[LemmyCaution]

This thread will be a technical discussion of the ProMag PM081a optics mount for the M14/M1a.

Let's please keep the discussion to the technical/factual aspects of the mount. This is a budget scope mount. There's not much to be gleaned from 'buy the real thing' or 'buy a quality mount' posts here.

To start, the PM081a is an imitation of the ARMS #18 mount. They are similar in appearance, but attach to the receiver in different ways.

As one can see, the resemblance to the ARMS #18 is uncanny. Same low rail. Same cut outs for scope ring wrenches. Same circular lightening cuts. Same basic shape. Same relieved areas, in which ARMS put their identifying marks. Those areas are blank on the ProMag mount. It literally looks as if someone took a mold of the ARMS mount, ground the identifying marks from the mold, and sent it to a foundry to be cast en masse.

Here's the ARMS mount, for comparison:

The differences:

1: The finish of the ARMS mount is Parkerized. The ProMag mount appears to be a sprayed on epoxy paint. This is in large part to cover the crudeness of the casting and roughness of the finish machining on the Picatinny rail and receiver mating surfaces.

2: The ARMS mount uses a milled dovetail to attach the rear of the mount to the groove in the receiver where the charger clip guide is normally mounted on the M14.

When the fixing screw in the mount is tightened into this dovetail, the rear of the mount is drawn down onto the receiver. Ideally, the mount will be in contact and alignment with the corresponding face of the receiver, such that when the screw is tightened, the mount is not pulled out of alignment, but is tightly secured to the receiver.

On the ProMag mount, there is no dovetail. One must still remove the charger clip guide from the receiver, but instead of the dovetail, the mount employs a 1/4-20 cup point socket set screw. This set screw is tightened against the face of the groove in the receiver and serves as a point of contact, not as a point of attachment.

This set screw is secured in place with an 8-32 socket set screw, which is tapped into the side of the mount.

3: The third point of contact on the ARMS mount is via a hollow screw at the front of the mount. Through this screw is inserted a large face pressure pad, secured by an e-clip. The screw is tightened into the mount, pressing the pad onto the top of the receiver. The large bearing surface is intended to increase friction on the receiver, in order to keep the front of the mount from shifting its axis, relative to the bore.

The adjustment screw is secured in place with a socket cap screw tapped into the front of the receiver. The head of this screw protrudes from the mount and can often interfere with the rifle's handguard.

On the ProMag mount, the method of contact is the same, but instead of the large face pressure pad, another 1/4-20 cup point socket set screw is used. The cup point will exert a high pressure in a small surface area on the top of the receiver in an effort to keep the front of the mount from shifting, relative to the bore. This screw is secured by an 8-32 socket set screw tapped into the front of the mount. The socket cap screw does not protrude from the front of the mount.

4: The ARMS mount is stated by the manufacturer to be made of 8620 case hardened steel. The ProMag mount is made of steel of unknown alloy. It is doubtful the steel is hardened. On my particular sample, one of the dogs on the Picatinnny rail came with a dent in its edge, as if the mounts had been shipped from the machine shop in a barrel, as is common for bulk steel parts. It looked at though it had been thrown in the barrel, or the barrel had been dropped or roughly handled in shipping. The damage is visible in the third groove from the bottom in the photo above.

Other characteristics:

The ProMag mount is secured to the receiver using the 12-32 screw hole in the side of the receiver. Alignment is achieved via the horizontal and vertical grooves in the receiver. The slots on my mount fit the grooves on my receiver appropriately. The 12-32 screw supplied with the mount is secured into the mount with an e-clip, just as the ARMS mount is. The head of the screw is 3/8" across the flats, such that the same wrench is used for the gas plug and the mount. The manufacturer specifies 90-100in/lbs torque to tighten this bolt, as well as the use of 'thread locker such as Locktite Grade B'. I'm guessing this refers to MIL-S-22473E Grade B, or Loctite product 085. Loctite 242 should work adequately and affordably, if necessary at all. I don't see any reason to tighten the screw greater than the 22in/lbs Bassett Machine specifies for their mount. The attachment method is exactly the same, with the addition of two additional points of contact, which should further tension and lock the screw, beyond the applied torque.

A consideration in using the ARMS mount with a Springfield receiver is loose tolerance in the receiver between the center of the side mounting bolt and the face of the charger clip guide slot. If this dimension is too short, the front of the mount will be pushed down, relative to the bore axis. If too long, the opposite. On the ProMag mount, due to the lack of a dovetail, the mount does not need to come into contact with the receiver at the rear. Provided the receiver and mount align properly at the side attachment point, the mount will be aligned with the bore axis. On my rifle, the rear of the mount floats about 0.02" from the face of the charger clip guide slot. This mount would therefore be appropriate for those who want the low ARMS type mount, but have badly out of spec receivers that will not accept the ARMS mount.

That's about all I have for now. I'll have follow up posts, as I get optics mounted and sighted in.

 

[NoExpert]

Another member picked one of these up and the lug above the side mount screw got deformed pretty quick under recoil. The metal isn't hardened so the receiver beat it up pretty bad. Hope it works out better for you.

 

[Grantman]

Coincidentally I've recently purchased for my m305 one of the Promag PM081A steel mounts. I've handled 5 of them to on-sell and they all look good in the necessary areas - the rail itself, the keys that fit into the receiver, and the faces that contact the receiver.

LemmyCaution I have a question for you. Bear with me as I try and describe this. You say the dovetail attachment of the ARMS 18 serves as an anchor into which the mount's rear screw tightens, drawing the mount in toward the dovetail attachment. This seems counterintuitive to dealing with the rotational forces on the mount which would initially push down on the front of the mount, as the scope and rifle bounce rearward, followed by force on the rear of the mount when the rifle's recoil is arrested. Therefore it makes more sense to me to have a screw pushing from the mount against the receiver at the dovetail region rather than pulling the mount in toward that area. What makes me wonder further is that per the ARMS 18 install instructions I’ve read (but cannot yet link to per forum rules), they state when inserting the dovetail attachment of the ARMS 18 to ensure it slides freely back and forth in order to line up properly. In my eyes this seems a liability. While lateral movement is necessary for its install, having it sound so 'free' per the instructions gives the impression it increases risk of up and down movement also. Even miniscule up and down movement in the ARMS dovetail filler would be extremely undesirable as it would fail to afford support to the mount when rotational forces push the rear of the mount down.

Therefore In theory I find the PM081A's dovetail design better in that there is a screw pushing the back of the mount directly off against the receiver which seems a better way of dealing with rotational pressure on the mount when recoiling.

As for the hardened steel vs high carbon steel it really is irrelevant when it comes to these mounts in my opinion. The life of aluminium mounts would be but a glimpse if high carbon steel wasn’t holding up. While aluminium mounts are rubbish as a generalisation, there are ones that will hold up for longer than a ‘glimpse’. Even if a mind numbingly sloppy job was done and there somehow happened to be rotational play in the steel mount it would take considerable force to even start to get deformation – say a guy incessantly smashing the front of your barrel with a sledgehammer.

 

[M1AallTheWay]

Thank you for the extremely informative and exceptionally well illustrated comparison of the two mounts. I would take issue with only one small point in your discussion.

...2: The ARMS mount uses a milled dovetail to attach the rear of the mount to the groove in the receiver where the charger clip guide is normally mounted on the M14...
When the fixing screw in the mount is tightened into this dovetail, the rear of the mount is drawn down onto the receiver. Ideally, the mount will be in contact and alignment with the corresponding face of the receiver, such that when the screw is tightened, the mount is not pulled out of alignment, but is tightly secured to the receiver...

The fixing screw runs up and down inside an internally and externally threaded plug which ARMS calls a "circular spacer" in step 7. The gap that should exist between the angled back surface of the mount and the guide dovetail is bridged by screwing the circular spacer down "until very light contact is made with the receiver." Then, the fixing screw is tightened, pulling the "adjustable dovetail" and the mount tightly together with the dovetail on the receiver without changing the orientation of the mount, which has already been established by the horizontal and vertical keys sitting properly in the corresponding receiver grooves.

That also solves this concern:

...You say the dovetail attachment of the ARMS 18 serves as an anchor into which the mount's rear screw tightens, drawing the mount in toward the dovetail attachment.
...What makes me wonder further is that per the ARMS 18 install instructions I’ve read (but cannot yet link to per forum rules), they state when inserting the dovetail attachment of the ARMS 18 to ensure it slides freely back and forth in order to line up properly. In my eyes this seems a liability. While lateral movement is necessary for its install, having it sound so 'free' per the instructions gives the impression it increases risk of up and down movement also. Even miniscule up and down movement in the ARMS dovetail filler would be extremely undesirable as it would fail to afford support to the mount when rotational forces push the rear of the mount down...

