If I were looking at buying a transferable lower and I want to make sure the sear pin is placed in the proper location, which measurement should I be more concerned about looking at?

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While you're here, what is the distance I should be looking on A or B? (I'm assuming center to center would be the proper measurement.)

Thanks

Edit to add: I used a photo posted by @far_right for the start of my A/B diagram, it's only for reference and isn't the lower I'd be looking at.

Since ive been summoned

I would think A, since B could vary based upon the selector. But I guess A could vary based upon the top deck.

Probably A, to much variable in a selector

The blue print uses B, the center of the selector to the center of the sear pin hole.  The A reference measurement I’ve seen online varies 0.022”.

This is why jigs, like Bowers, are popular.  It’s tough to accurately measure a hole.  If you eyeball the center with calipers, there will be a certain error & the reproducibility of it will likely be inconsistent.  The tolerances then become the issue:  how many thousandths off can you be & still have reliable function?

If you use a jig, you use proper sized pins to center it:  0.125” for the top hole & 0.375” for the selector, then the jig either fits, or it doesn’t.  Jigs are the “Go/No Go” gauges of the machining world, or they were when I ran a mill.

The only accurate way I know of to measure center to center of holes is on a vertical edge mill with a center, which is a tapered point that finds the center of the hole from the center of the chuck.

I’m sure a real machinist will come along with better info, but the older Bridgeports machines with X,Y,Z axis you crank by hand measured with a center pin and hand crank while watching the axis measurement.  CNC—I know nothing, but have seen laser measuring on TV’s “How it’s Made.”

My concerns with a jig are getting accused of something by ATF or having a manufacturer of the jig forced to turn over sales info and then having the ATF at my door demanding it be turned over and what not.  I've seen the new frontier armory jig, which also locates the hammer pin, and it would seem to be the best option as it would triangulate the pin location in reference to the hammer and safety that the sear interacts with.  Again, I'm. just not sure how much sense it makes to buy/own such a jig.

Alternatively if you have the item available to inspect you can check sear function with drill stock as gauge pins, the way one would check to shim/time a RDIAS.

If you’re buying a transferable lower, you have all the justification to have a jig to ATF.  They’re cheap, compared to a $10k plus registered receiver. And, if you’re really nervous about having one, you can send it to ATF or toss it in a lake when you’re done.

With the proliferation of 80% lowers, asking for measurements is more conspicuous & suspicious, IMO, which is why I don’t provide data on them, especially when eyeballing the center of a hole with calipers is inherently inaccurate.

Best of luck with your purchase.  The SOTs I’ve dealt with had jigs or were able to measure off their own legally owned lowers. YMMV.

B should be .225, +- .0015

In reference to asking for measurements being conspicuous because of 80% builds... there are tons of blue prints available online so if anyone was intrested in making their own, with mill access and all that you need to do an 80%, I suspect they're just going to look at a print or buy the M16 jig.

I had not considered that the FFL handling the transfer could gauge the lower on my behalf. If it's coiming in from out of state vs buying from a local FFL with the item in inventory/consignment, it seems like an in state FFL might have to go through a lot of work for a transfer that could fall through on arrival, with a good bit of money involved. Am I misunderstanding how most transfers from out of state are done?

When I started the thread, I was hoping to see a decent way a seller could snap a photo with a measuring device beside one or more of the holes to verify proper placement before committing to a purchase. As noted by another poster, centering on the holes would prove difficult.

After seeing folks in some of the threads talking about specs being off here and there causing minor issues with fitting other uppers, timing and so on, I thought it was worth asking as I am considering jumping into the FA world depending on how things look in the FA market as the economy roller coasters.

I am not a lawyer and the ATF likes arbitrary and contradictory decisions BUT it seems that if you're really shopping then a jig would be easy to justify and very helpful.

If there are tons of blue prints, do the math & answer your own question.  I’m not trying to be a dick, but it doesn’t sound like you know how to read a blueprint, or you would have done so.

Machining holes in a metal part are measured from a repeatable point on the part, then center to center on each hole on the appropriate axis; a pilot hole drilled & appropriate sized reamer used to accurately make the hole within tolerance of the drawing.

A picture of a lower isn’t going to going to get what you need.  A jig will in person.  My SOTs had jigs for common items.  Eyeballing a picture or even a part, hole center to hole center free-hand with a caliper is not accurate enough to answer your question.

I would ask your SOT what he recommends.

As an aside, 0.225 +/- 0.015” isn’t the right answer, unless you’re measuring differently than center to center, which isn’t accurate due to the curve of the holes.  But if accuracy doesn’t matter, that value is as good as anything else.

Personally, if I was investing $10K+, I would want to be accurate.

Good catch, yeah that is not even close. Good thing I use a jig. Not sure where I got that number from.

Thanks. I assume you mean barrel extension and not receiver extension...right?

Also, I am guessing that a perfect timing would be fully locked right as the sear trips and releases the hammer...?

Thanks

Quarter bore used to have a page it showed how to time a DIAS. You could do the same thing to make sure your sear isn't dropping the hammer too soon.

The info you need is about halfway down on the page. They specifically say you can use their instructions for both. That's how I checked my M16 when I got it.

http://www.quarterbore.com/nfa/dias.html

Thanks

"B" dimension is more important.

The hammer and trigger pin hole placement are
both indexed off the safety selector hole center.

The amount of sear surface engagement to hammer ear determines timing.  So sear pin placement should be indexed off the same
datum as the trigger and hammer.