Re: M40a1 build guide
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 10Xview</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kft101</div><div class="ubbcode-body">
Redfield states the ring screws are 6-40.
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That's interesting, because the MAXIMUM major diameter (Class 3A) for a 6-40 is .1380", while the bolts in mine measure .161" which is closer to a #8 size. I wonder if the size changed somewhere along the way. For whatever reason, the gun industry is notorious for using very odd thread sizes.
For example, the Wichita sling swivel stud's thread size is 7/32"-40 (or at least it was in the 1990s), and requires a #9 drill for the pilot. Try finding that thread size in the shop books! I don't know how I found the tap for it in 1990, but I did. </div></div>
I think you're right.
From US Optics, their repro mounts use 6-40 ring bolts, and from a reliable source, the Unertl mounts use 6-32 ring bolts. Comparing a bolt from each mount side by side, that sounds right, as they're both the same diameter but the Unertl has a more coarse (lower) thread count.
Comparing them to a bolt from an original Redfield bottom screw ring, I think I got bum scoop from the Redfield guy. Maybe he was thinking about their newer rings, but the bolts used on the bottom screw rings are definitely not 6-40. Just eyeballing the bolt side by side with the others, I'm guessing it's 8-40.
UPDATE: OK, confirmed that the Redfield bottom-screw rings use 8-40 thread pattern bolts. Swapped the bolts from a set of Badger rings (and they use 8-40) and they fit perfectly between the Badger and Redfield rings.