I had several problems last time out. Failures to eject failures to feed the bolt not locking back after the last round. I was thinking mag problems these were old range mags. Today put a new empty mag in the gun pulled the charging handle and released the bolt would not lock open tried several times. Then put the same mag in a different gun and it locked back every time. Now I am thinking bolt catch and mag catch. Would replacing these be the first thing to try? Any thing else

Is it a new Rifle?

No, M16a1

Need history on the rifle, and if your using after market mags or USGI mags.

It this a NIB NFA weapon, or is it a converted receiver.  Also, Is the rifle fully stock from COLT or have you added any mods.

You didn't say what kind of stock you have.  The short CAR stocks are more prone to problems.  What kind of buffer do you have and how much does it weigh?

Not locking back is a symptom of short stroking.  Check for gas leakage or anything which would bind or slow up the action.  Make sure the screws on the carrier key are tight and that the gas rings are intact.

Not locking back can also be a sign of too much gas.  If the bolt rebounds from the end of the extension tube it can fly past the mag catch before it has moved into place.  If you have a cheesy plastic bb filled buffer throw it out and get a real buffer.  The H marked buffer weighs more and is prefered for collapsable stocks.

A good test is to put pressure on the lower part of the bolt catch with your finger when you fire the last round to see if it helps catch the bolt.

It would be interesting to try this uppper/bolt on the other lower to see if the problem stays with the upper or lower.

if it won't lock back manually, I'd suspect the part of the bolt catch/release that the magazine follower is supposed to push up on is worn or broken. Compare it to the one in the rifle that is locking properly.  You could also try actually swapping them to to confirm if that is the problem.
also, check that there is something preventing the bolt from going all the way back, like something in the buffer group.
hope this helps!

I personally would not do this, as it can impede rearward movement of the bolt carrier group, resulting in a KB!!

I'd like to hear more about this. Care to explain how it would cause a KB?

I am going to the range now to check some things will post results later.

Went to the range only to find they were pouring a slab for a new clubhouse with only three volunters ended up spending most of my time helping. Shot some only problem was a failure lock back on the last round twice. I think I will go ahead and replace the bolt catch assembly and mark the mag and replace the spring and see if that helps

The reason that I asked for the configuration of the rifle was to check for stock style.

First off, pull back on the cocking handle and push the bolt lock into place to lock the bolt.  If the bolt is not coming all the way back to lock the bolt, chances are that the butt stock top screw is an A-2 screw that is installed in an A-1 stock.  The A-2 screw is longer and will enter the receiver extension and not allow the buffer to fully retract to the back of the tube, but instead make contact with the buffer short of end, and cause the carrier/bolt to not come all the way back for engagement.

Second is to check the bolt lock to make sure that it moves freely in the receiver. If it binds, then the mag pressure will not be enough to over-ride the added tension. If needed, work the hold open by hand to free it up.

The last thing I can think of is the mag lock.  Yours may be loose and not holding the mag high enough.  Use a pencil and press in on the mag release button, then reach over and turn the catch one rotation clock-wise to add more tension to the lock.

Dano523, The stock is a fiberite telestock with Colt H buffer it seems to be fine, spring appears good. The mags or USGI. I have ordered new Colt bolt catch and spring. I think the present spring on the bolt catch may be weak it does seem to stick sometimes it could just be dirty I will know more when I pull it out just waiting on the new parts. No problem locking bolt back by pulling charging handle. Compared to the cost of the gun a new bolt catch and mag catch seem like cheap insurance to just go ahead and replace and see if it helps.

flyfish,
Just punch the roll pin out, and clean the catch, spring, detent, and channel.

The fact that the spring is weak is not your problem.  If the spring was too weak, then the bolt catch spring would not hold the catch down and you would have problems with the catch coming up and stopping the bolt during cycle with the mag still having ammo. A stronger spring will only require that the mag follower to have more pressure to overcome the stronger spring to lift the bolt catch.

Chances are that the bolt catch channel is full of crap/brass flakes and needs to be cleaned.  Since the whole ordeal will take less than a few minutes, I would just pull the assembly, clean it out.

Food for thought.