7.5 ar pistol (new) bolt wont lock back last round

[Chunky_Lover]

Ive been sick for a while so havent had a chance to shoot my freshly built ar pistol
basically this same kit http://www.sportsmansguide.com/produ...race?a=1763881

Now I had limited time was able to shoot approx 150 rounds, no problems at all
but last round would not lock mag open

3 mags were brand new pmags, 1 mag maybe 5 year old pmag
Shooting wolf ammo I think 65 or54 grain I will have to go check, but I was happy it shot wolf no problem
I did pick up some of the shells I will post images later


Now I forgot about doing the 1 round first to see if mag locks the bolt
so on the very last shot of the day I stopped and took out the mag that had 1 bullet left inside the mag
inserted and shot and that one locked the bolt back



but then we had to leave and could not try that again, so not sure if that was just a coincidence or maybe everything was breaking in????


I was actually so much more concerned about FTF or double feeds, broken shells I really tried reading up on those problems more and never looked up bolt not locking back


I did figure since it ran all those mags just fine with no issues that was good

quick search shows maybe try the next buffer weight, or maybe do some mod to the mag lock lever but racking back on an empty mag it will lock open
I believe mine is the carbine buffer but looks like its made in china compared to my bushmaster buffer I have
I saw a video of someone weighing all of them
mine was off a touch from the video but maybe its the scale


anyone have any suggestions
since I bought the upper already assembled I have no info about that other then whats stamped on the barrel nato 5.56 and 1:7 I think but its from anderson arms so I could email to get more info
I wanted to buy a complete upper in the hopes it would be ported correctly and all that
which at least in my case it did shoot all the ammo with no problems

the lower was milled by me, and uses a mil spec lower parts kit and the hammer has the hook on it
another lower parts kit I have the hammer does not have the hook

I was planning on if I had bigger problems changing out a buffer from my carbine rifle, possibly the entire bolt carrier, maybe the spring and maybe the hammer but we ran into problems and had to leave early


I will get photos later of everything

Thanks for any info you can provide

[Iraqgunz]

I would start with a heavier buffer for sure. And there is no correct port size for a 7.5 as there is no standard for them.

[Chunky_Lover]

I would start with a heavier buffer for sure. And there is no correct port size for a 7.5 as there is no standard for them.

I found this sitehttp://thearguys.com/ar-facts/difference-ar-15-buffers/
I will again verify my weights of the 2 buffers I have available to weigh


so once I find out the exact weight, im probably just going for the next size up?

I will also take out the spring, measure and count the coils too
since im not really sure what that is at least I can do some searching im guessing its all carbine length parts


I also found some posts on one brand mag catch not being made correctly so I will check out the photos compare to mine just to see if thats the case too
As I have no idea what brand any of the lower parts kit I have they came in unmarked bags

[Chunky_Lover]

incase it helps any
my pistol buffer spring is 37 coils long from top to bottom
and buffer weighs 2.9 oz and I just noticed I can unscrew it
has few dings on the rubber end, like its bottoming out a touch?


the parts from my rifle is same 37 coil spring but longer
and the buffer is 3oz and looks to be much better quality made if it unscrews its too tight for me to do it by hand no marks on the rubber buffer tip, and it has probably 4500 rounds through it
and has a pin through the end and doesnt rattle as much

the pistol buffer even just moving the pistol some you can hear it hit and move back and forth inside
pretty loud as ive never had an ar that loud before
maybe because its so cheaply made?



the springs even though same coils the pistol spring is physically shorter




and just because I was wondering maybe the bolt carrier wasnt going as far back because of the pistol tube and the marks I saw on the buffer end
I took pics holding the charging handle all the way back, and how it sits locked

May be hard to see but compared to my rifle, which goes really far back when charging by hand
the pistol doesnt go as far back, but the rifle doesnt have a forward assist or dust flap and different bolt carrier so maybe the opening on the upper is cut out slightly different making it look like it goes further back?


pistol locked open


pistol pulled back by hand



rifle locked open




rifle pulled back by hand




I have 2 pistol tubes one is longer on the outside but a non precise measurement showed they both seemed to be the same length internally
so I dont think my pistol tube is any shorter but I have not checked it towards my rifle tube yet





so how will a heavier buffer work in making the last round lock the mag open
because it can travel a touch slower it can give more time for the mag follower to work its way up to lock the bolt carrier?

