Colt’s .170ness and HK416 uppers

[Hootiewho]

Ok,

****For those who don’t know I did learn that Colt’s accurized hammer is tall enough to activate the firing pin safety on the HK416/MR556. Just a tip of the day from Hootiewho, but onto my problem.

I have a handful of Colt 6933 & 6921 SBR lowers from the .170 period. I would really like to install Geissele HK MR556 triggers in these lowers for my uppers but we all know they are .154”.

Has anyone ever successfully bored out the pin holes on the trigger, hammer and disconnector?

I know Geissele makes SSA, SSA-E, and DMR triggers for large pin colts. Lets face it, for a long period in the mid 2000’s, if you wanted a factory SBR AR a Colt 6933 or 6921 was pretty much the most available choice.

I don’t want to bushing my lowers, I’ll just snag some MR556 lowers here and there and form one them before I do that. Just mainly curious if the smiths here know if it is possible to bore out the .154 fcg to work with the .170 holes?

Thanks in advance btw.

[scottryan]

The only person I could trust to do this correctly would be Ken Elmore if he is willing to do it.

[TMS951]

I would first ask Geissele their opinion of amount of material there to work with. They may also have a smith recommendation.

Personally if I nought a 6k 416 upper I'd buy the MR556 lower or complete gun and sbr it.

If you paid less than the 6k that HKparts has them for now thats awesome and I would be interested in where you got it.

[Sry0fcr]

Do you want to know if it's possible or advisable?

[khc3]

Carbide .170 reamer is $38.74 at Mcmaster carr.

[M Sadler]

J-Spring in the hammer is gonna be a problem.

If the hammer in a Geissele large pin trigger is tall enough, why not just pony up?

[ccosby]

m60joe will open up fire control groups for the larger colt pins as well. Might want to talk to him.

[GH41]

Why fool with the MR556 trigger. Geissele describes it as SSA like. Why not use the large pin SSA? Maybe I missed something.

[Hootiewho]

Why fool with the MR556 trigger. