Issues with my M4?

[tall_guy]

I bought it used but it looks new.  I fired a few rounds just to prove the weapon.  But I seem to be having some issues with it.  

For example, I can cycle the bolt a few times.  Then, with a snap cap in the mag, I can depress the cartridge drop lever, hear a small "snap", pull the bolt back, and the bolt locks open.  

But then again, I can cycle the bolt a few times, insert a snap cap in the mag, depress the cartridge drop lever, and instead of a small "snap", I hear some substantial mechanism snap forward.  Then when I cycle the bolt, the shell carrier moves the snap cap into the receiver and the bolt seats it into the chamber.  

I can perform the exact same procedure any number of times and I have no idea which of the above movements are performed. 

What am I missing?  Is it a malfunction?  Or am I doing something I don't realize?  I'd like to consistently be able to depress the cartridge drop lever, have the internal mechanism snap forward, pull back the bolt and load a round.  

 

 

[tall_guy]

To summarize.  Bolt in battery.  Insert snap cap/shell into magazine.  Depress cartridge release lever.  Sometimes the shell is moved into the magazine loading port.  Then I cycle the bolt and the shell inserts into the chamber.

But, sometimes, with the bolt is in battery.  I insert shell into mag.  Then depress cartridge release lever.  Then, sometimes, the shell remains in the mag, and when I cycle the bolt, the bolt simply locks open.  

I can follow the same procedure but get two different results. 

I tried to edit my OP but can't seem to find that option.   

 

[Steve Rose]

With the firearm pointed is a safe direction, and safety on, perform your test using real ammo.

Snap cap are designed to “snap” the hammer or action allowing the hammer or firing pin to fall and spring pressure to be relieved. The snap cap's shape and dimensions may, or may not, cycle through the action of the firearm as live ammo would. There are “dummy rounds” available which replicate the dimensions and weight of real ammo for testing cycling functions.

[tall_guy]

Thanks.  Will do.