viper PST gen II 5-25x60 FFP MRAD scope.

Just checking something first. Are you sure it's 50mm objective lense and not 50mm? I ask because for ring selection it will be important enough.

The rifle come with a 0MOA picitinny rail. I’d like to use it mostly on the range on longer distances out at least 600yrds to 1000yrds.

Most calibers (centrefire) will reach 600 yards with anywhere between 11 to 19 MOA of adjustment meaning unless you use a ton of elevation to get your zero you should easily make the 600 yard with the 0moa rail and standard set of rings.

For distances beyond 600 you start to need more elevation even for short bumps, say 800 to 900 can take an additional 10M MOA or more. Then another 12 to 14 MOA for 900 to 1000.

This is also a reason to have a well measured set of rings. The higher the ring holds the scope of the boreline (dead centre of your barrel) the more elevation you will use to get your zero. So try to measure the rings to fit about 3mm off the top of the barrel.

To pick the rings you need to measure the height off the barrel needed or clearance needed.

Usually what i do is;

• Take the diameter of the objective lense, say 50mm (that is why i asked was 60mm correct as this mistake will mean the rings will work or not).
• Add 3mm (top and bottom) for the width of the housing and 3mm for the minimum height off the barrel.
• So now you know you need 59mm clearance.
• Split that which gives our height from the barrel to the centre line of the scope tube. So 29.5mm but round it up to 30mm.
• Now measure the height of the rail. Do this by measuring the actual height of the rail from the flat underside to the top of the picatinny rail. Not in the "grooves", must be the top of the rail.
• Say its 7mm.
• Take the 7mm from the 30 mm leaving you with 23mm and that is the height of the rings you need.

Once you have this number you simply need to find out how the manufacturer measures their rings.

For example some measure their ring heights from the base of the ring (the flat piece that sits on/touches the top of the picatinny rail) to the bottom of the curve of the ring (where the tube of the scope makes contact with the inner of the ring).

Some measure from the base of the ring (the flat piece that sits on/touches the top of the picatinny rail) to the centre of the ring diameter (centre line of the tube if you will or where the two halves of the rings come together).


Once you figure this out you choose the correct size accordingly.

Would I have to change out the factory rail to a 20MOA or can you use a slightly higher ring to get around it it?

Simple test.

Next time you are at the range get your 100 yard zero. Mark or record the setting. Now dial the elevation turret all the way down to 0moa. Count the clicks/revolutions/MOA used to reach absolute bottom of the elevation settings. Now dial up the same amount and fire a shot or two to make sure you are still on zero.

If you are back on zero then this reading, we'll call it 23.5 MOA (or 94 clicks), is your zero reading. If the scope has 70 MOA (or 280 clicks) then you have just used 23.5 moa (94 clicks) to get your zero meaning you have at most 46.5 (186 clicks) left of elevation adjustment.

Now you consider the distances you want to shoot.

Out to 600 yards, and using at most 20 moa, you will use up half of your remaining adjustment. No problems there and no need for an elevated rail.

To get out to 100 yards, and this really depends on caliber, you may need another 14 to 20 MOA. Which you have if all the numbers hold true and the scope will do the 70 moa it claims. So you need not buy an elevated rail and can shoot away.

Now the other side of that. Some believe, myself included, that shooting a scope on almost maximum adjustment puts unnecessary pressure on the scope mechanics having it at or close to max adjustment when firing. Its why I recommend to anyone asking the same question as yourself to consider an elevated rail if this is going to be the case. There is no drawback from having one.

If you really don't need it then don't get it. This would be in a scenario where your zero is NOT 23.5 MOA but more like 5 MOA. However it's rare, actually i've never seen it, that a scope uses so little adjustment to get a 100 yard zero.

One other thing to remember is a 20 MOA rail (example) may not give you 20 MOA. Depending on make/model you may get between 14 - 20 moa. The other thing is if you use say the example of 23.5 MOA to get 100 yard zero and you stick on a 20 MOA rail it does not mean you will now only use 3.5 MOA. Because of the imperfections in a rail, the way it sits on the rifle, how tight it is torqued you may loose approx 2 - 4 MOA. So you may find yourself using 5 - 8 MOA with the rail to get your 100 yard zero. Don't panic, this is not a sign of a fault and you've still gained back over 15/16 MOA.

It all depends on your 100 yard zero and the adjustment needed to get it. The higher this number the greater the MOA rail you may need. So for example if you done that test and got 37moa for your 100 yard zero instead of 23.5 then a 20 moa will help but i'm confident you will run out of adjustment for anything past 900 yards and even if you make 1000 yards your scope will be close to or at its max elevation adjustment.

In such a case go for a 30 moa rail.


Sorry for being so long winded but its not a one word answer scenario.

A ballistic calculator would give a good approximation. They're not always right and some are not even close.

Its caliber specific, obviously, but also real world results can differ from what a BC will tell you. At shorter ranges a few clicks means an inch or two, but at longer ranges a few clicks mean feet or even yards off target.

But for the purpose of this thread and the the issue/question i raised about drops, yes a BC would be a helpful tool. I never considered it really because i rely on actual testing over a BC. Not meaning to sound like a smart arse, just a personal choice. Like a bore sighter. Never used one, but wouldn't discourage anyone from using one if they wanted.

Found this photo which helps visualize my points above about measuring the right ring height.