In some ways, I think the ARMS#18 can be compared to the Bassett, which LemmyCaution has already pointed out. The Bassett gets the job done, but the #18 is more of a "belt and suspenders" approach, in that in addition to sharing the same primary attachment method, the #18 further resists recoil forces with the front and rear supports. As long as the receiver is within spec, they can't hurt, but that can prove to be a big caveat.

 

[Grantman]

Great clarification M1AallTheWay, thanks.

 

[Grantman]

I mounted my PM081A several weeks ago but won't have a scope mounted and test fired till around Christmas.

Naturally the biggest concern is that the receiver and mount are sufficiently to spec that the mount will be securely in line with the barrel. The ARMS 18 and Basset style mounts rely entirely on the receiver being in spec, at the mount points, in order line up correctly. They do not offer windage adjustment like what is regularly seen on aluminium mounts with their steel stripper clip inserts and the shimming/screw-in spacer setup those mounts have with their rear side-screw setup.

It is mounted on a Norinco M305 / M14S which are recognised as having very good in-spec forged receivers. I found the mount's rear support screw just barely scraped up against the stripper clip dovetail lip closest to the muzzle. Per recommendations I'd read here: http://www.perfectunion.com/vb/showthread.php?t=67350 the forward and rear support screws should be screwed down till touching followed by a slight quarter turn. It was harder to judge with the rear screw due to the resistance caused by the contact with the stripper clip dovetail lip. As you can see I've got a larger gap between the dovetail and mount’s rear than LemmyCaution which I'm wondering was caused by over tightening the mount's rear screw or whether that’s simply how it is. Guess I'll find out when I mount my scope.

 

[Bimmer]

Thanks for doing this, LemmyCaution...

I hope this isn't too off-topic, but these are my (positive) experience with ProMag products:

I bought two scope mounts for my AR, a ProMag ($8) and a Leupold ($25, on sale).
The ProMag is just as nicely finished as the Leupold piece. It's not only perfectly serviceable, but also in some ways clearly superior (I wrote up a lengthy review over at M4C.net.

I also have a ProMag rubber buttstock pad on my AR. It cost $8 (less than half the cost of the comparable VLTOR piece) and is perfectly functional and more visually appealing to boot.

No, I wouldn't buy everything ProMag makes. However, I wouldn't buy everything from any other manufacturer I can think of, either...

Bimmer

 

[Bimmer]

To give an idea of how well my mount lines up I stretched a series of straight lines from the mount toward the barrel . Note the rail isn't directly centered above the barrel, rather it is marginally off to the left of barrel (viewing from shouldered position) but still in line with the barrel which is what matters.

This doesn't look straight at all.

It looks to me like your scope mount is aimed off to the left of where your irons are sighted.

Look at the red line starting at the rear sight aperture - it doesn't wind up at the front sight post.

Or trace the red line that does overlay the front sight post back to the receiver - it doesn't wind up at the rear sight aperture.

 

[LemmyCaution]

This doesn't look straight at all.

It looks to me like your scope mount is aimed off to the left of where your irons are sighted.

Look at the red line starting at the rear sight aperture - it doesn't wind up at the front sight post.

Or trace the red line that does overlay the front sight post back to the receiver - it doesn't wind up at the rear sight aperture.

I'm going to have to chime in here, as someone who works with computer graphics (as well as physical layout and measurement) as part of my livelihood.

Grantman-

It's extremely difficult to merge vector based drawing (the red lines on your photo) with raster based images (the photograph of your rifle).

The issue involved here is the extremely precise axial alignment of camera lens and subject (you're misaligned in both elevation and azimuth) required to be able to superimpose vector lines on the raster image to gauge alignment.

If I may suggest a better means of gauging the alignment of your optics mount, I'd recommend the following:

1: Obtain some fine thread and tie a non-slipping knot, such as a bowline, to form a loop in the end of the thread approximately 25mm in diameter.
2: Place this loop over the rear sight aperture and center the aperture, relative to the receiver. Better yet, if your irons are already sighted in, leave the sights where they lie and you'll be accurately gauging the bore line of the rifle for windage.
3: Lead the thread out to the front sight blade or post.
4: At a point approximately 100mm toward the rear sight from the front sight blade or post, tie a slipping loop knot such that the tag end of the thread slips and the dead end (toward the rear sight) is fixed.
5: Pass the thread around the front sight or blade and run the tag end through the loop formed in the slip knot.
6: Draw the thread tight, stretching the thread between the front and rear sights and secure with an overhand knot.

