measurement poll

[SimonSays]

On 4/3/2019 at 1:39 AM, 98Z5V said:

A manufacturer can put that mag catch placement anywhere they want, within obvious limits of the platform, brother.  As long as they account for that "higher or lower" placement in their exact placement of that BCG channel that they bore in the upper - on their own upper that mates to that lower.    They can literally do what they want, within functional limits.

When people try to match different uppers and lower from different manufacturers - is when we see these issues.

On 4/4/2019 at 11:54 PM, 98Z5V said:

The reason there MUST BE a correlation to that, is - the bottom of the bolt face has to strip the next round out of the magazine during the cycling process, when the magazine is seated and locked. The bolt face takes this force, because of the cam pin - the cam pin isn't allowed to rotate until it gets to the cam pin cutout in the upper - that bolt is locked in place in the BCG body, extended, while the cam pin is traveling down your charging handle groove, in the upper receiver - it's solid, not moving into the BCG body - and stripping the next round.

Now, there will definitely be some variance, or tolerance, in what's too high and what's to low, in the relation of those two parts - but it can't be much.  20-thou?  30-thou?  I don't know. 

Within limits, manufacturers can do whatever they want for mag catch placement in the lower - as long as they are within that range or tolerance in BCG centerline over the top of that mag catch.  And it will work.  They are working within the limits of their own upper AND lower receiver dimensions to achieve that distance between those two parts. If a manufacturer fuks that up - your gun doesn't feed/strip the next round, OR the BCG slams into the back of your mag body.  It can only be those two things. 

 

That up there ^^^ explains so much for me. I have gathered from some Wise Builders on this site not to mix uppers and lowers from different manufacturers, but I truly didn’t understand why. Some stated there may be functional issues, but I didn’t understand what those might be. Some stated the lines wouldn’t match up. I thought they ment it would be aesthetically unpleasant. That might be true, but “lines” could also mean just what you are talking about. I’m seriously considering abandoning building a 243 and 260 with 80% lowers. 

Thanks for clicking on the light bulb in my head. 

[ar-mountaineer]

58 minutes ago, SimonSays said:

That up there ^^^ explains so much for me. I have gathered from some Wise Builders on this site not to mix uppers and lowers from different manufacturers, but I truly didn’t understand why. Some stated there may be functional issues, but I didn’t understand what those might be. Some stated the lines wouldn’t match up. I thought they ment it would be aesthetically unpleasant. That might be true, but “lines” could also mean just what you are talking about. I’m seriously considering abandoning building a 243 and 260 with 80% lowers. 

Thanks for clicking on the light bulb in my head. 

If you get the correct stuff (meet spec) then there should be no problem with it functioning. I bought a .243 mutt upper (I don’t know anything about the components on it) and it went through a function check and works on my serialized lower. I plan on this upper going on my 80% lower and there is no reason it shouldn’t function the same with the same lower parts (buffer et. al.). Heck the gas tube in the upper is short and it still functions. 

Edited April 8, 2019 by ar-mountaineer

[panel77]

1 hour ago, SimonSays said:

That up there ^^^ explains so much for me. I have gathered from some Wise Builders on this site not to mix uppers and lowers from different manufacturers, but I truly didn’t understand why. Some stated there may be functional issues, but I didn’t understand what those might be. Some stated the lines wouldn’t match up. I thought they meant it would be aesthetically unpleasant. That might be true, but “lines” could also mean just what you are talking about. I’m seriously considering abandoning building a 243 and 260 with 80% lowers. 

Thanks for clicking on the light bulb in my head. 

you are correct.

There are many manufacturers that dont make an upper and a lot of  uppers work with them.

im my case I am using the upper that is made and sold by the same company that made the lower. 

BUT... the lower is apparently enough out of spec to have tolerance stacking.... to fail.... sometimes but again, 1 time is too many and makes it a great hammer that cant be relied on....

