I recently purchased a Bushmaster upper with a 14.5" barrel on a pre-ban Oly lower, with a 4 position M4 telescoping stock. The bolt carrier is a matte silver finish, marked with the letter D. The key is parkerized. The bolt has no markings. The extractor spring has a blue plastic insert.

My firing pin retaining pin is bent. After reading the FAQs, I have determined that my hammer is dragging on the firing pin. My carrier is of the open design.

I am also having a failure to eject problem. After disassembly, I determined the ejector was sticking. I punched out the roll pin, and removed the ejector and spring. After cleaning them, I polished the hole to remove any burrs. After reassembley, the ejector seems to function correctly. I have not test fired the weapon since cleaning, so I cannot say for sure that this has solved my problem.

I am now looking to replace my bolt/carrier with a Colt M16 bolt/carrier.

Two questions:

Will a M16 bolt/carrier function better than what I have now?

What's with the blue plastic insert in the extractor spring?

It will alleviate the firing pin retaining pin bending problem.  It should also alleviate your ejection problem.  The only problem that you have about a 1 in 39824920424 chance of running into is that if you are just barely getting enough gas as it is, the added weight the M16 bolt carrier group brings with it could enduce short-stroking.  This is HIGHLY unlikely.

If you want a protected firing pin with a minimal weight difference from your current bolt carrier group, get one of the RRA Enhanced Bolt Carrier Groups found in the EE forums.

If you change your bolt, you will need to check headspace if you value your safety.


I already answered this in the other thread, but if others care to add their 2 cents...

Sounds like you have a chromed carrier.  Some will pay a premium for that.  I believe they do it to enhance corrosion resistance.  I suppose it requires less lubrication as well (could be an advantage in sandy areas?).

I suspect you have fixed your ejection problem.  If it works, you have nothing to gain by changing the bolt/carrier.

My firing pin retaining pin slowly bends over 1000's of rounds.  I suspect when the bolt rebounds off the end of the receiver extension tube the firing pin whacks the retaining pin.  After lots of cycles the retaining pin starts to bend.  Never seems to get bad enough to affect removal or replacement when I clean the bolt though.  

The blue rubber bumper improves the extractor engagement.  All manufacturers use them as does the military.  They started using a heavy duty spring and black rubber bumper in the M4's to improve extraction.  Comparing the black and blue bumpers side by side the only differenc is color.  They both 'smoosh' the same.  I believe the black bumper just indicates a heavy spring was used.  

Extraction is a not uncommon problem with AR's but I think a lot of those are caused by rough or sticky chambers.  The problem can be solved with heavy duty extractor springs in some casses. Polishing the chamber would be a better approach.

When I bought this rifle, the upper had never been fired. I disassembled it prior to my first range session. The retaining cotter pin was already bent.

The underneath side of the bolt carrier is very rough, with deep machining marks in it. When comparing it to my Colt bolt carrier from a different rifle, there is a definite diference in the quality of the finish.

[b]How do I polish the chamber?[/b]

If it's a Bushmaster rifle the chamber should be fine.  If it's not broken...

Hard to comment on the rough bottom of your carrier without seeing it.

The bottom of the bolt is machined so that a notch machined into the top front corner of the hammer will engage the firing pin and stop the bolt in the event the disconector drops the hammer.  The purpose is to prevent full auto if some clever person removes the disconector.

It could be someone pulled the bolt part way back and let go allowing the hammer to catch the firing pin and bend the firing pin retaining pin.  Straighten the pin with pliers and see if it bends under normal operation.

Thanks for the replys.

As I understand you, the dimensions of the plastic extractor inserts are the same, they are only different in their color that designates a spring strength.