Rifle Scopes 20 moa vs 48 moa? S&B 5-25 mounting

[BillCosby]

Hey guys, so quick question, I have a sako trg 42 with a 20 moa badger rail, as well as the 0 moa factory rail, and I just picked up a 28 moa AI 34mm mount from another member here. Am I better off only using the 28 MOA mount with the flat rail or would the 48 moa combined rail and mount work well? I heard somewhere that S&B 5-25's are designed to use a 45 moa base, so I didn't know if 48 would throw it out of whack or just be negligible in difference, thanks!

 

[Aries64]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BillCosby</div><div class="ubbcode-body">Hey guys, so quick question, I have a sako trg 42 with a 20 moa badger rail, as well as the 0 moa factory rail, and I just picked up a 28 moa AI 34mm mount from another member here. Am I better off only using the 28 MOA mount with the flat rail or would the 48 moa combined rail and mount work well? I heard somewhere that S&B 5-25's are designed to use a 45 moa base, so I didn't know if 48 would throw it out of whack or just be negligible in difference, thanks! </div></div>
Bill -

Given the parts you have on hand, I would go with the 20 MOA Badger Rail and the 28 MOA AI Mount. <span style="font-style: italic">Theorietically</span> the combination of those two components will give you 48 MOA of cant, but in reality it may or may not add up to 48 MOA. You might end up a bit over or under 48 MOA. Either way you should be fine and will probably be close to getting the full range of travel out of your scope whether you have a MIL version or an MOA version. Don't forget that you'll probably burn-up several MOA zeroing (at 100 yards).

If you use the 0 MOA Sako Rail and the 28 MOA AI Mount you would <span style="font-style: italic">theorietically</span> have 28 MOA of cant, which definitely isn't enough cant to allow you to dial the full range of travel if you have a MIL version. If you have an MOA version scope, you won't be as far off of the total cant required to get full travel as the MOA version has less travel and therefore requires a less aggressive base cant. PMII's also usually have a bit of extra travel above the specified travel, but remember, you'll use up some travel when zeroing.

<span style="font-weight: bold">PMIIs' are designed and engineered to be used with aggressively canted bases. PMII variables' are preset at the S & B factory with their reticles' adjusted out-of-center by HALF of their FULL Elevation travel, giving them UPWARD Elevation bias. In order to compensate for this mechanical bias/offset, a base that approximates one half of each respective scope's full Elevation travel should be used.</span> However, its' a good idea to get a base with a few "extra" MOA built-in to allow for the travel you'll use to zero at 100 yards.

Selection of a base with the correct cant is all detailed in the <span style="font-weight: bold"><span style="font-style: italic">"2006 PMII User Manual"</span></span> beginning on page 3 under <span style="font-weight: bold"><span style="font-style: italic">"Section 4.2"</span></span>, <span style="font-weight: bold"><span style="font-style: italic">"Adjustment range and forward angle"</span></span> and concluding on page 4. Italicized and in quotation marks below is the section I am speaking of. Below the text of Section 4.2 is a diagram from page 19 of the PMII User Manual showing <span style="font-style: italic">"picture 2"</span>, <span style="font-style: italic">"picture 3"</span>, and <span style="font-style: italic">"picture 4"</span> as referenced in the (2006) User Manual. Keep in mind that the Manual does not allow for any travel used to "zero" the scope as I did in my paragraph above.

<span style="font-weight: bold"><span style="font-style: italic">"4.2. Adjustment range and forward angle

Scopes for high precision shooting are often used for shooting at great
distances. In this case the elevation adjustment is used to compensate for bullet
drop. To provide an extensive elevation range in these scopes Schmidt &
Bender has increased the main tube diameter from 30mm to 34mm.
Nevertheless the elevation and windage range like in any other scope is limited.

In scopes for hunting the reticle is generally centered optically and mechanically
in order to receive the same amount of travel in all directions and to make the
mounting of the scope to the firearm easier (see picture 2).