[Iraqgunz]

You have garbage components. Buffers are not designed to unscrew. Trash the parts and get a good spring. You need a heavier buffer as a start.

incase it helps any
my pistol buffer spring is 37 coils long from top to bottom
and buffer weighs 2.9 oz and I just noticed I can unscrew it
has few dings on the rubber end, like its bottoming out a touch?


the parts from my rifle is same 37 coil spring but longer
and the buffer is 3oz and looks to be much better quality made if it unscrews its too tight for me to do it by hand no marks on the rubber buffer tip, and it has probably 4500 rounds through it
and has a pin through the end and doesnt rattle as much

the pistol buffer even just moving the pistol some you can hear it hit and move back and forth inside
pretty loud as ive never had an ar that loud before
maybe because its so cheaply made?



the springs even though same coils the pistol spring is physically shorter




and just because I was wondering maybe the bolt carrier wasnt going as far back because of the pistol tube and the marks I saw on the buffer end
I took pics holding the charging handle all the way back, and how it sits locked

May be hard to see but compared to my rifle, which goes really far back when charging by hand
the pistol doesnt go as far back, but the rifle doesnt have a forward assist or dust flap and different bolt carrier so maybe the opening on the upper is cut out slightly different making it look like it goes further back?


pistol locked open


pistol pulled back by hand



rifle locked open




rifle pulled back by hand




I have 2 pistol tubes one is longer on the outside but a non precise measurement showed they both seemed to be the same length internally
so I dont think my pistol tube is any shorter but I have not checked it towards my rifle tube yet





so how will a heavier buffer work in making the last round lock the mag open
because it can travel a touch slower it can give more time for the mag follower to work its way up to lock the bolt carrier?

[Clint]

If a buffer will unscrew...it's screwed.

They're not supposed to do that.

The bolt should retract about 1/4" beyond the bolt catch when held back.

The crazy buffer may be at fault.

Next step is to replace the buffer with something mil-spec.

I'd like to suggest an H2 buffer, as many pistols work best with that weight, but an H buffer is probably a safer bet.

Do the lock back tests with the new buffer and try some full power ammo as well.

[Chunky_Lover]

yeah I thought from the second I saw that buffer it looked really cheap

I ordered a h so will see how that works

If I do change more parts I want to do one change at a time
so I can see what different parts do


I also checked the mag follower touching the bolt catch
seems when the mags are inserted the catch goes up as far as it can
meaning the followers press it up fully

Now is the catch tall enough, I will have to check my rifle for comparison
I notice the pmags dont have much contact where my old abused metal mag really has a large foot for the catch to touch

[Chunky_Lover]

unfortunately I broke my rules and did 2 mods at the same time today


I got in the h buffer and installed the thordson enhanced buffer tube cover, which required a carbine length buffer tube to fit properly
Now I was all excited to show when I pull the charging handle all the way back my bolt carrier is even more further back inside the buffer tube then before
But I just realized maybe that carbine tube I put on could be different in length so maybe thats why my bolt carrier now goes in farther
But if that was my issue before, not going all the way in then hopefully that may solve the problem and if not I still have both buffers to mess with, plus the buffer from my rifle of better quality

I will see very soon when I go back to shoot it again


here are pics

how far it goes back now



and how it was with the crappy buffer and pistol tube

[sinlessorrow]

Make sure the carrier key is not striking the lower and get a H2 buffer at a minimum. I would say a H3 honestly.

[Chunky_Lover]

Make sure the carrier key is not striking the lower and get a H2 buffer at a minimum. I would say a H3 honestly.

do you mean not striking the upper?

so the photo I had when it was pulled all the way back, the first photo from my last post
if I take the upper apart, pull out bolt carrier by hand it will go back maybe 1/4 more then my photo shows until it hits the charging handle
the charging handle wont allow it to travel further, and I would assume the spring or buffer would also stop it before reaching that point

now maybe there is a chance when firing it moves futher back then I can pull it
so I will be on the lookout for any wear, since this is still brand new there are no wear marks yet

If I didnt post in the very first couple of posts the crappy buffer it came with already had signs of the rubber end damaged, making me think it wasnt allowed to travel back further maybe because the pistol tube wanst as long as the rifle tube I have on right now
But I wont be able to know until tomorrow when I go test it out


if you did mean hitting the lower can you explain that as I dont understand how it could do that