 

"98Z5V" knows what he is talking about.

he like many others have been able to mix/match uppers as long as measurements are taken and wisdom applied, most will interchange. in the hot rod world its called blueprinting-the process of measuring everything, knowing what the spec is supposed to be and getting parts that make that spec.

as if it hasnt been stated enough-308 PSA is NOT compatible with anything that is not PSA(in large frame-223 stuff works perfect every time interchangeably)

308 and PSA can NOT be used in the same sentence if you want to talk about interchange.

 

go ahead and build your 80% any way you want-BUT... buy uppers to match at the same time and I would use this message list to get a lower from a a company that is at the lower end of spec range... eliminate one piece of the problem.

[98Z5V]

6 hours ago, SimonSays said:

Thanks for clicking on the light bulb in my head. 

Somehow, I've found a way to explain complicated mechanical shiit in laymen's terms...   That has got to be from Army time...  If you can teach a Private how to break an anvil in a sandpit, you're explaining it right...  

I'm humbled by the comments most of the time, truly.  I'm just a normal dude.  I've been building MX race engines since 2002, and teaching others how to do it.  There's a LOT of complicated shiit into making a stock 30hp 400cc engine into a 60hp 434cc engine with a 14.25:1 compression piston, cams, crank, carb swap, and carb tuning, fuel (octane) requirements - that build is race gas all the way, too.  On those engines, I've been playing with EFI systems in MX since the 2006 model year - but I was playing with EFI systems since my 1987 RX-7 and 1993 Jeep Wrangler.   I can explain all that like reading a children's book.  I've literally written papers on 4-Stroke Engine Calculations, just as tutorials for people, and can have you understanding the differences between Static Compression calculations, and Dynamic Compression Calculations, Piston Speed Calculations, and (more importantly) Piston Acceleration Calculations...  It doesn't matter if it's a 4-Stroke dirtbike, or a Mustang GT-350 engine - things you must do, in order to not blow it up.  You build it wrong, and it's gonna blow.  That rap evolved into turbo systems, on things that didn't have turbos from the factory, then it went to things that had turbos from the factory, and making them WAY better and more efficient.  Like 230 RWHP on a EFI turbo one-liter engine with a CVT clutch system (that has 25~28% parasitic loss) - that's 300 crank horsepower from a one-liter engine.  Extrapolate that into a 5.0L HO Mustang engine...    That evolved into tuning CVT clutching...  it never ends...

Now, in looking at that for the last 17 years (11 on turbos) - look at my second passion...  the AR platform, and the travesty that is the .308AR marketplace, with manufacturers doing whatever they want. Of course, I'll get involved in it, deep. 

I'll lay out the nuts and bolts of it all - whenever the topic arises.  Whenever it does, I try to provide input that makes sense, and is easy to understand.  If you really knew me, though - like some of these guys here - I'm just a knucklehead looking for the next adrenaline rush that I can trick myself into.  There are guys on this board that are closer to me than alot of my own family members - true Brothers.  I'll give you the shirt off my back, if it will help you out, and I'll try to work through issues that guns have - they're mechanical, they cannot out-think us. We just need to know what they're doing, accurately, to solve issues with them. 

I'm - literally - here for you, man.  Seriously.  If I can't figure it out, I'll find out why I can't figure it out, what went wrong, and solve it, eventually, somehow.  Most of the time, though - here - I don't say anything... These guys got this. 

Edited April 9, 2019 by 98Z5V

[DNP]

I think that's a pretty accurate reader's digest version of the autobiography. 

[panel77]

Pic

first 4 are broken arm lower with bear creek upper and amt upper

last 2 are amt lower and upper

fit is ‘fine’

the broken arm was supposed to be black and looks more like a greenish dark black.

it fits snug with pins and doesn’t rock and has rear lug pressure screw. But along the whole middle on both uppers it has a slight gap. Seller told me he uses aero uppers with no problem but also only uses lancer mags.

the broken arm measures at 0.669

[98Z5V]

1 minute ago, panel77 said:

 

Why are the pivot and takedown pins on this one going in from this side of the gun?...

