Hey. I just put together a new AR pistol build (my first AR pistol). It's a pistol kit on an Anderson lower.

http://www.karrisguns.com/catalog/product/66bd55c433164084987995eb68ec8104

I took it to the range today, and I had multiple issues with it. The first issue I had was that the bolt would not hold open after the last round was fired, but when I have an empty mag inserted and cycle the bolt by hand, it will stay back. The empty mag also appears to be engaging the bolt catch fully.

The second issue I had was multiple failure to feeds. I was using a mix of american eagle and federal 55 gr 223. I had several instances where, when I cycled the bolt by hand to chamber the first round (because the bolt did not hold open as stated above), the first round would not be stripped from the magazine. After re-cycling the bolt and re-inserting the magazine several times, the first round would feed.

In addition to this, I had multiple rounds fail to feed throughout different pints through a magazine. The bolt would appear to be mostly forward, but it must have been slightly out of battery as pulling the trigger just produced a click from the hammer dropping, and I had to apply a lot of force on the charging handle to get the bolt to retract at which point a live round would eject that had apparently not been fed correctly. There were also some instances where the bolt would jam about halfway back and a round would be stuck smashed partly up with the nose against the feed ramps. I did not have any extraction failures at all.

I only had one magazine with me, a gen 2 pmag that I've used in my Colt 6920 and another AR build without issue. I did notice that the magazine was a tight fit and would not drop free. Once I got home, I tried a few US GI mags I had laying around, and while they sometimes drop free, it's not consistent and they will sometimes just sit in the mag well after the mag release is pressed. I did try backing the mag release out a few turns and that did not seem to help any. The pmags require actually pulling down on them after pressing the mag release to get them loose.

Prior to going to the range, I had applied what I thought was a good amount of oil to the BCG and charging handle.

Unfortunately, I did not think to save any of the spent cases or live rounds that jammed, nor did I think to pay attention to how the rounds were ejecting. Sometimes I could get through 3 rounds at a time if I loaded just three into the magazine, but any more than that and I always had a FTF, and had quite a few FTFs with just three rounds.

At this point, I am just trying to figure out if all the issues I have seen (bolt hold open, FTF, mag well tightness) are related, and if the issues are related to the upper, lower, magazine or all three. I used only the parts provided in the LPK from the kit in the above link. I can provide any sort of pictures if needed. I'm just trying to get a troubleshooting strategy together for next weekend when I go back to the range. I know I will be bringing additional mags for sure. This is only my second AR build, and my first pistol build. My other build was a 16" delton on a PSA lower, and I never had any issues with that build.

Thanks for any help.

It sounds like a magazine or ammo issue.  Since it is brand new,  I would suggest taking your upper off and inserting your tightest fitting magazine rseveral times and hitting the magazine release. That will tell you if the problem is with the lower or not.  If it goes in easily and drops free then you have a problem could be with the upper.  Since you had it somewhat working it just may need breaking it.

Magazines that are a tight fit can cause feeding issues similar to what you have described. Another test is to place your rifle on safe, insert a loaded magazine and keeping your finger well away from the trigger point the rifle in a safe direction and work the charging handle to load and empty the magazine.  With new builds, I generally lube the shit out of the upper and BCG in order to help break the rifle in.  I will run some two or three loaded magazines through the rifle to identify any potential feeding or functioning issues before test firing it.

Finally, I have found with most do-it-yourself builds it may take about a 100 rounds or so to break it in properly.  You can also try different ammo.

Thanks for the info. I did try inserting the magazine with just the lower, and I had the same result with a tight fit. I also noticed when was at the range and hand-cycling the bolt, that rounds were failing to strip and chamber from the magazine. I put probably about 70 rounds or so through it today without seeing much of a change, but maybe it still needs a bit more. The issue of the bolt not holding open in addition to everything else does concern me though. It never held open once except when I cycled it manually with an empty mag. Is that something that would suddenly start working after it "breaks in"?

If you have another upper available, put it on the pistol lower and try the hand feeding exercise again (Pull the charging handle back and let it go forward freely, don't ride the handle forward), and see if you still have a feeding issue.  

Does the bolt lock back on a empty magazine when hand cycling?  

Did you install the bolt catch yourself? Sometimes the bolt catch is binding in the lower and can cause issues with the magazine fitting into the lower, and the proper functioning of the bolt.

Finally, I would lube the hell out of the upper and work the BCG back and forth to get the oil everywhere.  AR pistols are known to be difficult to get functioning properly at first and generally require some tweaking, especially home builds.


With regard to the bolt not locking back after firing rounds, that could be gas issue or a buffer spring/buffer issue.  The best way to test this is by loading a magazine with one or two round and fire it and see if it locks back. Take several different magazines to the range with you and try each one the same way.  Another consideration is to change out the ammo and try two or three different types of ammo.  AR pistols are more ammo sensitive than rifles.

Thanks for the info.

I did install the bolt catch and build the lower myself. I do have another upper to try, and had not thought about putting that onto the pistol lower. However, the upper I have is a 16" mid length; will putting it on a lower with the pistol buffer tube or buffer cause problems (not sure if the buffer used in a pistol vs carbine is the same or not)? The bolt does lock back on an empty magazine when I hand cycle it. I did try firing only one round at a time when I was at the range, and the bolt was not locking back.

When I get to the range next, I will take a few magazines, that 16" upper if it will work, and I will save the spent brass so that I can take pictures of it. Thanks again for all of the help. Luckily the upper does have a full warranty on it, but I want to rule out something dumb like the lower/mag/break in before I send it back.

Carbine and pistol buffers and springs should be the same. I would take the 16" upper to the range and test it on the pistol lower.  What you are doing is trying to determine the source of the malfunction.  You could try the hand cycling of the pistol lower with the 16" to see if the feed problem is still there. If it isn't then there may be a problem with the upper you purchased working properly with the lower. It could also be a gas issue as discussed above.