HOME

The Trijicon RMR 08A, in a Burkett Offset Mount. Not a common selection for a .338LM Accuracy International AWSM, but an excellent choice as a secondary sight for a precision rifle that also needs to be capable of rapid snap shots at close range targets.

Introduction

Even for a precision shooter there are some time critical shooting situations - Jungle Lane competitions, and some of the short range Gunslinger competition stages - where there may only be a few seconds to take several shots at close range targets. Something many long range precision rifles are not well suited for. And even with a variable magnification scope, the time to make any magnification adjustment and the reduced field of view often makes a high magnification telescopic sight less than ideal for snap shots at short range, or for multiple rapid short range engagements.

In my case I often fly to shooting locations and travel with a; carry-on bag, pack, Pelican hard case, and a rifle case. That amount of luggage means being restricted to a single rifle. There is a simple solution, and that's to add a secondary sight to the precision rifle. Often seen in 3-gun competitions on 'black rifles' - where a secondary sight is there as a back-up, or as an alternative aiming solution - secondary sights such as the Trijicon RMR are becoming much more common. And on some firearms eg pistols and shotguns, reflex sights are becoming more popular as the primary sight.

This article discusses using a Trijicon RMR 08A as a secondary sight on an Accuracy International AWSM (using .338 Lapua Magnum 250gr Scenars). 

Other information on this page:
Introduction
Targets
Trijicon RMR 08A
Point Of Aim And Sight Pictures
Burkett Offset Mount
Point Blank Range
Danger Range

For information about reflex sights, and how they work, visit Wikipeadia at:

http://en.wikipedia.org/wiki/Reflex_sight

Targets

When selecting any rifle sight consideration needs to be given to the likely targets that will be be engaged and the ranges likely to be encountered. There are several close quarter targets that will commonly be shoot in competitions with the AWSM, often with only a few seconds to take the shot or shots, and usually from standing positions:

1. At ranges less than 100m
• Military "E" board target
2. At ranges between 100m and 200m
   • 12" square metal target
   • 8" round metal target
3. At ranges between 100m and 300m
• Full sized IPSC metal target (18"wide x 30" high)

To be clear, using the AWSM for CQB is not ideal, and it is in no way competitive against AR style rifles. But in these scenarios, compared to using the primary rifle scope, the RMR provides a much faster aiming solution, has a significantly better field of view, with excellent sight pictures, and consequently arguably much better hit probability.

The RMR 08A can be mounted above a telescopic sight, or, in this case offset in a mount at 45 degrees.

Trijicon RMR 08A

Key to the selction of the Trijicon RMR (Ruggedised Miniature Reflex) 08A was that it had a 2.9 moa amber triangle reticle and only 1x magnification - basically sight it in and then don't touch it!

"The RMR is designed to be used with both eyes open", and the RMR 08A does not have any battery, but has fiber optics and tritium as a dual illuminated version. "This automatically adjusts reticle brightness to the ambient light during the day using the fiber optics, and illuminates in low light or no light conditions using tritium".

The Trijicon RMR is available with various other reticles, and a more common choice would have been one of the red dots. In this case the RMR 08A will be mounted at a 45 degree offset and the advantage of the triangle is that it can be aligned parallel with the bottom of the square or IPSC targets - and that will eliminate cant in what will be a canted rifle.

That means when the AWSM is aimed using the RMR it'll be canted to a 45 degree angle. A word of caution here - the AWSM is chambered in .338 Lapua Magnum, so it has a bit of recoil - that means the rifle must still be shouldered.

For information about the Trijicon RMR scopes visit Trijicon at:

http://www.trijicon.com/na_en/products/

To purchase a Trijicon scope in New Zealand visit Serious Shooters at:

http://www.seriousshooters.co.nz

Point Of Aim And Sight Pictures

Given the likely scenarios for using the RMR 08A are short range rapid engagements - where there is no time for a precision shot - the point of aim with the 12.9 moa triangle reticle will always be at the top of the target.

With the four likely targets to be engaged, the point of aim is the following:

• Military "E" board target
  • The apex of the triangle at the top of the "shoulder" and in the centre line, and the bottom of the triangle parallel to the bottom of the target
• 8" round metal target
  • The circle centred within the triangle
• 12" square metal target
  • The apex of the triangle on the top of the square and each bottom corner slightly overlapping the edge
• Full sized IPSC metal target (18" wide and 30" high)
  • The apex of the triangle at the top of the "shoulder" and in the centre line, and the bottom of the triangle parallel to the bottom of the target

The AWSM is a heavy rifle and is difficult to hold a steady standing position, so in all cases the POA references a physical element of the target, so that when taking a shot the POA simply doesn't drift down or around the target. And in this case that's a key advantage of the triangle reticle over a dot reticle - the triangle provides three reference points for all the targets likely to be engaged.

