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I purchased a Precision Firearms (6.5 Grendel Neptune 5 ) a couple of years ago and put it on my Rock River Varmint AR15 and fit up fine. Due to health problems I sold the upper to a friend. The uppers' lugs would not fit or even insert into his Rock River lower pin area. His lower was a new Rock River lower unit. I removed just enough off the side of each upper lug until it inserted into the lower unit. Very tight fit and hard to lock up. He took it home and tried to put it one several other (inexpensive lower units) and it you not fit any. The upper lugs are to wide I think.
So my question is what is the best way to fix this (take more off the sides of the upper lugs or what?
Thanks
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Order a matching lower from PF?
You're dropping a tuned Ferrari chassis and motor onto Pinto frames.
Is this their billet Neptune 5 upper?
Really deserves a matching billet lower if you haven't felt how the 2 mate up with each other.
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LRRPF52 Wrote:Order a matching lower from PF?
You're dropping a tuned Ferrari chassis and motor onto Pinto frames.
Is this their billet Neptune 5 upper? Yes, when i purchased it they said it would match up. Well it did with mine but seem not on others. although the others are cheap lowers. well try a colt lower this afternoon and see how that fits.
Really deserves a matching billet lower if you haven't felt how the 2 mate up with each other. I really did not want to spend more money just to sell it.
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did you put a micrometer on the lug and the lower?
are they both within spec?
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i did. the upper was a little wider than the RR lower. i did take some off each side of the u[per lugs. still too wont fit most forged lowers that we have tried. will try a colt next. i hope that was correct way to fit up
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LRRPF52 Wrote:Order a matching lower from PF?
You're dropping a tuned Ferrari chassis and motor onto Pinto frames.
Ouch, you really know how to hit a guy when he's down! 
OP: If it were me, I'd be changing the cheap lower before I went to grinding on that upper. And I wouldn't do that much. A tight fit from upper to lower is a good thing, make use of it. They make cheap delrin punches to push out tight pivot pins, way better option than filing off material on PF's stuff IMO.
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02-17-2018, 12:19 AM
(This post was last modified: 02-17-2018, 12:49 AM by Klem.)
In your shoes I would buy either a billet or forged stripped lower from PF and make it a set. Their billet is $200 and the Forged is only $100, plus another, say $50 for transfer to a FFL Dealer and postage. Then populate the lower with parts or swap the parts over from the RRA lower. Both the PF forged and billet lowers are 7075 grade aluminium so best quality and your Ferrari upper is crying out for a similar quality lower. It will sell quicker if the receiver is a matching set.
After you've filed the lugs on that upper it won't be as tight a fit but this can be offset by screwing in tight the Upper-to-lower tension screw.
You will end up paying for someone else's lower anyway so you may as well source the PF lower for best fit.
The anodising will be shaved off where you filed the lugs and this needs to be disclosed when you sell it. Unfortunately some buyers will see that as damage. A PF lower will likely still be a tight fit and this will help.
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Odd. My Neptune VIII fits just fine on my $49 blem PSA lowers, and Noreen Billet lower.
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What are the military specifications of those upper lugs? Are the PF lugs within spec?
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bj139 Wrote:What are the military specifications of those upper lugs? Are the PF lugs within spec?
Not an isolated case that a quality upper does not to fit an average lower. I have seen this with Arsenal Democracy uppers (Seekins) that do not fit Colt lowers.
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Klem Wrote:In your shoes I would buy either a billet or forged stripped lower from PF and make it a set. Their billet is $200 and the Forged is only $100, plus another, say $50 for transfer to a FFL Dealer and postage. Then populate the lower with parts or swap the parts over from the RRA lower. Both the PF forged and billet lowers are 7075 grade aluminium so best quality and your Ferrari upper is crying out for a similar quality lower. It will sell quicker if the receiver is a matching set.
After you've filed the lugs on that upper it won't be as tight a fit but this can be offset by screwing in tight the Upper-to-lower tension screw.
You will end up paying for someone else's lower anyway so you may as well source the PF lower for best fit.
The anodizing will be shaved off where you filed the lugs and this needs to be disclosed when you sell it. Unfortunately some buyers will see that as damage. A PF lower will likely still be a tight fit and this will help.
well, it would not fit at all. neither upper lug would even start to insert into the lower. So I already took off the black paint from both side of the upper lug and it fit good and a little tight now into the his RR lower. lock up was tight also. we did not paint or anodize the upper lugs and left them as they are. it was already sold - he was there when we did it. thanks for all the info.
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A general guideline that makes a lot of sense when fitting parts is to take material off of the cheapest part, not the most expensive and well-made part.
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You did whaaaaaatt? I almost choked on my lunch reading that. Now I'm not hungry either. I guess as long as your buyer is satisfied. I might have some Arizona beach property available just in time for spring break.
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This might help in determining which part is out of spec.
http://www.m4carbine.net/showthread.php?...Dimensions
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I would have sent the PF back to PF and tell them to make it to industry specifications! Don't sound "precision" to me! If they only make their uppers to fit their lowers then so be it but they need to specify that! I have built a lot of uppers and those that went on RRA lowers did so without any problems! Of course I am just a "common man"
OJD
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I am no fan of PF, however... their upper I have fit on 3 different lowers. Two PSA lowers, 3 years apart circa, and one Noreen Billet.
Could be that there is a slight tolerance stacking going on with your PF upper against your RRA lower. Don't know, and since it has been tweaked to fit, it is no longer a matter of concern.
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old jeep driver Wrote:I would have sent the PF back to PF and tell them to make it to industry specifications! Don't sound "precision" to me! If they only make their uppers to fit their lowers then so be it but they need to specify that! I have built a lot of uppers and those that went on RRA lowers did so without any problems! Of course I am just a "common man"
OJD
I've seen PF billet receivers that fit so tightly together, you almost didn't need pins.
Billet receivers made for that level of precision fit are a bad match to mount onto forged lowers from the bargain tier.
Rock River machines their takedown lug recess mate-up differently so that their receivers will initially feel really tight.
They will often wear out their fit if you apply dynamic tension when shooting a lot, and get sloppy.
I should have caught that earlier about this not fitting on an RRA lower.
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