Hello everybody, and thank you in advance for any advice. I recently finished a build which consisted of a S&W MP-15 lower and armalite m15 upper. I bought three Magpul pmags for it and three c products aluminum mags. When you pull the charging handle and BCG back and release it on an empty c products mag it correctly locks the bolt back. However, an empty pmag will not lock the bolt back. I had heard prior to my purchases that the older S&W’s would not accept pmags. Mine does however accept pmags and they drop free pretty easily. Why does the bolt not lock back on the pmags? FYI it looks like the c products mags sit higher in the mag well then the pmags do. I have not shot it yet either.
Any other details about your build? is this a S&W factory assembled lower or a stripped lower? What brand LPK are you using?
Look into the ejection port while pulling the charging handle and compare where both mags (empty) sit in relation to the bolt catch. That way, you’ll also see if your PMAGs cant reach it or simply cant push it up.
Had a similar problem with my DPMS (i know… :eek: ) LPK, except I couldnt lock back on all my mags. I found that the spring was too stiff.
It is just a S&W lower reciever with a DPMS LPK. When you pull the charging handle back and look into the reciever the c products definitely sit higher. This is what I don’t understand because S&W told me pmags should work in it and they do go in and drop free easily. By spring to stiff do you mean the buffer spring?
That Cprod junk is more likely to be a problem than the Pmags. Look to see if the Bolt catch is touching the back of the follower on the pmags. There have been some out of spec bolt catches in the past.
It looks like the bolt catch is possibly touching the follower on the pmag. Also if you lock the bolt in the back position, by pulling the charging handle and holding the bolt release, and set it down not so easily or tap on buttstock the bolt releases. Does that mean I have a piece of junk bolt catch? If so where can I pick up a good bolt catch?
That’s normal. Does the bolt catch move somewhat freely if you work it manually?
I’ve had bolt catches in the past that I’ve needed to trim the plunger spring on and smooth out the surfaces of the bolt catch itself to get it to move right.
It moves pretty freely when you play with it.
By spring too stiff, I mean the plunger spring. I had to replace mine with an older, softer one. It solved my issue but I also had to sand the plunger down a bit bec it was bottoming out.
Does it fully lock back with your C prod mags?? IF you manually engage your bolt catch, does it go up all the way?
I agree with MarkM, you may have to hone the bolt catch a bit and/or the plunger as I did with mine to
It locks back every time on the c products mags and it never locks on the pmags. It you hold in the bottom of the bolt catch after pulling the charging handle and release it it stays locked back. Then when you hit the bolt release it releases the bcg. It works the way it is supposed to with everything except pmags.
It’s hard to say without getting the gun in front of me. Having another known good working rifle to compare next to it is helpful too. Do you have access to another trouble free rifle?
If it is the bolt release holding the follower on the pmag down would I need to grind a little on the front edge of the bolt release to allow the follower break free and go all the way up?
I will try to get my pictures on here, but the spur coming off of the front of the bolt release is definitely holding the follower down.
The follower should be PUSHING it up. But I’m quite reluctant to blame the mags. DEFINITELY do NOT grind the spur down.
I am in the process of loading a video on youtube of both mags being loaded. Will post the link when it is done.
Don’t know if you can tell anything from these pictures. The mag follower does push up on the spur. Don’t know why it is taking so long to upload my video on youtube.
Why is the bolt in the half way postion in the first 2 pics? It looks like the bolt catch is grabbing the carrier. Did you do that on purpose?
I did that by holding the bottom of bolt release in while releasing the charging handle because the bolt won’t lock on the pmag. I didn’t do anything else.
But will the gun lock all the way back if you do it manually? I’m not sure if the bolt catch should even be able to “grab” the carrier.
This may be pretty noobish but is the bolt catch supposed to lock on the bolt or on front of the carrier because mine definitely locks on the carrier.
It’s supposed to be at the front of the bolt, not on the carrier like your pictures show.
How would I go about fixing this?