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"Floating" Mepro M21 Mount Does "Spring Back" Well

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calinb View Drop Down
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    Posted: October/28/2020 at 18:23
The Meprolight 21 with picatinny AR mount is a tactical red dot so I'll post my question here.

I have it mounted on an Alexander Arms Precision model Beowulf. I've always had problems with accuracy and I think I've found the problem. I clamped my upper in my milling machine vice and setup a dial test and dial indicator on it. When I displace and release the floating sight in windage (twist it on the gun), the spring-driven return to zero position is unreliable to 0.007". The error in elevation is 0.0015" This is consistent with the targets I'm shooting (windage wanders much more than elevation).

Given about 3" of mounting base radius the elevation deviation is about 1/2 milliradian (easy to calculate, because a mil is 1:1000). The deviation in windage is 7/3 mils (2-1/3 mils).

I've had the sight for many years but only now became interested in getting this upper dialed-in (so no warranty support, I'm sure).

Anyone have any ideas, before I start to hack the quick release mount/base?

Thanks!

-Cal

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calinb View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calinb Quote  Post ReplyReply Direct Link To This Post Posted: October/29/2020 at 00:23
Update:

I think I found the problem.

I unscrewed the four screws from the picatinny quick release mount and discovered a workmanship defect. The screws attach the mount to a plate that connects the sight housing to the mount. The plate is integral to the elevation/windage adjustment mechanism. The housing and optical parts of the sight "float" on the plate and they rotate, relative to the plate and mount, in E and W by the rotation of the E/W adjuster screws. However, the E/W screws do not rigidly fix the position of the housing and optics contained therein. Rather, the mechanism moves against spring tension and its zero stability and repeatability relies on the springs to return the sight to the same position with sub-thou inch accuracy.

The problem was my M21 sight was assembled with giant blobs of thread lock compound drenched onto the four screws and into the plate's threaded holes. There was so much excess white goop that it gummed-up the floating E/W mechanism by flooding into the gap between the plate and the housing and into the hard contact points that establish the return to zero stop.

Cleaning it out was a pain! I used denatured alcohol, Q-tips, paper towels and, most importantly, those little square 22LR bore cleaning cotton patches. Using dental picks to "thread" the patch, I then pulled them back and forth through the tight gap between the plate and the housing to remove the goop. I did not want to disassemble the sight further to gain better access.

I didn't use any thread lock in reassembly and I'm not sure that I'll ever use it for this application. If I use it, it will be only a very small amount (far less than a drop) of blue or purple Loctite on each screw. The plate has through hole threads and it's pretty thin so it would take only a very small excess of thread lock to contaminate the mechanism.

I also think the mechanism should be assembled dry (no oil), because oil just collects dust and dirt (and I found no signs of factory lubricant.) After seeing how the E/W mechanism works, these M21 sights are not as immune to contamination (mud, dirty or frozen water, etc.) as most people assume, because the E/W mechanism could fairly easily become contaminated and then the sight will be prone to losing zero!

I actually have a second M21, new in its box, that I've never used. By manipulation and feel, I found that it has the same problem, though to a lessor extent so I'll need to clean it out too. I purchased both sights at the same time nearly 9 years ago.

The sight I cleaned now floats crisply and freely on its plate and snaps back to a solid stop with a sharp "click." There's no subtle sense of gumminess when moving it by hand and it does not discernibly move past the hard stop point (as it did before when rotated past its subtly mushy stop point). I'll set it up with my dial test indicators and then I'll really know how much it helped. Only after that will I do a live fire test, because I've wasted too much ammo due to Mepro's crappy quality control and assembly processes already!

I can't believe that the design of this sight is flawed (given Meprolight's IDF and other mil/LEO customers). I bet Mepro even knows about the problem but, rather than correcting the problem with a recall, they probably ignored it and continued to ship defective units to civvy customers!

-Cal
Co-founder (retired), Smart Firearms Training Devices, LLC
www.smartfirearms.us

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Scrumbag View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Scrumbag Quote  Post ReplyReply Direct Link To This Post Posted: October/29/2020 at 05:57
Interesting stuff. Always good to be reminded that it pays to check for wiggle!
Was sure I had a point when I started this post...
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calinb View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calinb Quote  Post ReplyReply Direct Link To This Post Posted: October/29/2020 at 15:50
My dial indicators gave me the bad news! The return to zero feels crisper with more of a hard stop, but the floating sight doesn't actually return in E or W any better than it did with the screw locking goop.