The RMR 08A is 1x magnification, so the target size will rapidly diminish in relation to the reticle as the range increases. The sight pictures on the right illustrate how much the target size diminishes for the likely shooting scenarios - the first row at around 100m, the second row at a little over 200m, and the third row a little over 300m. The point is that the POA (based on Danger Range) remains the same regardless of the distance.

If the POA was the apex in the centre of mass, at distances of 200 yards with the heavy AWSM, it is quite likely the poa would drift down the target, resulting in a miss (particularly with the 8" round). For example, a POA centre of mass at 200 yards puts the point of impact at 1" from the bottom. Allowing for 2 MOA precision that leaves no margin for error (and is also ouside the desired danger range).

Sight pictures for centre of mass poa on 8" round, 12" square and full sized IPSC targets using the 12.9 moa yellow triangle reticle of the Trijicon RMR 08A.

Burkett Offset Mount

Trijon have numerous mounts available for the RMR, including; picatinny rails, weaver rails, and ACOGs.

While the RMR 08A is a very sturdy item, in this case the AWSM sometime gets carried in a drag bag and the in the back of vehicles over rough ground - so a little more protection was sought. Consequently, the Burkett Offset Mount was selected because it has side protectors, and also has a version specifically for mounting the Trijicon RMR.

This AWSM has a picatinny rail running along much of the stock, but the front half doesn't have the slots cut. Fortunately, the Burkett Offset Mount has a little pin that can either sit in the slot, or it can be slid to the side.

The primary rifle scope on this AWSM is a USO 5-25x58 TPAL in an Accuracy International one piece scope mount. Unfortunately there wasn't enough room to simply slide the locking nut of the Burkett Offset Mount under the scope tube, so a couple of the knurls had to be filed down (as it wouldn't be ideal to take the primary scope off every time it was necessary to add or remove the RMR).

To purchase a Burkett Offset Mount visit Predator Tactical at:

http://www.mattburkett.com

Point Blank Range

Point blank range is a term often misused term by television and newspaper media, but it actually has a specific definition for shooting, and is a concept all shooters should become familiar with.

Point blank range is the maximum range between the muzzle and the target for which a bullet will hit that target, when it is placed at any distance between the muzzle and that maximum range. It has an associated point blank zero range which provides the optimum zero range for a specific sized target, based upon the rifle and ammunition - so a smaller target will have a closer point blank range than a larger target. For a hunter the point blank target zone would be based on the animal vital size (target zone), rather than the total animal size.

It is an extremely important concept because it means that when the point blank range for a given target size is known, it is not necessary to do any range estimation or calculations. Just aim and shoot and the target will be hit.

The difficulty with point blank range is that it requires zeroing at a range that is often inconvenient, and not suited to most box shooting ranges (where equipment may be only at a specific 100 yards or 200 yards).

Example 1: A target with a zone height of 8" (eg an 8" round) has a point blank zero range of 271m, for a point blank range of 318m, and where the maximum ordinate is at 153m.

Example 2: A target with a zone height of 12" (eg a 12" square) has a point blank zero range of 317m, for a point blank range of 373m, and where the maximum ordinate is at 175m.

Example 3: A target with a zone height of 15" (eg full sized IPSC) has a point blank zero range of 460m, for a point blank range of 544m, and where the maximum ordinate is at 247m.

In all the above examples the apex of the triangle is the zero, and the point of aim is always the centre of the target. In other words, if the AWSM was zeroed at 317m, aim the top of the triangle at the centre of a 12" square target and if it's at any distance of less than 373m it will be hit.

The RMR 08A reticle is an equilateral triangle (all sides the same length), so the point for the point blank zero within the triangle is at the middle of the triangle, which is 6.5 moa below the apex.

Delta IV of the Field Firing Solutions ballistic calculator includes path metrics for Maximum Ordinate, Danger Space, and Point Blank Zero Range. To purchase FFS from Lex Talus Corporation visit Field Firing Solutions at:

http://www.precisionworkbench.com

To read an article by Michael Haugen on Alternate Target Engagement Techniques (including Point Blank Zero) download the article from US Armorment at:

http://usarmorment.com/pdf/DA%202005.11MH.pdf

Danger Range

But zeroing at 317m is not convenient, and typically the AWSM is zeroed at 100m, so in this case the concept of danger range will be used. For simplicity, and in keeping with MOA calculations, the RMR 08A will be zeroed at 100 yards.

