The OSI Drilling fixtures

Here are some pictures of the OSI drilling fixtures.

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Special notes on full auto conversion and use of the jig for M16 milling and drilling. 



The picture to the left shows the jig prepped to drill the trigger group holes.  For best results, alignment would be checked and the jig held in place by large C clamps.  The order of drilling the holes is important.  

Pins are inserted in the selector and takedown pin holes as soon as they are drilled and checked for alignment.  This will reduce or eliminate any error attributable to the shifting of the setscrews.   Nails can be inserted in the smaller fire control holes when they are finished.  

In order of drilling, first is the selector hole, then check for alignment.  Once verified, insert pin and drill the front takedown pin hole.  Insert pin and then drill the rear takedown pin hole.  

Then working from front to rear, drill other trigger group holes as needed.  


This is not the best photo to illustrate the pin locations, but the camera crapped out before more could be taken.  Here you see the three lynch pins in place on an early prototype jig.

Note that the trigger pin hole may end up far enough forward that you will need to remove a little bit of metal from the front of the inside of the trigger housing in order to clear enough space for the movement of the hammer.  This is normal as this part is thicker on the Tannery lowers than on many factory lowers.  It is best to use the sanding drum on a Dremel tool to remove this material.  Another option is to mill it out a little more, but do not be too  aggressive as this will break through to the passageway for the magazine catch assembly. 


A factory lower is used as a template for the drilling jig.  Where measurements differ from the blueprints, I decided to use the working lower as the model.  The top plane of the jig matches the top plane of the lower but is not quite an exact match. It is close enough to use as a guide for filing, milling or sanding the top plane of a Tannery lower.  But final fit should be done by hand. 

Note, this picture relates to an older style jig that is now out of production.

The drilling plate is the main component of the system.  Individual drilling plates are available  and can be used without the jig in a variety of ways.  Once the top plane is set close, final fitting with an upper should determine how much more material should be removed.  be careful not to remove too much material before you begin final fitting
Here is a test fit with upper in place

Here you see holes centered and ready to drill

Center the existing buffer hole on the 80% lower with buffer hole in the jig.  You can check alignment as shown here by installing an upper receiver assembly on the lower while in the jig and checking bolt carrier travel.   The new jig has a tapped hole for the buffer tapping operation to ensure that the tap goes in straight.  As a byproduct of this, note the new style buffer holes are more vulnerable to damage when drilling by hand.  


Here is a top view of the improved fixture.  It can be used for both milling and drilling operations in different machines and clamping arrangements.  The heavy 3/8" steel plates have setscrews countersunk so that the fixture can be clamped or laid on its side (on either side) and still remain parallel to the work. UPDATE:  It may also be clamped on end for drilling the buffer tube hole in the lower.  Manually check level and square when it is in the clamps.  The front edges may have a little angle on them or can shift in a vice.  It is steel, so crank the vice really tight once you know you are hitting the lower square, level and straight.   Do not crank the setscrews too tight, especially the ones at the magwell.  They can bend in the magwell.   FURTHER UPDATE: The new style jig centers itself off the magazine well and this operation will be much easier.  You will have to do some fitting on the magwell so that the lower slides onto the jig because the tannery shop lowers come with a tight magwell. 

Alignment is set in relation to the hole for the magazine catch and a machine screw that passes through it to a nut on the other side of the jig.  This is not a tight tolerance, so there is enough wiggle room to make the adjustments you need to precisely align the front of the magwell.  



The jig is usually laid on its side for most operations including drilling and using a sanding drum on a drill press to trim down the top plane of the lower.  Note the auto sear hole on the left plate but not the right plate.  It is not drilled all the way through, but can be finished by authorized parties who wish to use the jig for lowers to be compatible with M16 parts.   UPDATE: Simple drilling of an auto sear pin hole is insufficient to make most factory AR type lowers compatible with GI pattern M16 parts.  This includes the Tannery shop lowers.  Anyone who says that my jig or anyone else's jig is used to drill "one simple hole" to convert a lower to M16 configuration is simply lying.  The process also involves milling the trigger well area and renders the "conversion" permanent with or without the internal M16 parts.  This means that if you perform this operation on any brand of lower or one that you have made yourself, the lower is by legal definition a full automatic weapon whether or not you have installed any M16 parts.  


The new drill jigs are made from wider plates and include holes for the trigger guard.  I have noticed that the design of the fence at the bottom of the tannery shop magwells is larger than on GI lowers, so you may or may not want to use the jig for drilling these holes, but the jig will help you set the distance between the holes.  If by some chance they will not perfectly align with your trigger guard, drill the front hole slightly larger in small increments until the front plunger of the trigger guard fits.  UPDATE: I am not including the front plunger pin hole on these since I have found that the lower looks better when these are drilled to fit on a case by case basis rather than at the milspec location.  

