panel77

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.

i will add a pic of the upper. the AMT upper fits the AMT lower nicely. i dont have any other lower and the PSA upper is for PSA lower ONLY and is useful for target practice only. Broken Arms lower coming sometime all slots are at 0.2500

lower

I had one jam but it was an obvious error and related to flawed testing and a repeatable error. Extreme... LOL redneckness to test theory-failed. used clamp with modified trash/test mag. should tried clamp with reg mag but i just need 2-3 hours(only had about 15 minutes) and more ammo...

so 100 rounds later and no definitive answer. when i jammed mag up in and pushed HARD on it while operating, it appeared to have zero issues with AMT lower and Bear creek (BCA) 18" OEM upper. then i did a few mags right at teh end(limited timeframe) and without holding them in, it worked fine.... but now im skittish, i dont want to lug a hammer around the woods, a real hammer is lighter... gonna mill the new 80 and see what happens. in all cases- that slot is in the wrong place compared to everyone who has responded both here and via manufactures contacts. wrong by at least 0.020 and most often wrong by an average of 0.030 im throwing up my hands for now-its a 45 minute drive at least to reach a place where i can test, no back yard range...

I will find out tomorrow when I hit the hills. got 3 boxes of gear and a yellow pad for notes. if the redneck vise grips jamming the modified mag up into the POS lowers makes it run, im getting new lowers. already ordered one from a vendor with superior customer service who provided a dimension.

Yep-you helped me make significant corrections to the upper setup and I got the correct matching upper and swapped all the good parts onto it with the same issue but slightly improved. right length buffer spring, right length buffer tube, 5.4 buffer, right length gas tube, gas port size drilled out to 78 then 96... mags are all 'brand new' pmags 10rounders. with less than 30 through each mag. also went to local gunsmith who knows more than me and he drilled one of the POS PA-10 gas ports to 96 and it didnt help.

so to respond to 98Z5V who was super helpful in prior post, trying not to get too off-topic... yes I started with AMT lower and PSA upper assembly only to experience this problem almost all the time. With his help and insight, I grabbed a matching AMT upper and used teh PSA barrel and handgaurd and parts with a new proper length gas tube and drilled out to i forget maybe 78 or so from 65? and put it back together, things worked better by a 10x factor but still had same problem but not as often. then i got a little more measury with the calipers and able to look at 2 different lowers... long story short is i havnt been too scientific and thought the mag catch was too low and not allowing things to sit in proper place so i started the original post as a poll. both the AMT and the non name but identical matching lower measured at 0.695 and the one i hadnt used in forever but worked 100%(in my memory) was at 0.665? so I wanted data after digging in google and using a calculator-i asked you professionals for real world data and the consencus seems to be the 0.695 is wrong and a doorstop. tomorrow will be a science day. oh and based on 98xx i got a bear creek complete upper cause i was tired of the drama and it fits very very nicely with the existing lowers and it had the same problem. which led me to think it was a lower problem(not saying 98xx wasnt right about PSA p.o.s. uppers...)

sorry-my wife says i am terrible in writing and come across wrong. prob has to do with my dis-like of writing and expounding on thoughts in general... sorry-you have in my previous PSA post been above extremely helpful and of a superior and scientific mind-I dont know if I ever got around to thanking you properly and publicly. Thank you. oh-and it looks like I didnt look up your profile properly-I wrote 94xx and what I meant was you-98Z5V-you got me pointed the right direction and now that ive found a more accessible spot to test fire stuff, im going to be a little more scientific this trip. I can guarantee the PSA uppers had problems and you helped point me the right direction but what I am expecting to find is compounding errors all inside a non-standard platform.

thanks-not interested in modding xxx mags when i can do one lower mod and if it doesnt work-its a throwaway anyway. or an extreme backup with bright neon sign that says only use with visegrips... LOL pmags.... piles of...

no measurable gaps, havnt thrown a feeler gage in there but its not a gaps issue. based on data received so far its a design flaw in the lower, out of spec(as if there is an official DPMS spec) by 0.020 compared the average of other specs. Im gonna mess around with creating a modified lower-specific mag catch just so the lowers arent lawn ornaments... nothing to lose at this point as using visegrips to jam a mag up the hole isnt a realistic or useful or viable option even though its the most appropriate redneck solution. hey-i resemble that remark!

the 80% utilized that returned a 0.695 are recent and from AMT. also ordered one from Blitzkrieg and will be returning-it looks IDENTICAL to the AMT and measures identical. I doubt this is gen 1 DPMS and its not Armalite. Where my options are to shoot, bench rest is not an option also I do not want a bench rest only rifle, unless a rifle works each and every time, its not a rifle, its a hammer. You guys are great, keep the data flowing

-American Made Tactical measured at 0.695=fail.

