Re: S&B PM11 Zeroing Question
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: signut</div><div class="ubbcode-body">Just got my S&B back from S&B Service; had it changed to a CM turrent system to get some more elevation. Mounted in a TRG series 3 ring mount, think these are 30 moa or so. Shot at range , zeroed at 100, but here's the question: the original turrents bottomed out right at 100, loosened turrents and set, got the full 64 moa elevation.</div></div>
The reason that you got the "full" travel out of your original (1/4 MOA) turrets is because the 30 MOA cant of the Sako mount you have isn't too far-off from 32.5 MOA, which is really what is recommended by Schmidt & Bender for a 65 MOA travel PMII. S & B mechanically offsets PMII reticles out-of-center by 1/2 of their total travel, which is why S & B recommends that PMIIs' be used with base cants equal to 1/2 of the total Elevation range. The aggressive cant is required to compensate for this mechanical offset.
In a nutshell, in order to be able to dial the advertised 65 MOA Elevation range of a 1/4 MOA turret PMII 5-25X[56], a 33-35 MOA mount or base is ideal (1/2 of 65 MOA is 32.5 MOA). A couple extra MOA aren't going to hurt you, as you're going to use-up several MOA to zero. Also, PMIIs' typically have a bit of "extra" travel so there is a bit of "wiggle room". <span style="font-style: italic">However, when you had the turrets converted to 0.1 MIL increments, your 30 MOA mount became inadequate for the 26 MIL turret.</span> In order to be able to dial to 26 MILs' ideally you want a 45 MOA mount or base <span style="font-style: italic">(remember, you need to compensate for the PMII's mechanically-offset reticle)</span>.
S & B's base cant recommendation is explained in the PMII User Manual - see text from said manual below. Not to be flippant, but people should try reading instruction manuals. A lot of people use a "blanket recommendation" and tell people to use a 20 MOA base. Since most people never need to dial beyond 12 MILs' or so, a 20 MOA base <span style="font-style: italic">usually</span> works. Unfortunately, there have been quite a few people who - based on people's recommendation for a 20 MOA mount or base, bought a 20 MOA mount or base, then discovered that they can't dial the full range of advertised travel. However, a 20 MOA base, and even a 30 MOA base will not provide enough cant to allow a PMII 5-25X[56] to be dialed all the way to the 26 MIL mark. You need a 45 MOA base (roughly 13 MILs') for that.
As an aside, FYI both the 0.1 MIL and .25 MOA versions of the PMII 5-25X[56] have more than 100 MOA of <span style="font-weight: bold"><span style="font-style: italic">internal travel</span></span>. The trick is that the Elevation travel of both versions is artificially-limited by the stop pin in the DT Turret. If you force the turret past it's stop you're going to break something. (The 0.1 MIL Elevation turret terminates at 26 MIL, while the 1/4 MOA Elevation Turret terminates at 65 MOA).
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: signut</div><div class="ubbcode-body">The new turrents are zeroed at 8+ cm up alreday, this loses a third of the 26 cm elevation that this setup is supposed to be capable of. is there an adjustment for this/ Going to call service and ask them but if anyone knows feel free to unload here !! And thanks </div></div>
Don't call S & B - there's nothing wrong with your scope's travel. Again, your 30 MOA base just won't cut it for a MIL version PMII 5-25X[56] scope, which was designed to be used with a 45 MOA base. If you had chosen a mount or base as recommended by Schmidt & Bender (45 MOA or very close to it) you would be able to dial to the full 26 MILs' indicated on the Elevation Turret. The only "adjustment" you need to make if you want/need more travel is to buy a mount or base with the correct cant for your scope. Check out the <span style="font-weight: bold"><span style="font-style: italic">
SPUHR ST-4801</span></span>, which has 44.4 MOA of built-in cant and a height of 35mm.
Italicized and in quotation marks below is S & B's recommendation for selection of a base with the correct cant as 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). 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.
<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 dont 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="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. Likewise, the position of the reticle in the tube is exaggerated.
I've posted this info mamy times here, and I know that it's helped SH members and others because I've been thanked in posts, via PM, sometimes emails. Several people have also told me that they wish that they had seen my posts about S & B base cants before they bought their mount or base, as they would have followed S & B's recommendations for selecting base cant and bought the correct cant the first time.
Keith