This should give you as close to a straight line between front and rear sights as you are going to get without a laser or some very expensive jigs. It will also give you a physical object to measure against.

At this point, you can use a caliper to measure the distance from the thread to the edges of your Picatinny rail on the left and right and at the front and rear. These measurements will gauge the windage of the rail relative to the iron sights.

If you wish to gauge the elevation, you can place a straight edge (such as a long ruler) on top of the front sight blade or post and run it to the rear sight aperture. Line the bottom corner of the straight edge up with the center of the aperture and secure the ruler in place with a small clamp that grips the end of the straight edge and rests on top of the rear sight aperture.

You should now be able to use your caliper to take inside measurements between the bottom of the straight edge and the top of your Picatinny rail. measure at the front and the rear of the rail to gauge elevation misalignment. Keep in mind that the sight line of the iron sights should converge toward the bore line. The sight line and the bore line will converge at the zero range. If you intend to zero your optics at the same range as your irons, it would be nice to have your mount parallel to the sight line. It will give you more elevation adjustment on the optic than if the mount is parallel to the bore line. It's also a whole lot easier to gauge.

Please note that this technique will not work with mounts that ride above the sight line, such at the Bassett high mounts.

I'll have data of this type for my rifle and mount later in the week and will report back.

-LC

 

[Grantman]

I should have mentioned the front sight is slipped to the right on it's dovetails which I haven’t adjusted and may be marginally canted, nullifying the old string from rear sight to front post trick. On a $380 (USD rate) NIB Norinco it doesn't worry me much - since I'm scoping the rifle. Also the centre line drawn on the mount was a best guess. And yes, there are significant issues with the pincushion, barrel affect distortion of compact camera lenses on top of the viewpoint issues, but I thought I'd give it a stab out of curiosity. In the end from my perspective, the mount lines up as I'd expect it to. The problem I have with further examination is that I don't have the rifle kept with me currently due to my renting situation. Thus it won't be till Christmas when I get the time to mount a scope and practically test the mount.

 

[Al Mack]

OK here's my .02, get a 2 foot dowel 3/8ths inch round. set it in the mounts center groove. You will see clearly if your mount is true. If not you'll know where you are off and how much.

 

[Grantman]

Yeah or a meter ruler. Unfortunately I won't be able to test that out for over a month. That's why I was stuffing around drawing lines heh.

 

[Grantman]

Well after testing a couple of things to note. Due to the retaining clip that holds the mount's side bolt fully extended I had simply lined up the bolt with the receiver hole and tightened. Turns out the mount's side bolt hole didn't line up properly with the receiver by about 1mm, so when I lined up the bolt with the receiver and tightened it down the mount's keys were not properly mating with the receiver's slots.

Took a round file to the mount's side bolt hole and widened it accordingly. Snugged the mount on good and tight and found it does indeed point slightly left. My scope was sufficient to accommodate for it, but it was a significant amount. Irrespective of the windage issue, the mount does seem very solid indeed.

 

[led boot]

One thing about a cup point hardened set screw.
They are designed to dig into whatever metal they are screwed against.
When I tried the mount, I ended up turning the set screws flat in the lathe.
YMMV

 

[hubb1953]

I have one of these mounts. I got it at a very good price.
When I first attached it I noticed that the bottom edge was hitting the wood right below the keyway. I thought maybe that my keyway might be machined out of spec (I have read that some SAI recievers have problems in this area.) however I also have a Scout -Squad SAI and I measured them against each other and they are the same.
I milled .050 off of the bottom of the mount and solved that problem.
When I torqued the mounting bolt down I got to about 35 in./ lbs. and the screw just kept turning. I thought that was it- I had stripped the threads out of the reciever. I took the mount off and the bolt was stripped. The reciever threads were fine. I called Promag and asked if I could buy a new mounting bolt, They were very nice but explained there was no part number for this item. The gentleman I spoke with asked me for my address and in a few days two new bolts showed up in the mail.
The new bolts were longer than the original by about one-sixteenth of an inch an when screwed in and torqued down the threads stuck out into the groove where the bolt ear rides blocking the bolt. A quick trip to the lathe fixed that.
All in all I think that the mount is ok and would buy one again as long as the price was right.