machining bore in AR upper receiver
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  1. #1
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    Default machining bore in AR upper receiver

    hey guys, i'm about to start programming a run of upper receivers. what would you guys recommend for the main bore on these? gonna be on a VF3 with trunion. no TSC unfortunately.

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    A lot depends what you intend to use them for, For long range competition a tight one inch, maybe a half thou under one inch, to the other extent if they are to be used on top of class 3 lowers (M16's) a little loose would be more reliable, maybe 1.002. I kinda like them when it takes just a little heat for the barrel to fit in, although some barrel extensions are made a whisker oversize for this reason. The tighter the better for accuracy but a little looser for a blaster, just my opinion I'm sure others have different ideas, Jon P.

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    Quote Originally Posted by JonP View Post
    A lot depends what you intend to use them for, For long range competition a tight one inch, maybe a half thou under one inch, to the other extent if they are to be used on top of class 3 lowers (M16's) a little loose would be more reliable, maybe 1.002. I kinda like them when it takes just a little heat for the barrel to fit in, although some barrel extensions are made a whisker oversize for this reason. The tighter the better for accuracy but a little looser for a blaster, just my opinion I'm sure others have different ideas, Jon P.
    these are gonna be 9mm uppers built for USPSA competitions. my question is like what kinda tools should i use? drill+ream?

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    I think you should use a boring head and boring bar. That will give you the best control over location. Drilling/reaming location will be worse.

    Although I see you plan to use a CNC mill. In that case, I would rough drill the hole and then mill the hole to desired dimension. That should give you good control over location. If milling doesn't give you the desired diametral precision, you could mill undersize and ream.

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    Quote Originally Posted by GGaskill View Post
    I think you should use a boring head and boring bar. That will give you the best control over location. Drilling/reaming location will be worse.

    Although I see you plan to use a CNC mill. In that case, I would rough drill the hole and then mill the hole to desired dimension. That should give you good control over location. If milling doesn't give you the desired diametral precision, you could mill undersize and ream.
    are you aware this is a 1" bore 8" deep?

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    From what I understand the best uppers are drilled, bored undersize, reamed, then honed to final size.
    FWIW, make the bore true 1" not over, not under. reason is that barrel extension to bore fit is critical for accuracy and function. If you are worried about to tight a fit of the bolt carrier in the bore, its pretty easy to polish the humps on the bcg that they ride on in the bore, but Ive never ever seen a bcg that's too tight cause problems.
    What I have seen is a barrel extension that was too sloppy a fit in the oversized upper to give good accuracy without putting a shim in it or using an oversize extension.

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    Quote Originally Posted by akajun View Post
    From what I understand the best uppers are drilled, bored undersize, reamed, then honed to final size.
    FWIW, make the bore true 1" not over, not under. reason is that barrel extension to bore fit is critical for accuracy and function. If you are worried about to tight a fit of the bolt carrier in the bore, its pretty easy to polish the humps on the bcg that they ride on in the bore, but Ive never ever seen a bcg that's too tight cause problems.
    What I have seen is a barrel extension that was too sloppy a fit in the oversized upper to give good accuracy without putting a shim in it or using an oversize extension.
    Good to know, thanks for the advice!

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    Kennametal makes a drill for that has replaceable tip I use a carbide tipped reamer. Do you have thru spindle coolant ? It takes a tall z to drill that. I mount right to table and I use up all of a 20 inch z no 4th axis. I’m not sure you can do it with 20” and a trunion
    Thanx Don


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    Quote Originally Posted by D Nelson View Post
    Kennametal makes a drill for that has replaceable tip I use a carbide tipped reamer. Do you have thru spindle coolant ? It takes a tall z to drill that. I mount right to table and I use up all of a 20 inch z no 4th axis. I’m not sure you can do it with 20” and a trunion
    Thanx Don


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    no TSC unfortunately i believe our VF3 is the extended Z. believe it should fit.

    never mind, just checked, VF3's have 25" of Z standard.

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    I made fixtures for each op. If you don’t have thru coolant I think you will have to peck the shit out of it. But the ream does leave a nice finish. The worst part of the deal is the hole for the bolt cover. That is a long way down to it


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    Quote Originally Posted by D Nelson View Post
    I made fixtures for each op. If you don’t have thru coolant I think you will have to peck the shit out of it. But the ream does leave a nice finish. The worst part of the deal is the hole for the bolt cover. That is a long way down to it


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    ya, gonna peck for sure. we're looking into a better machine in the near future, that one will have TSC for sure.

    which hole are you talking about? not sure i'm picturing it right.

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    The small hole that holds the bolt cover on


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    Also you will need a self centering vise for them


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    Quote Originally Posted by D Nelson View Post
    The small hole that holds the bolt cover on


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    AR9's dont use those so no biggie.

    i use a kurt 420D 5 axis centering vise.

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    The ones I build have them


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    here is one I built


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    Quote Originally Posted by D Nelson View Post
    here is one I built


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    wtf, why would you need a forward assist with a PCC?

    that just looks like a standard AR15 upper on an AR9, the uppers i'll be making are simplified, no forward assist, no ejector cover and the ejector port is much smaller.

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    Empwoer that sounds like a fun project, yea I think drill, ream and hone if you can, I assume you are making from billet, do you use the side charging handle ? Yea no forward assist no brass deflector and no cover makes it simple and functional for competition. A snug 1 inch bore will give a lot less carrier tilt witch really helps, although there are other ways to minimize bolt carrier tilt. It is really amazing the range of tolerance in different carriers etc.I think if I was making some uppers from scratch I would drill ream and hone to size as I got the carriers. There is no argument though that the barrel extension fit is the most important, although carry rarely is it loose. Good luck. Jon P

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    The big boys drill and then bore with a custom boring head. The head is just below nominal diameter with two opposing cutters. One roughs, the other finishes. They make the hole in seconds and hold tenth tolerances. Not cheap, but if you wanna play with big dogs, you gotta pee in the tall grass.

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    Gw Schultz makes all the tooling for the Ar uppers there is a bunch of custom cutters for the uppers. They also need to be broached if you use a standard charging handle




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