In order to make the elevation adjustment range usable in its full extent it is
necessary to preset the reticle of the PMII scopes out of the center already at
the Schmidt & Bender factory (see picture 3). As a consequence the gunsmith
is obliged to consider the preset position of the reticle in the elevation range
when mounting the scope to the firearm (see picture 4). With this setup the full
elevation range is usable in one direction allowing to shoot at distances up to
2000m depending on the used calibre and scope type.

Determining the correct forward angle
The necessary forward angle is depending on the used type of elevation
adjustment. At the Schmidt & Bender factory the reticles of PMII scopes are
adjusted out of center by half the amount of the full elevation range. This value
must be compensated in the mount system.
Forward angled mounts or rails for every Schmidt & Bender PMII scope type
are available from all renowned mount manufacturers.

Example for determining the required forward angle:
A standard elevation turret (Single Turn) with an elevation range of 13mrad
(equals 130cm at 100m distance) requires a forward angle of 65cm at 100m
(equalling the half of the full elevation range). For a gunsmith compensating for
this value using the mounts the following rule of thumb applies: If the space
between the two mount rings is 100mm the front mount should be 0.65mm
lower than the rear mount.

If the gunsmith is using a forward angled rail standard mount rings without
forward angled can be used."</span></span>

<span style="font-weight: bold"><span style="font-style: italic">"picture 2"</span>, <span style="font-style: italic">"picture 3"</span>, and <span style="font-style: italic">"picture 4"</span> as referenced in the <span style="font-style: italic">2006 PMII User Manual:</span></span>

<span style="font-weight: bold"><span style="text-decoration: underline">NOTE:</span></span> <span style="font-weight: bold"><span style="font-style: italic">In the diagram above:</span></span>

<span style="font-weight: bold">"<span style="color: #FF0000">Picture 2</span>"</span> shows a scope mounted on a rifle with a flat (non-canted base). The drawing illustrates a typical scope with optically-centered reticle and erector. Personally, I always use canted bases, as they have no downside - they don't adversely affect 100 yard zeroing and shift a bit of Elevation travel to the upper end of the scope's adjustment range. <span style="font-weight: bold"><span style="font-style: italic">PMIIs' are actually designed to use agressively-canted bases.</span></span>

<span style="font-weight: bold">"<span style="color: #FF0000">Picture 3</span>"</span> shows a scope mounted on a rifle with a flat (non-canted base). The drawing illustrates the effect that the PMII's Reticle offset (PMII reticle's are preset at the factory <span style="text-decoration: underline">biased towards the top of the scope tube</span>) has upon the PMII's operation and the PMII's requirement for specific cant scope bases. This mechanical offset is engineered into the PMIIs' and requires use of specific cant bases in order to get optimum Elevation travel out of respective PMII scopes. The Reticle's off-center orientation (towards the top of the tube) means that you're actually looking very slightly upward when you look through a PMII at the bottom of it's travel, and that the Reticle's LOS (Line-of-Sight) won't intersect the projectile's trajectory without a lot of "UP" adjustment. Obviously, the scope's offset over the bore will require the shooter to use a lot of "UP" adjustment to zero the rifle. Use of a correct cant base eliminates these issues and allows the PMII to function as designed.

<span style="font-weight: bold">"<span style="color: #FF0000">Picture 4</span>"</span> shows a scope mounted on a rifle with a canted base. The drawing illustrates how the use of a base with the correct cant affects the PMII Reticle's LOS (Line-of-Sight) and allows the respective PMII access to it's full Elevation adjustment range. The Reticle is still oriented at the top of the scope tube, but the canted base has angled the scope (and Reticle) slightly downward in relation to the axis of the bore, allowing the Reticle's LOS (Line-of-Sight) to intersect with the projectile's trajectory. The scope can now be "zeroed" with minimal Elevation adjustment. After the scope is zeroed, the Elevation Turret shoud be set to "0". Scopes with DT (Double Turn) Elevation Turrets should be reset.

Also, the illustrations show the bullets arcing upward from the axis of the bores. This, of course - does not occur in real life. I'm sure that the gravity-defying bullets were drawn in that manner to simulate the arc of the rounds as they travel downrange.