[panel77]

17 minutes ago, 98Z5V said:

Why are the pivot and takedown pins on this one going in from this side of the gun?...

because its 80% and hasnt been touched or parts applied and they are the closest pins at hand-15 pins with no detent and too short to stay in from the other side...

 

BA lower just came in teh mail today.

[Lane]

22 minutes ago, 98Z5V said:

Why are the pivot and takedown pins on this one going in from this side of the gun?...

It's an unfinished 80% lower; they are just holding the two parts together for fit; and haven't been installed in a formal sense. What really confuses me is those hexagonal bumps on the other side (and it looks like pins are going to be short as well). 

On 4/8/2019 at 5:46 PM, SimonSays said:

I’m seriously considering abandoning building a 243 and 260 with 80% lowers. 

I don't think this should totally dissuade you from building 80% lowers. It really depends on the upper you wanted to begin with though. There are some Armalite style 80% lowers available; but then the compatible uppers are much fewer. I'll admit; I love 80% builds myself. If you pick either an upper or a lower to start with; it shouldn't be too difficult to figure out which other components will work. As you can tell from this measurement poll; it's not hard to get a whole bunch of numbers from people who have a few of these things around. 

[Lane]

17 minutes ago, panel77 said:

closest pins at hand-15 pins with no detent and too short to stay in from the other side...

That explains it. I wasn't half as worried about which way the went in; only that they were too short. Carry on.

[panel77]

9 minutes ago, Lane said:

It's an unfinished 80% lower; they are just holding the two parts together for fit; and haven't been installed in a formal sense. What really confuses me is those hexagonal bumps on the other side (and it looks like pins are going to be short as well). 

I don't think this should totally dissuade you from building 80% lowers. It really depends on the upper you wanted to begin with though. There are some Armalite style 80% lowers available; but then the compatible uppers are much fewer. I'll admit; I love 80% builds myself. If you pick either an upper or a lower to start with; it shouldn't be too difficult to figure out which other components will work. As you can tell from this measurement poll; it's not hard to get a whole bunch of numbers from people who have a few of these things around. 

I REALLY wish the broken arms came with its own upper but it didnt.

Frankly-I only bought it cause the seller spent time on the phone with me really trying to figure out the issue that is happening...

I will probably end up with 3-4 digits worth of upper/lowers before i find one setup that works and will have to run through 1,000 rnds before i dont worry about every rnd not working...

arrggg... PITA

 

the hex is a decoration.. like i said-i bought it to support superior customer service from the owner...

[Lane]

2 minutes ago, panel77 said:

I REALLY wish the broken arms came with its own upper but it didn't.

I will probably end up with 3-4 digits worth of upper/lowers before i find one setup that works and will have to run through 1,000 rnds before i dont worry about every rnd not working...

I don't think it's that bad in this case. I'll admit; I will need to read through this entire thread again to try and get a handle on where you are with this 80%. Looks like it's not milled yet, so you might want to save task in terms of flexibility. What were your plans for that? A jig? Blueprints of some sort?

I mentioned some time back; there is a way forward with a modified magazine catch if that is really your only issue here. It's not a perfect solution; but I think it's much better than modifying every magazine you have (or want to use). From what I can see, the lower will need a minor "enhancement" as well to make it possible to install. The maximum spread I saw across the mag catch height was ~0.060" (from memory); which is a bit more than a dime in thickness.

Hell; if you want the simple solution? JB Weld to thicken the top, and some way to grind it back down flat (a metal file would be fine). Same amount of adjustment needs to be done to the bottom of the catch (not even the whole profile) if the magazine catch slot in the magazine doesn't fit. Then you would need an access port where it plugs into the upper; on the left side so you can install it. That would also require a bit of filing. I don't think you need anywhere near that full 0.060" though, so in both cases it's a very tiny adjustment.

Here is a rough CAD sketch of the maximum deviation listed. As I mentioned; I still need to re-read all this thread to figure out how far off each of your lowers is. A modified catch for you would look a lot less extreme from what I can tell (red part shifted down, smaller amount of modification needed). 

