I recently put a Bushmaster M4A3 upper on a new condition preban Eagle Arms Lower. I fired 80 rounds of Selier and Bellot (an older, safe lot)this weekend. Using four different magazines, one adventure line 20, one simmonds 20, one Okay 30, and an orlite 30, the bolt failed to lock back each time after the last round.  Besides this problem I had none others. I am wondering if anyone could shed some light on why this might be happening.  One thing I have noticed is that the bolt catch release feels a little sticky when I press it.  Thanks for the help guys.

When I first started reading this thread I was leaning to towards a bad mag. Since you used several and had the same results it may be the latch.
Have you used these mags in another firearm?
What results did you get?
Try cleaning the latch and check to see if it may need to be fited better (too tight).

I used only the 30 rounder previously and had no problems. I would be suprised though if all the mags are bad. The adventure line mag is almost new. Before I put the Bushmaster on I had the origional Eagle Arms upper and did not have problems with the bolt catch.  

First off, check to see it the carrier/bolt is coming all the back and allowing the bolt to travel past the bolt catch.  You can check this by pulling back on the cocking handle and looking up threw the mag well.  

Now, insert a empty mag into the rifle and pull back on the cocking handle, the bolt should lock back.  If it will not lock, reach over and pull the release button(top) a little father out and see if it lock.  If you can get the bolt to lock back manualy, then the catch is a little stiff and can be worked in by hand.  Add a litle CLP and just work the catch in and out until smooth.

Hope this helps.

Thanks for the advice. My rifle is not in Philadelphia with me right now so I will have to check again when I get back home for Thanksgiving break. Before I took the rifle out to the range I had tested it in the fashion that you describe.

The bolt will lock back if I pull back the charging handle and press the bottom of the button down manualy. When I put an empty mag in it would not reliably lock back, so I think that the catch might be a little stiff.

Where should I oil it to eliminate this problem, and does it merit a possible replacement of any parts?

Oil the catch at the sides of the slot/roll pin, and at the bottom of the lever/spring, then work the catch by hand.  It should loosen up. It also helps to use a empty mag in the mag well to add in the process.  Once worked in, the empty mag itself should push up on the catch.  

If you find that even after the catch has been lossened, the catch will still not rise with any of your empty mags, then you may a problem with the catch spring, spring detent pin, or spring channel.  If this is the case, use a punch and punch the roll pin out of the catch. The 3 pieces will be a catch lever, a small spring and a detent pin.  Check the spring channel/hole for crap that may be in the hole and causing the catch return spring to be too strong for the mag to overcome.  Most of the time, it is just a flake from the anodizing process that is causing the spring to be over pressurized, but depending on the receiver, the spring/detent may be too long or the hole drilled to shallow.  Also, any small flakes in the hole will be crushed when you break in the catch by hand, so don't worry about having to pull the catch to clean them out.

Note:The spring can be trimmed if needed, but this is a worst case scenario.

Hope this helps.

Thank you Dano for your time and help. I will give this a try when I return home tomorrow, and post the results next week when I get back. Have a great holiday.

No Worries!!!!

Myself, I will be off hunting pheasants until monday, So it sounds like both of us will be MIA until after the week end.

Before taking the rifle to the range I liberaly applied CLP as directed. The situation improved somewhat but not completely. At this point the bolt locks back about 50 percent of the time. Any other suggestions?

 Take the bolt catch out, deburr everything, catch and the slot in the casting. The catch must be completely free without the upper on.
What may have happened at one time or another is that someone might have pulled the trigger and let the hammer drop a few times without the upper on the lower. This can sometimes bend the bolt catch *slightly* and cause it to drag.


Or, if it is free, you may have to clip a (1) coil off of the spring under the catch to tune it if you're sure it's not short stroking and the magazine springs are all OK.

What stock do you have on the tail of your new upper?  It souns more like there is something in the upper that is preventing the bolt/carrier fro going fuly rearward.
Usually when this happens, my first stop is the incorrect sbutt stock screw...a longer A2 in an A1 stock.
Are you ejecting OK?

Not sure if you've tried this, but remove the butt stock and attempt to lock the bolt back with an empty magazine.
The bolt catch should catch in front of the actual bolt. Not the bolt carrier.
Sometimes the screw that holds the end cap on the buttstock on is too long.
Good luck.
-Steve

Currently I have a RRA six position collapsible stock on the lower. I have tested the catch by pulling the bolt back and manually locking it. The bolt locks back correctly, not on the carrier.  In addition, I am not having any ejection or feeding problems.

I will have to try and remove the catch when I return home. I have not done this before. Any tips?  Thanks again guys for all the help.

I had the same problem with some S&B. It was hot enough to cycle the rifle but of insufficient power to reliably lock the bolt back.

At this point you really need to take a thorough look at the entire gs system...from barrel gas hole and back.  The hole may be undersized unlikely, or off-axis to teh gas port in the front sight tower (possible)

Interesting about your experience with S&B, that is what I have been using while experienceing this problem. If it was of insufficient power to lock the bolt back wouldn't it fail to cycle properly as well?

You would think so, but that did not prove to be the case. No failures to cycle yet the bolt would only lock back some of the time. After switching to Q3131A and some 70s vintage LC,two things were observed,a noticeably stronger recoil impulse and 100% lockback on an empty mag. It is my opinion that the S&B was at just that point powerwise that it would reliably cycle but not have that little bit of extra power required to push the carrier assembly that  extra distance to lock the bolt back.
The S&B I had was marked M193. Well, you can put a tutu on a pig but don't plan on it dancing Swan Lake.
YMMV

DF, I think the problems may all begin when you put that Tutu on your pig.  Now, my dog will do  the Nutcracker, but he won't put on the Tutu. And he'll only perform this ballet during the Xmas season, so tickets are very hard to get!

GVIDON, just try a few other types of ammo and see what happens.