Keith

 

[jasonk]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

Very detailed explanation from A64.

My big concern with the AI mount on the 20 moa rail or any rail is that it "could" make the cheek weld uncomfortably high. AI mounts aren't low mounts to begin with, but I'm not a TRG guy so I don't know how high it will make it.

FYI, those 28moa mounts don't work with the 5-25 SB from the factory as far as I know. I believe they still have to have a little milling done to them to allow the bottom of the tube to clear under the turrets. Be sure to check that as you'll damage your scope if you manage to get it in there and there isn't clearance between the scope and mount. Your's may be milled already, can't say without a good picture or having it in hand.

 

[BillCosby]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

Appreciate the awesome responses guys, and thanks for the heads up on the mount. I checked around and indeed it looks like it won't fit, but unfortunately I already committed to purchase so I guess I will start looking for a machine shop in the area. From what I found it looks like I need to have it cut by 0.053" in order to safely clear.

 

[Aries64]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BillCosby</div><div class="ubbcode-body">Appreciate the awesome responses guys, and thanks for the heads up on the mount. I checked around and indeed it looks like it won't fit, but unfortunately I already committed to purchase so I guess I will start looking for a machine shop in the area. From what I found it looks like I need to have it cut by 0.053" in order to safely clear. </div></div>
It's great that Jason brought up the issue that the 28 MOA AI Mount wouldn't work with your PMII 5-25X[56m] without being machined - I thought that your mount had already been modified since you were planning on using it with your PMII 5-25[56m]. The 28 MOA AI Mount was designed to be used with the PMII 4-16X[42mm], which has 13 MILs' of travel in the MIL versions and 56 MOA of travel in the MOA versions. Great catch Jason!

Anyway, since your 28 MOA AI Mount won't work without being machined and the machining will cost you a few bucks, instead of having it machined and then stacking it on top of the 20 MOA Badger Rail why don't you sell both your un-modified AI Mount (once you get it) and the 20 MOA Badger Rail, then buy an appropriate mount such as the <span style="font-style: italic"><span style="font-weight: bold">SPUHR ST-4801</span></span> and use it on top of the Sako 0 MOA Rail? The SPUHR ST-4801 has 44.4 MOA and will mount your PMII lower to the bore and be a much more elegant solution.


Keith

 

[BillCosby]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

Not a bad idea!

 

[justinblake]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

A 48moa cant will allow for a 100 yard zero with Pm2 5-25x56?

 

[bengates]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BillCosby</div><div class="ubbcode-body">Not a bad idea!</div></div>

Keith is right. The Spuhr is beautiful and a much simpler solution.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TexasTRG</div><div class="ubbcode-body">A 48moa cant will allow for a 100 yard zero with Pm2 5-25x56?</div></div>

Is that a question, or a statement? If it's a question, then the answer is no - the 48 MOA cant is likely too much for the SB 5x25 since about 44 MOA/ 13 Mils is it's maximum one way travel from center.

 

[Opticsspecialist]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

usually, you can't use a mount with moa built in in and a rail with moa built in, something doesn't geometrically mess. so flat base with the ai mount

 

[Aries64]

Re: 20 moa vs 48 moa? S&B 5-25 mounting

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TexasTRG</div><div class="ubbcode-body">A 48moa cant will allow for a 100 yard zero with Pm2 5-25x56? </div></div>
<span style="font-style: italic">It should, if not it will be very close</span>. 13 MILs' = 130 "clicks" x .36", which is 46.800"/44.699 MOA. That's why a 45 MOA base is recommended for use with the PMII 5-25X[56mm] - <span style="font-style: italic">it's part of a system</span>. However, the 45 MOA base recommendation doesn't take into account any travel that is used during zeroing, which for a typical rifle requires several MOA at 100 yards, so you can usually get away with a few "extra" MOA of cant on a PMII. 3 MOA at 100 yards is equal to 3.141", which is less than 0.9 MIL (3.240"). Remember also that a PMII 5-25X[56mm] probably sits 1.8" or more above the bore and a bit of cant helps the bullet path and the LOS (Line of Sight) converge.


Keith