Danger range is also the maximum range between the muzzle and the target for which a bullet will hit that target when it is placed at any distance between the muzzle and that maximum range. But danger range is based upon any specified zero range. Danger range is related to both the zero range and the size of the target, and can be identified by calculating the bullet drop.

Danger range is often the more practical concept to apply, because it is often easier to zero the rifle at a box range, say at 100m. Whereas Point Blank Range will give you the longest maximum range because it is based on the optimum (maximum) zero range.

Some precision shooters zero at long distances eg 600m, and this is also (similar to point blank range) results in two zero ranges. In these cases the calculation must add both the climb and the drop to get the total drop.

The Lapua ballistic charts for the 250gr Scenar, indicate that for a rifle zeroed at 100 yards the bullet drop is 3" at 200 yards, and 10" at 300 yards.

Given that these close targets will often be shot standing there is an expectation that precision will be reduced to 2 MOA. Using the sight pictures above, for example with the apex at the top of the square, the zero therefore needs to be dropped 1 moa at 100yards. That means a shot at 100 yards, which is the highest point in the trajectory of the bullet, and even with a 2 MOA error will still hit the top of the target. For simplicity the RMR 08A will be zeroed at 1" below the apex of the triangle, at 100 yards.

Example 4: Aiming at the top of an 8" round metal target

• at 100 yards, will hit at 1" below the top of the target (that's 1 MOA below the top)
• at 200 yards, will hit at 5" below the top of the target (that's 1 MOA / 2", plus 3" bullet drop)

Example 5: Aiming at the top of a 12" square target

• at 100 yards, will hit at 1" below the top of the target (that's 1 MOA below the top)
• at 200 yards, will hit at 5" below the top of the target (that's 1 MOA / 2", plus 3" bullet drop)
• at 300 yards, will miss at 1" below the bottom of the target (that's 1 MOA / 3" plus 10" bullet drop) ie exceeds the danger range

Example 6: A 30" high target eg full sized IPSC, has a danger range of 333m (that's 30" less 6" to the shoulder, less 4.67" and 1.5" at the bottom).

• at 100 yards, will hit at 1" below the top of the "shoulder" (that's 1 MOA below the top of the "shoulder")
• at 200 yards, will hit at 5" below the top of the "shoulder" (that's 1 MOA / 2", plus 3" bullet drop)
• at 300 yards, will hit at 13" below the top of the "shoulder" (that's 1 MOA / 3" plus 10" bullet drop)

Therefore, using the RMR 08A with the sight pictures above, and with this zero:

• Both the the 8" round and 12" square targets can be engaged out to about 220m
• The military "E" and full sized IPSC can be engaged out to about 333m

Those that realise the primary scope reticle and adjustments are in mils, may think having a secondary sight in MOA will be confusing. But once the RMR is sighted in there are no range calculations at all. Simply looking at the sight pictures will indicate if the target is within danger range. For example, the 12" square is 5 MOA high when at 229 yards. So for the target scenarios listed at the top of the page, if the 12" square is not less than half the reticle height the target is well within danger range.

The reality is that with 1x magnification at around 200m, as shown above the sight pictures for the 8" round and 12" square, there isn't much margin for error, and a lot of practice will be required to consistently hit those targets out at 200m.

Given the RMR 08A is a 1x magnification, and that any rapid fire engagements past 200m will be done from the prone or supported firing positions, any rapid fire engagements on the 8" round or 12" square targets past 200m will be done using the primary rifle sight.

None-the-less the examples demonstrate that the .338 AWSM has a formidable point blank range, although at around 10kg loaded it's not particularly suited to firing from non-prone positions. But with the Trijicon RMR 08A as a secondary sight it's now set-up for rapid snap shots at close range targets.

On the AWSM, using a similar cheek weld as with the telescopic sight, provides an excellent sight picture with the RMR 08A.

Note: Danger Range and Danger Space are not the same thing. For a discussion on Danger Space read the Gunslinger PLRS article Vectronix PLRF10C and Field Firing Solutions: Integrated Ranging Out To 2500m at:

http://www.gunslinger.net.nz

Disclaimer:  Shooting is potentially dangerous and damaged equipment can be expensive to fix.  You should confirm all changes to be made to your rifle system and shooting practices with your local gunsmith or gunshop owner.  The example and data above are provided for information purposes only.  No warranty is made for its suitability or use with your rifle.