Gunsmithing AR 15 upper build problem

[Oakland Tactical]

I bought a couple of the inexpensive upper receivers with the M4 ramps from George at GAP that they bought from a closing business. I have a M4 barrel extension on the new barrel. My problem is that now the mag does not go into the lower receiver and lock in when the upper is on the rifle. This happens with the bolt held back and when it is closed. When I open the rifle up by removing the back pin the mag goes in and locks in but when I go to close it the feed lips of the mag bind on the upper, well before it can close. The bolt seems to lock in right and moves back and forth as would be expected for a new upper.

Any ideas of what I could have done wrong? Thanks in advance.

 

[pwc001]

Re: AR 15 upper build problem

Did you try more than one mag?

 

[toolmaker]

Re: AR 15 upper build problem

inexpensive upper receivers bought from a closing buisness. They are probably out of spec. My guess is that the pivot pin ''lugs'' were machined off center in relation to the horizontal center line of the receiver. This may cause the wall of the upper receiver to protrude into the mag well causing the mag to bind up.

 

[Outsydlooknin75]

Re: AR 15 upper build problem

Are you using a known GOOD lower reciever ... sounds to me that hte mag catch is either too high on the lower or the mags are out of spec and the cut it too high.

 

[Hellbender]

Re: AR 15 upper build problem

Do you have another upper you could stick on the lower, and rule out lower problems?

 

[Oakland Tactical]

Re: AR 15 upper build problem

I have tried more than one mag, good quality GI and Magpuls. I also had another flat top upper on this before changed it to put a barrel on with m4 feed ramps. I think I have a decent lower it is a R guns forged.

I guess the upper it is out of spec, I put the old upper on with the new barrel and it works fine.

Can I hit the old upper with a dremmel to make the little cuts without causing any problems? I have heard this is something that some smiths do.

Thanks all for the replys.

 

[Oakland Tactical]

Re: AR 15 upper build problem

toolmaker64 you may be right the back of the upper was sloppy in the lower, the back lug slide side to side about 1/16" each way in the lower but was tight up and down.

 

[toolmaker]

Re: AR 15 upper build problem

I have seen AR uppers that were so far out of spec that the Bolt carrier would not even enter the buffer tube.