I have a Armalite lower with a RRA national match upper. The bolt closes intermittently(30% of the time) on the last round. It will occationally even close after a pull on the charge handle. This happens on all mags(20 and 30 round USGI's, all makes). I think the bolt catch spring is too strong but don't know how to approach it. Any ideas would be appreciated.

You are saying that it fails to chamber the last round right?  Just making sure I understand you...

Or is it the bolt hold-open with an empty mag that is failing?

After the last round is fired the empty case is ejected and the bolt still closes.

The bolt latch is nearly at the end of travel.  It may be short-stroking or it may be rebounding off the end of the receiver extension too fast.  As you suggest, a bolt catch spring that is too stiff or a mag spring that is too week can also cause this.

One trick is to push on the bottom of the bolt latch on the last round and see if the added force get's it to latch open every time.  If it works that way, it's not short-stroking.

Sounds like a problem with the bolt hold open assy.  Could be simple, like dirt under the spring plunger.  Has the lower ever functioned properly? Other causes are : wrong spring installed, hole not properly drilled in lower, damaged bolt stop.

Doesn't Armalite have a lifetime warranty on their rifles?  If so, and it is not a dirt problem, let them fix it.  

It's not short stroking. This happens even when the bolt is pulled back with the charge handle. When I cycle the bolt with the charge handle then remove the mag I can push the bolt hold open assembly up quite a bit. I have removed the hold open assembly and cleaned and oiled it. Still does the same thing. I had not thought about the spring hole. Maybe it's not deep enough. I will compare to another ar.

How many rounds have you put through this gun?  I know on a couple of my ARs and a friends AR it took a couple hundred rounds for the bolt to hold open right.  You just have to wait for the surfaces to wear together a bit.  You can also try working the bolt hold open by hand a bunch and see if that helps.  I know it helped me some.

I almost always take a coil off of the bolt catch spring, had the identical problem you are speaking of with a rifle a few weeks ago.
Robert
www.rbprecision.com

The bolt catch spring needs only to be stiff enough to hold the catch down during recoil. That is the bolt catch spring's only job.

The bolt catch hole should be somewhere near to 0.47" deep. Go with a +/-0.005" tolerance, the spring has some play.

Make sure there isn't a disco spring in the bolt catch plunger hole. That's pretty common. The disco spring has a one end that is of a larger diameter.

rbprecision,
I had considered removing enough spring to make it function. Do you have any tricks to make the spring flat on the end like it is from the factory?
Thanks

roundfile,

If by "flat" you mean "ground and finished ends" the bolt catch spring does not have ground ends but the disconnector spring does. I say again, is one end of the spring larger than the other?


Pic's not the greatest but the price was right.
[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=16143[/img]

Bolt catch springs are usually silver to white and have finished ends. A finished end is when the coil turns back into itself and creates a flat surface. On a ground end the wire is actually flat on the outside edge. Disco springs are usually brown to gold. I'd equate the tension of a bolt catch spring to about that of the long one in a ball point pen. Disco springs are much stiffer and involve a mirrored ball hanging from the ceiling.

Yes, I mean finished.

OK, you mean finished but not ground. It's a simply matter to slightly bend the end of the spring back into the proper orientation. It won't look as pretty as a factory job but it'll work. Have you checked the depth of the hole? Usually, (depending on the spring length) the spring does not extend from the hole.

I checked the hole depth and it was 4.9. Plenty deep. I trimmed two coils off(because the first one was really flat and it works perfect. Thanks for the help.