got replies from vendors I emailed results so far 5DTactical thinks they are too awesome to send me a spec. direct quote, "Customer Care Agent (5D Tactical) Apr 1, 15:59 PDT Great question! I see you haven't placed an order with us! I can't just provide specs of our lowers of course, but I can tell you that they follow DPMS Generation 1 pattern! I would recommend ordering one, and then you would be able to measure everything you like! Customer Care Agent" -nope-never doing business with them. ---------------------------- -Noreen firearms asked for clarification-always been happy with their CS-update-they supplied a spec of 0.659 -Hellfire armory responded almost immediately with a picture AND a dimension of 0.685 -Broken Arms in San Jose responded with critical thinking and problem solving along with a phone number and request to call. He was great to talk to, helped with information, clarity and provided a spec of 0.6665 along with great customer service. They immediately got my business. -0.650 from a forum member-brand unknown, possibly Diamondback -Xantos @ 0.669 and PSA 100% @ 0.700 from forum member -Fulton @ 0.671 from forum member -Colfax doesnt make 80% anymore but forum member measured at 0.663 -Ceratac @ 0.650 and Belt Fed @ 0.650 from forum member

So the mag thing can be an issue I am learning BUT... what I am leaning on is the theory that the 0.695 pushes the mag too far down to where a sub-par mag or a jostle is enough. all mags are PMAG with less than 50 through each mag. my next more scientific visit to the range will include a c-clamp to jam that mag as deep as possible and if it 'fixes' it, these lowers are going to be scrap while I replace them with ones more in spec.

looks like .5000 but then you state.65. in all cases-better than what i measured. and I assume you have zero feeding issues. care to state buffer weight, buffer length, tube length, and spring length? gas system length?

and I assume you have zero feeding issues on the Xantos. care to state buffer weight, buffer length, tube length, and spring length? gas system length? have you ever had mag feed issues on your PSA? is it PSA upper AND lower?

and I assume you have zero feeding issues. care to state buffer weight, buffer length, tube length, and spring length? gas system length?

Hellfire got back to me and stated they have a .685 distance Colfax doesnt make 80% anymore but they stated theirs are .663 +/- 0.005 I have a theory that appears to be playing out to be at least a component of a problem.

Sorry for not being more clear. YES, I am having PITA with a 308 AR. tried multiple complete uppers from 2 different vendors(PSA fail-fixed with help of 94x? member) but after swapping lowers with good uppers, mix/match, swap with about xx different combinations-the lowers appear to POSSIBLY be the problem and POSSIBLY not be allowing the mag to sit high enough into upper to get stripped(insert joke here) of the next round properly. from all the online spec drawings I can find and attempting to do math(ouch) it looks like the spec(yeah i know there isnt one) is supposed to be .685-MAYBE... but thats the conclusion from drawings. 2 lowers that mo matter what upper is on get the bolt riding over the round too intermittently to be useful for more than a hammer(if it doesnt go bang E-V-E-R-Y time, its a hammer)lead me to see how deep the mag sits hence the poll. mag catch location dictates mage insertion depth which dictates how effectively the bolt will strip a new round. 'I think" and since I can guarantee I know less about this sort of thing, I am working to get all your brainpower on it. Thank you to all who supplied info, I am hoping to accumulate a statistical sample size to determine the folly or validity of my theory.

Sorry I thought this was the AR 308 Forum. on a 308/large pattern AR lower. Long story short, I am trying to eliminate variables in a feeding issue

Please measure with accurate tool from top of lower to upper side of mag catch slot.

Shot it after doing all that stuff = fail threw away all that work and time and parts money. bought 18" bear creek complete upper w/BCG... came quick and nice and lubed all to heck-great. got 2 rounds through it before bolt rode over round and jammed. took it to gunsmith, he got excite and took the A2 upper I had with all teh PSA fixes on it and reamed that gas hole out to #40=.096 nope first round bang, second round stack. so its not teh upper, not the upper parts, not anything to do with upper... what is it? BTW, have 2 complete lower setups. one from AMT and other unknown in raw from gunsmith but ALL dimension match AMT.... HOWEVER, I measured a poly 80-10 and there is ONE difference between the lowers. the poly 80 mag catch is 0.661 from upper edge of mag catch slot to top of lower, the other 2 lowers are 0.695 also ordered one from blitzkrieg and they get them from same place as AMT and if i can ever get them to reply to my email or phone calls-returning it, dont need 4 of the same useless brick. so next will be to take all the uppers out and borrow the poly 10 and see what happens. if anyone has a 10-can you measure from top of lower to top of mag catch slot? thanks

Shot it yesterday. Got 3-4 mags(10rnd) through it before near teh end of one mag, the bolt rode over the bullet as bullet was feeding. wasnt the mag. tried single slow shots and rapid fire-didnt make difference. MIGHT chalk it up to ammo(cheap bulk) but... it did better after; proper length gas tube port to .0785 long spring 5.4x buffer 2 quarter stack ran out of full mags and was 50 miles from the ammo can... ideas before i run another $40 through it?

of course youre right-by the time i read my post, i couldnt edit. this one is rifle length but doesnt note port size. which would you prefer-18 or 20"? https://www.bearcreekarsenal.com/bca-ar-10-complete-upper-assembly-20-4150-parkerized-socom-barrel-308-1-10-twist-rifle-length-gas-system-w-15-m-lok-rail?utm_source=Bear+Creek+Arsenal+Newsletter&utm_campaign=172c3b3265-12%2F29%2F2017-+NEW+Rose+Gold+Glock+Barrels_COPY_01&utm_medium=email&utm_term=0_f428505653-172c3b3265-132848813&mc_cid=172c3b3265&mc_eid=3c39da930a