If the front of the site is bumped or twisted the wrong way (easy to do in the woods by bumping against a tree or even just laying the rifle down) it will not fully return to zero, to the degree I described in my first post; it will simply stay "stuck" in the non-zeroed position. (Perhaps after an unknown number of shots it might return, but I've not experienced any evidence of it returning during Beowulf live fire.

The "wrong" way (down and to the right at the front end of the sight / up and to the left at the aft end of the sight) is away from the floating spring-opposed direction.

If the sight housing is deflected to compress the E/W adjuster return spring and quickly snapped to release (rotating it in directions opposite the "wrong way"), the sight will return to zero after one "snap."

This handicap is not something I'd prefer to have to do when assuming each and every shooting position following carry or recovering an arm from storage...and I'm not even a military or LEO operator!


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Post Options Post Options   Thanks (0) Thanks(0)   Quote calinb Quote  Post ReplyReply Direct Link To This Post Posted: October/29/2020 at 15:53
Originally posted by Scrumbag Scrumbag wrote:

Interesting stuff. Always good to be reminded that it pays to check for wiggle!


Yes--and given that even wiggle "by design" (Mepro M21) may not be adequately addressed, I think it's generally a bad thing! I even worry about AR/M16 iron rear sights, but they seem to work well unless they are really jammed up by something.

Also, tools like a milling table, vice and sub-thou dial indicators help one to check too.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote koshkin Quote  Post ReplyReply Direct Link To This Post Posted: November/01/2020 at 11:06
Keep in mind that M21 was never intended for the recoil of the Beowulf and that likely extends to damn near every spring loaded mount on the market.

I do know it holds up quite nicely for the IDF, but they do not shoot far with it and do not subject it to a ton of recoil

ILya
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calinb Quote  Post ReplyReply Direct Link To This Post Posted: November/01/2020 at 13:26
Originally posted by koshkin koshkin wrote:

Keep in mind that M21 was never intended for the recoil of the Beowulf and that likely extends to damn near every spring loaded mount on the market.

I do know it holds up quite nicely for the IDF, but they do not shoot far with it and do not subject it to a ton of recoil

ILya

Thanks very much for your take on it, koshkin. I was wondering about the 'wulf's recoil.

I also learned that the M21 parallax is terrible (at 150 yards anyway). It is worse in elevation but still pretty bad in windage. The parallax can be managed, however, by using a sight picture with the reticle very low in the window and perfectly centered too, of course. Unless it's a close-in shot, I just put my 12 MOA triangle right above the black outline of the lower housing. The elevation parallax error is far less the lower the reticle is "placed" in the window in one's sight picture. It's easy to bob one's head around and determine the parallax effect with sight picture changes. A very low reticle sight picture is also best for my A2 stock and my cheek weld.

I have a Trijicon RMR that I might try. This Beowulf has no provision for an iron front sight and I'd like to use it to hunt white tail in the woods, but 200 yards shots are still a possibility. I could also temporarily mount a scope on it, just to check the gun's accuracy, but I've never had problems with iron sights on my 10/22s or M1 Garands. I'm an Appleseed "Rifleman" who can easily qualify on the AQT with iron sights any day, but I'd fail with this Mepro 21 and Beowulf!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote calinb Quote  Post ReplyReply Direct Link To This Post Posted: November/03/2020 at 20:45
Update:

I removed the picatinny QR adapter again and worked harder to remove more white debris from the over-zealous factory use of thread lock. It seems to have helped. Shooting prone at 100 yards with a sling, I'm shooting around 1.5 to 2 inch 5-shot groups. The 2 inches was with declining and less than ideal daylight--not because the reticle is difficult to see (it's far easier to see than iron sights) but because it becomes difficult to get a repeatable "pumpkin on a post" sight picture. Anyway, that's about the accuracy that I should be getting. I need to move back out to 150 yards, which is the desired zero range with my 1436 fps 500 gr. Hornady loads, and continue to check accuracy.

Maintaining a sight picture with the reticle at the bottom of the window and perfectly centered remains important to minimize parallax error, I'm certain. I'm still nervous about the return to zero capability of the springy adapter interface though.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Scrumbag Quote  Post ReplyReply Direct Link To This Post Posted: November/04/2020 at 12:24
Good stuff, let us know how you go?
Was sure I had a point when I started this post...
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