[panel77]

3 minutes ago, Lane said:

I don't think it's that bad in this case. I'll admit; I will need to read through this entire thread again to try and get a handle on where you are with this 80%. Looks like it's not milled yet, so you might want to save task in terms of flexibility. What were your plans for that? A jig? Blueprints of some sort?

I mentioned some time back; there is a way forward with a modified magazine catch if that is really your only issue here. It's not a perfect solution; but I think it's much better than modifying every magazine you have (or want to use). From what I can see, the lower will need a minor "enhancement" as well to make it possible to install. The maximum spread I saw across the mag catch height was ~0.060" (from memory); which is a bit more than a dime in thickness.

Hell; if you want the simple solution? JB Weld to thicken the top, and some way to grind it back down flat (a metal file would be fine). Same amount of adjustment needs to be done to the bottom of the catch (not even the whole profile) if the magazine catch slot in the magazine doesn't fit. Then you would need an access port where it plugs into the upper; on the left side so you can install it. That would also require a bit of filing. I don't think you need anywhere near that full 0.060" though, so in both cases it's a very tiny adjustment.

Here is a rough CAD sketch of the maximum deviation listed. As I mentioned; I still need to re-read all this thread to figure out how far off each of your lowers is. A modified catch for you would look a lot less extreme from what I can tell (red part shifted down, smaller amount of modification needed). 

That is exactly my plan to salvage the current lowers.

I have skills and machine tools of various kinds. 

im not gonna mod the new 80, but the fail ones already failed.

1 at a time though...

at the end of the day-it might not be the problem.

too bad im not next door to '94' i know he would laugh at what ive done and also know in 2 shakes how to fix it.

hes given superior advice and ive made every change he suggested-corrected buffer, spring, gas tube, port size.. with almost no change. next is to swap working lowers to see if that can be the problem.

[Lane]

3 minutes ago, panel77 said:

I have skills and machine tools of various kinds. 

1 at a time though...

Sounds good. I mentioned JB Weld as the simple solution... I wouldn't trust that with stainless steel mags; but it should hold up to Polymers just fine. Finding someone to weld on a bit of material should also be no problem. A MIG or TIG could do that in seconds. Not sure I would torch a part that small; but perhaps there are welders out there that could do that.

That's my problem lately; this "1 at a time thing"... Sometimes it helps to have more than one for comparison sake.

[panel77]

Taking the problem thread back where it started.

trying to leave this one as a spec thread.

sorry for rabbit trail

https://forum.308ar.com/topic/17168-psa-awesomeness/page/2/?tab=comments#comment-273267

 

[panel77]

Thunder guns at 0.700

 

[98Z5V]

Never heard of that lower - that's their mag catch height? 

[panel77]

11 minutes ago, 98Z5V said:

Never heard of that lower - that's their mag catch height? 

yep-top of lower to top of mag catch-right in the range of others.-straight from teh maker.

 

got another lower-going to take teh guts out of one of the matched pairs and use it in teh new lower.

mess around with identical parts and see if the lower is the fail-if it is, then just going to take teh 2 defective lowers, machine the slot closer to the upper, then weld 2 mag catch pieces together, machine off to match the correct height and adjust teh actual catch part to teh correct height and make them proprietary catch for those very specific lowers allowing me to run any upper and any mag without thinking.

[98Z5V]

Instead of the trouble of welding mag catches together to take up space of a re-machined slot...,   Machine the slot upwards, to what you need. Fill the bottom with HTS-2000 alu brazing rod, re-machine the bottom until the slot is 0.250" from the top.  That would be alot easier, and it's also a permanent fix for that lower.

[panel77]

1 minute ago, 98Z5V said:

Instead of the trouble of welding mag catches together to take up space of a re-machined slot...,   Machine the slot upwards, to what you need. Fill the bottom with HTS-2000 alu brazing rod, re-machine the bottom until the slot is 0.250" from the top.  That would be alot easier, and it's also a permanent fix for that lower.

What about the new centerline for the mag button?

wont that cant the screw downwards and create drag/resistance?

[98Z5V]

Take a look at that thing close - that should be a pretty narrow section that the mag catch passes through, and the hole that it passes through is not a tight fit.  That's something to look at, but it's not going to have a giant impact in the mag catch function.  How many -thou do you think you'll have to machine, when moving that mag catch placement upwards, on these lowers that are giving you issues?

[panel77]

20.

not measurably a lot but im grasping at straws...

still havnt made the time (and ammo money) to take a banquet table out, stip everything to nuts,

measure it all

and break out the yellow pad and start trying to make a fail.

 

the stinker is that i cant have a fail even now cause it will make me think that when it counts(charging hog) i will have a gash instead of a bang.

unwieldy hammer... and i have yet to see a hatchet as an MLOK attachment.

 

the PSA 223/80% has never, ever failed. steelcase, brass, really poor handling skills...

[Lane]

11 minutes ago, panel77 said:

i will have a gash instead of a bang.

unwieldy hammer... and i have yet to see a hatchet as an MLOK attachment.

An MLOK hatchet seems like the easy way out given these constraints. 

29 minutes ago, 98Z5V said:

HTS-2000 alu brazing rod

Never tried this one; got SUPER angry at some "welding" rod that was actually for braising though. Never enjoyed messing around with impure alloy blends that way; but they sure are easier to mill off later. 

It's not much of a weld on the mag catch fix. One simply needs to add a bead to the top, and then file/mill back to the new spec.

Personally; I'm still most interested in what everyone has going on in the uppers. I have a feeling there are more variations than anyone cares to think about (or admit). Is this actually caused by not being ABLE to copy the Armalite AR-10 designs legally?

[panel77]

so with the Broken arms lower and the BCA complete, untouched upper as unboxed, its had 100+30+30+30 with all PMAGs but a pile of them-all different.

zero issues. None. (1 round issue but that was a round issue not a gun issue-steel cheapo ammo)

all ammo was steel cheap tula/wolf

 

100% function. I didnt even change the gas tube to mate into the middle of the cutout as instructed by 98

 

All the lower parts came directly from the other non-working AMT lower-buffer, spring, tube...., LPK

 

so to conclude, either the BCA has a lower center allowing it to 'grab' rounds easier OR the Broken Arms lower puts its mag catch higher by 0.02x to put the rnd in the right place.

 

I kinda doubt the BCA upper is the change... and its because i used a different upper with the PSA upper assembly installed on a AMT upper with the Broken Arms lower and it functioned fine, sorta... with my "buddy" drilling out the gas port to 96, now it suffers from other issues relating to massive over-gassing. 

It never suffered from failing to grab a rnd but it ejected 1 rnd so hard it flipped and tried to load the empty shell in line but backwards....(no pic)

So the centerline of the AMT upper with PSA parts worked fine and will work perfect once i get an adjustable gas block on there and crank it back down to where it should have been in the 76-80 port hole size range.

This leads to a conclusion that the mag catch sitting too low in the lower makes the rifle not function.

I already took one of the lowers and threw it on the mill and slotted the catch out to allow it to sit higher. I havnt dragged my welder home from other buddys house to add onto a mag catch and install yet but i think it will be the solve for these 2 AMT lowers that have the mag catch slot in the wrong spot-too low.

 

AMT when contacted when i pressed(multiple phone calls) them to change the spec, they finally revealed they do not make the lowers(and prob not uppers) and would pass along my info to maker. They did give me credit and were above and beyond. I still would not buy any lower from any company without making sure that mag catch is in the 0.65-0.67 range from the top to the top of the mag catch slot.

I would have really preferred 0.65 from Broken Arms and others were in that closer range but his customer service was such I had to buy from him. The fit was average to the AMT upper. The AMT upper and lower fit together VERY well, flat and no gap and very very thin parting line-but the mag catch slot location is a fail.

I would buy from a company that has 0.65 AND has their own matching upper. Buy as a package deal and dont be cheap.(it always costs more to get right)