AR-15 Bolt Catch Problem Resolved

I assembled my first AR-15 lower several months ago. It has cycled and fired probably less than 500 rounds with no misfires or failure to feed issues. However, the bolt catch does not hold the bolt open very well without an empty magazine inserted. In other words, it holds perfect with an empty magazine inserted. I can bump the firearm from any angle and the bolt catch holds the bolt back out of battery with empty magazine inserted…

It takes very little jarring of the firearm to release the bolt into battery once the magazine is removed. Since I bought an “oops” kit, I have 2 trigger disconnect springs, 3 bolt catch springs, and 2 bolt catch plungers. Either the lower kit or the oops kit must have thrown in an extra bolt catch spring. I have compared the 3 springs and the 2 plungers and see no visible discrepancies. I have blown the hole out (in the receiver) to be sure there is no debris. I have tried multiple combinations of the springs and plungers and the results are always the same; I even tried the spare trigger disconnect spring for comparison, and still same results.

Based on this, I am considering cutting one coil out of one of the bolt catch springs, test, and cut another coil and test until it “seems” right.

Any other ideas or has anyone else had this problem?

I have never encountered this, but I’ll offer a guess…

Since the above would seem to imply it is not the bolt catch/spring. Is the buffer spring maybe too soft (weak? loose? not sure the right term here). The buffer spring pushes the BCG against the bolt catch, so if its not pushing it hard enough maybe thats why it slips out?

Do you have a spare BCG to test with?

No spare BCG around here; this is my first and only AR-15. I bought it as a kit from , it is this one:

Edit: the receiver is an Aero Precision M4E1

From Aero Precision website:

“Upper Tension Screw - Allows users to fine tune the fit of the upper and lower receiver using a nylon tipped tensioning set screw inserted in the grip tang of the lower receiver. This provides a tight fit with any standard AR15 upper receiver.”

Could it be that I have the tension screw too tight forcing the bottom of the BCG up too high? At current adjustment the rear take down pin moves in and out freely and there is no up and down wiggle slop between the upper and lower.

Another update, I tried it with no spring nor plunger, looks like @Harvey is right, the problem is with the buffer assembly.

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Yep, Harvey is correct. One of the guys I shoot with every week had the problem you described on his AR build. Long story short, he installed a heavier Colt buffer spring, 4.6 ounces I believe, and it fixed his bolt hold open problem. I think they also have a 5.4 ounce as an option, but that may be over kill.


WOOOOOO! I was right!

I mean, um, hopefully you can get that fixed up @Gary :wink:

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Thanks @Harvey, finally got some time to research a little more. Ordered some tungsten weighs so I can swap out and make existing buffer into an H1, H2, or H3.


Please keep us posted, I’d like to know how it turns out.

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Will do, shipping is a couple weeks out, just ordered this morning.

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Tracking shows the tungsten weights should arrive today - way ahead of original schedule :+1:

So, I took the current buffer apart to get ready to convert it to an H2 for first trial. What I found inside was not as expected:

I was expecting 3 steel weights with rubber spacers. The single weight is 2 ounces, and the whole assembly is 3 ounces:

So, will have to decide if I want to put together an H3 when the new weights arrive, or cut the 2 ounce weight to make the originally planned H2. The removed spacers appear to be 0.031" Aluminum, both stacked together on the plug end of the original assy. Will have to do some measuring of the new weights and figure out what spacers to scrounge together.


Turns out the spacers are 0.010 plastic. Putting in the 3 tungsten weights plus the 2 plastic spacers still rattled, so threw in a thin SS washer to tighten it up. It ended up almost 2x original weight @ 5.6 ounces:

Hope to try cycling on Wednesday, bolt catch still releases, but requires more aggressive jarring.

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An update for @Harvey, I rearranged a lot of heavy equipment in the garage on Tuesday, so hands were too sore to bother going to the range on Wednesday. I have been doing some more poking into what others have reported worked for them. Most were related to the buffer weight, but I am intrigued with a couple of reports regarding shorter Carbine/Pistol buffer tubes.

Apparently many have encountered this when accidentally putting a rifle spring in a carbine buffer tube. A carbine spring is a couple inches shorter than a rifle spring. Not to believe everything I read on the internet, but I ran across a post that said a carbine spring should be 10-1/16" to 11-1/4". I measured mine at 11-3/8". Not far out of tolerance, but certainly at the longer end of the spec.

