Joined
·
4,248 Posts
I've been planning on replacing the take-down pins in my MR556 lower with standard mil-spec pins. Why? Because I consider them a PITA and I hate having to get that little tool or a punch anytime I want to break the gun apart or get at the BCG. Well, tonight I got bored and swapped 'em out. I learned a few things in the process that I feel might be helpful to anyone attempting to do the same.
First, for those not familiar with front takedown pin removal/installation, here you go:
DIY AR-15 Build: Pivot Pin Installation
You will notice the installed milspec pin and the HK pin both have holes in them, but in different places:
![]()
To remove a milspec pin, the pin is pulled out and the hole in the pin body should line up with the detent/spring assembly. You insert a 1/16" punch and depress the detent/spring. You can then rotate the pin so that the so the detent does not extend into its respective channel in the back of the pin. The take down pin can now be removed easily. (NOTE: care should be taken as the detent and spring are under tension and are easily lost if not contained).
Notice in the pic that the HK takedown pin (the loose pin) does not have this hole in the body; rather, it is in the "head" (for lack of a better term) of the pin. This presents a bit of a problem since even if a punch is inserted and the detent/spring is depressed, the pin can not be rotated and removed due to the flat side of the pin head contacting the detent spring housing portion of the lower. What I did instead was to depress the detent/spring with my punch and shim a very thin feeler gauge between the pin head and housing, keeping the detent/spring captured in the housing. I could then pull the pin out and rotate it so the channel faced up. At this point I removed the shim and the takedown pin.
The rear takedown pin also presented a slight challenge. The castle nut on the receiver extension is staked. I simply used a small punch and a light hammer to "un-stake" the castle nut.
![]()
I have seen tutorials on other forums that advise using "cheater" bars and just muscling through this process. DO NOT DO THAT!!!! For the most part, there really is no good way to securely place an AR lower in a vice, apply great amounts of torque, AND not cause damage. If it is necessary (which it has yet to be for me), I would wrap the extension/buffer tube in leather and put that in a barrel clamp, and realize a new buffer tube may be a possibility. Since HK buffer tubes aren't cheap and or commonly in stock, I wanted to avoid this. I don't recommend the magazine style blocks for anything other than holding the lower in a workable position where no significant force is applied.
After I got as most of the staking material pushed back from the castle nut, I put on a pair of leather gloves. I gripped the buffer tube in one hand had my spanner wrench in the other. I was able to break the castle nut free. I recommend using a zip tie to keep the backplate from slipping on you and damaging your rear takedown pin spring. (NOTE: this spring is also under pressure and is easily lost) Since I was swapping out the rear pin, I only had to move the backplate enough to gain access to the spring. Take care when reinstalling the spring and back plate not to bind/break the spring.
RESULTS:
My rear pin was replaced with a new Colt pin, but as you can see form the wear on the front pin, I had to find a loaner from an LMT lower that isn't currently in use. I do have a slight amount of receiver play now, but it's no worse than standard AR receiver play. I will see if a new front takedown pin makes any difference. If accuracy suffers (which I doubt it will), I would consider reinstalling the front pin only, but leave the rear pin to allow quick/easy access to the BCG/upper internals.
UPDATE: The front pin was replace with a new Colt pivot pin. This helped to reduce the play slightly. While a slight amount of "play" still exists between the 416 upper and MR556 lower, it is significantly less than any of the factory Colt or LMT rifles that are in my possession or care. There has also been no loss or change in accuracy going between the HK to milspec pins (new or used).
First, for those not familiar with front takedown pin removal/installation, here you go:
DIY AR-15 Build: Pivot Pin Installation
You will notice the installed milspec pin and the HK pin both have holes in them, but in different places:
To remove a milspec pin, the pin is pulled out and the hole in the pin body should line up with the detent/spring assembly. You insert a 1/16" punch and depress the detent/spring. You can then rotate the pin so that the so the detent does not extend into its respective channel in the back of the pin. The take down pin can now be removed easily. (NOTE: care should be taken as the detent and spring are under tension and are easily lost if not contained).
Notice in the pic that the HK takedown pin (the loose pin) does not have this hole in the body; rather, it is in the "head" (for lack of a better term) of the pin. This presents a bit of a problem since even if a punch is inserted and the detent/spring is depressed, the pin can not be rotated and removed due to the flat side of the pin head contacting the detent spring housing portion of the lower. What I did instead was to depress the detent/spring with my punch and shim a very thin feeler gauge between the pin head and housing, keeping the detent/spring captured in the housing. I could then pull the pin out and rotate it so the channel faced up. At this point I removed the shim and the takedown pin.
The rear takedown pin also presented a slight challenge. The castle nut on the receiver extension is staked. I simply used a small punch and a light hammer to "un-stake" the castle nut.
I have seen tutorials on other forums that advise using "cheater" bars and just muscling through this process. DO NOT DO THAT!!!! For the most part, there really is no good way to securely place an AR lower in a vice, apply great amounts of torque, AND not cause damage. If it is necessary (which it has yet to be for me), I would wrap the extension/buffer tube in leather and put that in a barrel clamp, and realize a new buffer tube may be a possibility. Since HK buffer tubes aren't cheap and or commonly in stock, I wanted to avoid this. I don't recommend the magazine style blocks for anything other than holding the lower in a workable position where no significant force is applied.
After I got as most of the staking material pushed back from the castle nut, I put on a pair of leather gloves. I gripped the buffer tube in one hand had my spanner wrench in the other. I was able to break the castle nut free. I recommend using a zip tie to keep the backplate from slipping on you and damaging your rear takedown pin spring. (NOTE: this spring is also under pressure and is easily lost) Since I was swapping out the rear pin, I only had to move the backplate enough to gain access to the spring. Take care when reinstalling the spring and back plate not to bind/break the spring.
RESULTS:
My rear pin was replaced with a new Colt pin, but as you can see form the wear on the front pin, I had to find a loaner from an LMT lower that isn't currently in use. I do have a slight amount of receiver play now, but it's no worse than standard AR receiver play. I will see if a new front takedown pin makes any difference. If accuracy suffers (which I doubt it will), I would consider reinstalling the front pin only, but leave the rear pin to allow quick/easy access to the BCG/upper internals.
UPDATE: The front pin was replace with a new Colt pivot pin. This helped to reduce the play slightly. While a slight amount of "play" still exists between the 416 upper and MR556 lower, it is significantly less than any of the factory Colt or LMT rifles that are in my possession or care. There has also been no loss or change in accuracy going between the HK to milspec pins (new or used).
