An accurate lower receiver drill jig can also be used in place of the upper receiver. Clamp or stabilize the upper and lower receivers in this offset relationship. Next position the upper in position with the lower, but offset from the lower with the mating surface of the upper resting flat on the edge of the lower receiver's “deck.” Insert a pin through the upper spacer adapter and into the lower. Remove the pin and detach the upper from the lower without disturbing the spacer adapter (it should be a tight press fit). Insert a pin (a 12D nail can be cut and polished) and check for fit. Drill similarly from the other side of the lower receiver, If possible, first drill with a #31 drill bit and final drill with a 1/8” drill bit. Drill though the lower 1/8” rear takedown hole and halfway into the spacer. Make adjustments to the plastic, if warranted. Ideally, the lower reveiver's “buffer tower” should have no gap with the aft curved surface of the upper receiver. Assemble and tightly clamp the upper to the lower in the correct position. Scale and reprint the spacer to obtain a tight fit, if necessary. Print the upper rear takedown hole adapter spacer and drive it into the upper rear takedown pin hole. The lower can be best fitted to the upper in assembly with the upper. The hole is located in the correct location to optionally support the retention of custom slotted takedown pins (non-standard 1/8' pin required). Rear takedown pin detent/spring hole was added for completeness.This change is required to accommodate the correct capture of the buffer retention detent and spring when assembled with an A1 or A2 style butt stock. Model was corrected to match the AR15 blueprint and permit the use of A1 and A2 butt stocks.Brims attached to models to facilitate printing and reduce the likelihood of the part lifting at corners.1/8” to 1/4” rear takedown pin spacer adapter included. 10-24 caps screws and 3mm ABS filament 'pins' added to connect and bond the two lower receiver sections together.Buffer tube 'tower' is oriented on the printer bed for maximum strength (FFF print layers are perpendicular to the buffer tube).Model was split to permit printing of the lower receiver 'deck” down on the bed, resulting in better use of the most accurate printer axis (Z-axis) and a very flat deck.
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