The New jigs take the major alignment off of the location of the magazine catch hole in the Tannery lowers as this is correctly located in relation to other dimensions on the lower.  That does not make the system foolproof, but helps greatly.  If you have one of the older jigs without the provision for this alignment (whether or not you are the original purchaser) contact me and I will instruct you on how to do the modification and provide the add on parts if necessary.  On the new jigs a long 10-32 screw is included.  you place the lower in the jig and pass the screw through the hole in the jig that corresponds to the mag catch and align the pilot hole in the buffer tube with the buffer tube hole in the jig.   There is some 'wiggle room' in this fit to allow for fine adjustments of the set screws before drilling.  Check the alignment of the lower in the jig for square and level.  Perfect alignment should be checked with an upper after you have filed the front ears that hold the front takedown pin in place.  As with the older jigs, the critical fine measurement is the front of the magwell.   All is not lost if it is not perfect, but it will save you lots of headcaches if it is right.   It is normal for the lower to be closer to one side of the jig than the other and to require some shaving or filing of the ears that hold the bolt catch pin or the re-enforcement fence at the top of the magwell.   These are oversize on the tannery lowers compared to a GI spec lower.  Only shave what is necessary, since the extra material contributes to the structural integrity of the lower. 

General instructions:

The instructions below are mainly notes from and some emails and notes from people.  I have not set them all in perfect order, so read the whole thing before cutting any metal. 

Note that the drilling fixture does have a hole for the auto sear pin.  As is visible in the lower in the sample pictures, drilling for the auto sear pin is not sufficient for the installation of an auto sear.   This operation has not been performed on the sample.  Material must be removed from the trigger well for the sear to fit, as is with any commercially made AR type lower.  The sear hole on the fixture in the picture is also slightly off center and the drilling operation for this hole works differently from the drilling operations for the other trigger group holes.  Correct alignment is to be directly above the selector.  I don't have an exact spec for how far below the top plane of the lower, but from measuring off a GI M16, It looks centered about 1/8" down, which leaves very little material above it to hold the pin on location.  There are some close up pictures of an M16 in the gunsmithing section so you can see this.  

I am waiting for some more pictures to get back to me on the drilling and machining processes so that we can have a better technical page.  Anyone with pictures of their jig or plate in the building process email me at a location where I can download them from (unsolicited email attachments get wiped by my firewall).   

One thing a few people are bringing up is the dimension on the top plane of the lower.  Tannery lowers have extra material on this and you are expected to trim it down to where it will fit.  There is a little notch near the slot for the bolt catch that gives you an idea of how much is to be trimmed down.  It will roughly correspond to the top plane on our jigs and plates, but do not take this as the full precision measurement.  Make it close, but leave some material for final fitting with an upper as a guide.  This is important in setting the location of the takedown pin holes and the contour of the back of the lower where it meets the back of the upper.   That, in turn will affect the way the bolt carrier engages the buffer tube area.   

In setting the forward to rearward fit of the upper and lower with the pin holes, first open up the area between the "ears" that hold the front takedown pins on the lower.  This largely determines your left/right play on the fit.   The rear channel is already set in the casting and should not need to be fitted.   Once you have the left right fit in place,  the forward fit is set by placing your finger inside the magwell and feeling where it fits with the small part of the magwell that is in the upper.  You have the right fit when you do not feel a "step" between the front of the lower and the front of the upper ON THE INSIDE.  That's right, the critical measurement is the inside front of the magwell, as this is what will determine magazine fit and function.  Mark both the side of the upper and lower with a marker so that you can see these dimensions from the side in later fitting.  

One good guide for positioning the lower in a big jig (the one with the buffer tube hole) is to center the existing undersize hole in the Tannery lower in the guide hole of the jig.  This should ensure the correct relationship between the location of the buffer and the location of the takedown pins, all critical to the fit of the upper and lower.  Again, check and re-check everything for square and level.   The Tannery lowers have a lot of extra material on the back and you may need to shave some off.  Do not shave off any more than is necessary to fit the jig, as you want to leave some material to shave later.   The very back end tends to get beat up in the tapping process as you fight to put the tap in straight and make the first few threads.  That beat up part is generally shaved off in final trimming and the final angle is set to ensure the buffer tube goes on straight without rubbing on bolt when the bolt is all the way back.  

For removing metal from the top of the upper:  If you have one of the OSI jigs, you can hold the lover in the jig while you do this and get pretty decent results.  Just align the desired upper plane of the jig with the desired upper plane of the lower and then clamp it in place with the setscrews, making sure that it is all straight and level.   You can check this by looking through the front of the jig.   On a side note, you can avoid marking up the jig with the setscrews by putting a layer of thick tape on the lower before inching it in the jig.  Duct tape and metallic tape are decent choices, packing tape is not.  You may want to play it safe and put the lower in the jig just a tad (maybe 1/2mm) below where you want the final plane to end up, that way you can have something to work with if the trimming goes uneven.   Using a large flat file, or piece of coarse sandpaper glued to a 2" wide board, start filing/sanding away on the top plane until you get close to the right dimension.  You may also use a spindle sander or a sanding drum on a drill press.  Use the drilling jig as a guide.  At first, you will be just sanding on the lower, but eventually you will be getting close to the jig.  The jig is steel and the lower is aluminum, so your sandpaper is more likely to hit the aluminum than the steel, but a hard file or power tool might bite the steel.  When you do get close to the steel, switch to a finer grit and slow down.   

The rear curvature is best dealt with by wrapping sandpaper around something with about the same circumference of the curve you are dealing with and working it across with a side to side and rotational pattern.   A large dowel or even the buffer tube can work.   You can wrap the sandpaper around the buffer tube and hold it on with masking tape and use the buffer tube as the tool.   Another way to deal with the rear curvature and the top plane of the lower is to use a sanding drum mounted on a drill press.   The sanding drums come in various sizes and while it is not necessary to have the exact size, don't use an oversize drum for the rear (although it is fine for the top plane).  An undersize drum is fine for the rear.   Simply lay the jig flat on the drilling table and set the sanding drum at a fixed height close to the table and shape the lower by hand as it is held in the jig.   The top plane of the jig will act as a general guide for the top plane of the lower since it is a much harder material than the aluminum which the sanding drum will easily cut.   Avoid contact with the jig itself to help it last longer and apply oil to the surfaces of the jig to minimize damage when it does contact the sanding drum.  

When you reach the point where you are going to start drilling, check and re-check the fit with your upper.  If you are going to use a Tannery upper, you will want to drill the takedown pin holes in both at the same time as this will give you the best fit.   Make any fore/aft adjustments in the jig that make sense.   One way to check is to hold your upper right against the jig to see how the holes line up, then check that against the fore/aft location of the lower in the jig and check that against the inside front of the magwell and the way they fit.  One way to work with this is to mark the jig at a point an inch and a half behind the front takedown pin and line that mark up with a similar mark made across the upper and lower when they are in alignment.  

Follow the drilling pattern laid out in the text above.  First is the selector hole.  Once this is done, insert the large lynch pin and drill the rear takedown pin hole.  Insert the pin and then go to the front takedown pin hole.   You may want to put the upper on at this point, putting the upper in place and passing the lynch pin through both the jig, the lower and the upper.   Then drill the front takedown pin hole.  Having the upper in place while doing this will ensure that the two will be compatible with each other, but if the top plane is off, you could be ruining both the upper and the lower, so it would also be a smart idea to leave the upper off for this operation, it is your judgment call on how you want to handle it.  Once the hole is drilled, insert the pin.   

Once all of the side holes are drilled out, you get to the big bugger: the buffer tube hole.   If you have been shopping for the right tools for the job, you have probably discovered that the necessary 1-1/8" drill bit is one rare and expensive creature.   You might be able to get away with using a high quality hole saw if you are using the big jig, otherwise, you need the better quality bits.   Go very slow and careful when setting the location of the cut, and make it dead center to the hole that is already there.   The longer you take on this, the better it will probably turn out.  Chances are that you will have to force the bit back on center more than once.  If you end up with a slightly chewed hole, all is not lost.  You can correct for a lot of that error while threading the hole.   Be careful when the bit clears the ring and is just cutting on the one edge of the lower, as it will try to force its way back toward the thinner metal on the top of the ring.   

In tapping the buffer tube hole, take special care to make sure the tap goes in straight.  Back up a bit from time to time and go slow.  Don't worry too much about the threads being mangled at the rear end of the lower.  This part is extra thick on the Tannery lowers and will be trimmed off the back anyway.  If you end up with threads that are mangled enough through the length of the hole to give a loose fit of the buffer tube, you may have to use RTV sealant or in extreme cases, permanent JB weld to affix the tube in place. 

More to come on other aspects of finishing a Tannery Lower. 

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One finished project, pictures provided anonymously.