Further investigation indicates a carbine buffer tube is usually between 8-1/2" to 9" long. My pistol buffer tube is 7". All this adds up to I think my spring might be too long for the buffer tube. I don’t like to throw multiple variables into an experiment, so plan on staying the course and testing the H3 buffer weight this coming Wednesday. I will probably buy another carbine spring at the range if they have one in stock and test cycling of it as well. I will possibly start cutting off coils after that.

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Interesting. Did the kit you got maybe come with a mismatched buffer/spring? Certainly sounds plausible. I know I can’t tell the difference, and I just assume any kit will work “out of the box” without tuning

Interesting, I have an Outdoorsportsusa pistol tube that came with my kit and it measures 7 1/8" a carbine tube from a completely different mfg and build comes in at 7 7/8". I pulled the smooth buffer tube from my pistol and installed a regular carbine buffer (from a different vendor) for install an adjustable “brace” and that one measures 7 1/8". Since I have not had this issue I never bothered to check spring lengths. I’d have to do some serious safe diving to find my AR-15A1 and A2 but I know that there is a ~1" spacer in the AR-15A2 stock assembly to screw into between the buffer and the stock. I A$$Ume that the buffer tubes on my A1` and A2 are the same because I built them at the same time (1992 ish) but as noted I don’t know the length. Lemme see what Brownell’s says

From Brownell’s:

Rifle A1 & A2 the “Receiver Extension Tube” is 10 1/8" long
Mil Spec Carbine = 7 1/8"
Commercial Carbine = 7 3/8"
Pistol is anywhere from 7 1/8 to 7 3/8"

Rifle: 13 1/4"
Carbine: 11 3/4"
Couldn’t find a pistol specific spring length

I’ve run across this hundreds of times with range guns and always attributed it to mass abuse, never from a new kit.

Looking at how the bolt catch actually works it is DESIGNED with the spring being positioned under the fulcrum to push the bolt catch down and out of the way. That being said on a fully charged magazine the bolt catch dose not come in contact with anything on the magazine until the follower comes up. I would consider taking a look at the bolt catch and see if a little light polishing on both sides where the pin goes through would free it up to a more stable position. Pretty much if you put an AR down too hard with the bolt to the rear it will go home but too light is a safety issue.

That said cutting coils on the spring may be counter productive if you are looking for more “holding power”. I would consider closing the length by adding appropriate size fender washers to the bottom of the tube. If that works you can glue them together so they don’t rattle. A long time ago I knew a 3 gun guy who was more into the science of the gun than actually shooting the match. I want to say that he would “tune” his buffer springs with weights and washers. I would have liked to pick his brain but the one time I stopped by his place he had like 10,000 cats and I could smell the house before the door opened.

Will be interested in your experiments, please expound and document your efforts.



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Took the AR pistol back to the range today. The good news is it cycles nicely with the 5.6 oz. buffer. My perception is the recoil is reduced.

The gun range I go to is sold out of buffer springs, but I stopped by Home Depot on the way to the range and picked up a bag of 1" OD SS washers to give the @Craig6 idea a try. Unfortunately, either the ID of the buffer tube is less than the 1.000" spec, or the washers are a couple thousands over. Regardless, the washers were not going in that hole, and if I forced them, I’d probably never get them out :frowning:

Which brings me to next steps. I can unscrew the buffer tube several turns out of the lower receiver, which will effectively lengthen the buffer tube. At some point the buffer tube will be too far out to capture the buffer retainer, so will be a bit of a challenge to hold the buffer in place and close the upper into place, but I am sure it can be done. This should give an indicator of what shortening the spring will do. If it’s going the wrong way, I can grind some of the washers down to fit and try additional compressing of the spring.

Will post results …

Did this fix the bolt catch issue? unclear…

I’m not great at reading those diagrams (i took some engineering classes in HS… in the 90s :grimacing: ), but is the 1.000" pointing to the outer diameter? and the wall is 0.073"? So that would mean the inner diameter is 1.0-0.073 ?

You, sir, are a braver man than I :laughing:

I will say it helped, but it still doesn’t take much jarring to make it release without an empty magazine inserted.

You can hit the back of the buffer tube really hard with a rubber mallet and it does not release if an empty magazine is inserted. It has to be laid down very gently for the catch to hold if the empty magazine is removed.

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@Gary_H take your washers and run a bolt through them then lock them together with a nut. Chuck it in a drill and take a file to it that way you get all of them the same size at once.



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Good idea, thank you :+1: