Posted: 1/4/2020 5:01:07 PM EDT
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In the early '90s Colt introduced the R6830 Sporter Lightweight chambered in 7.62x39mm. It had an A2 upper receiver, fixed A2 stock, 16" barrel, and a lower receiver with the dreaded (gasp) sear block. Colt also offered 7.62x39mm rifles and conversion kits with the A4 upper receiver. The R6850 had a 16" barrel, and the R6851 had a 20" barrel. After a few years, Colt dropped the 7.62x39mm ARs from their lineup.
In 2018, about twenty 7.62x39mm 6940 upper receivers found their way to the secondary market. These upper receivers had been part of a foreign military sales evaluation program. Colt LE6940- 7.62x39 Upper Receivers. Being a big fan of the Colt 6940 series, I picked up one of the upper receivers. With my Colt OCD, it never felt right to pair my 7.62x39mm 6940 upper receiver with a 5.56mm rollmarked lower receiver. Several months ago I finally caved and began a search for a 7.62x39mm rollmarked Colt lower receiver. I bit the bullet and paid the "pre-ban" premium for a very gently used R6830 lower receiver. Upon receiving my R6830 lower receiver, my Colt OCD quickly confirmed that my new to me lower receiver was anodized in classic "pre-ban" gray and my 2014 vintage 6940 upper receiver was anodized in modern Colt black. To add an additional challenge to my project, the R6830 lower receivers left the factory with a steel sear block pinned into the lower receiver. In order to re-anodize the lower receiver and use a full-auto bolt carrier in the upper receiver, the sear block had to go. Compass Lake Engineering worked their magic on the sear block and the pins. U.S. Anodizing worked their magic on the anodizing and hole in the right side of the lower receiver left by the removal of the sear block. The end result was an up to date 7.62x39mm rollmarked Colt 6830 lower receiver for my 7.62x39mm 6940 upper receiver... in effect creating the Colt 6840 
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Quoted:
Yes, very nice. Looks like they did a good job with the sear block holes When Colt installed the sear block, they used a large pin that protruded through the right side of the lower and a second pin that extended into the bottom of the lower. (Some sear blocks also protrude through the left side of the lower.). When Compass Lake mills out the sear block, they also remove the part that protrudes through the side(s). As part of their service, they will install a steel plug where the sear block protruded through the side (just above the selector). If you are going to have the lower re-anodized, you will want to install that new steel plug after it has been re-anodized. If you don’t want to install the steel plug provided by Compass Lake, U.S. Anodizing can install an aluminum plug into the offending sear block hole. U.S. Anodizing will do this before they re-anodize the lower so as to make the hole/plug less noticeable. One last thing to consider, if you are going to have your lower receiver re-anodized after the sear block has been removed, all remnants of the bottom pin need to be removed from the lower receiver. That pin is also made of steel and cannot remain in the lower receiver. |
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Quoted:
why did you pull off the numbered white tape from the trials? not wise since you seem to be a collector. just saying. |
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The clear packing tape covering the white cloth tag with the upper number didn't impress me as being particularly robust. Even with only modest use, handling, and/or cleaning, I didn't feel the protective tape would have survived very long and protectrd the white cloth underneath it. The white cloth tag is safely in a bag inside the box that came with the upper receiver. i would not have and have not on the two that i bought. |
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Quoted:
Cool. But why not start with a Colt 901 lower? 
(Photos courtesy of modrenfirearms.net.) |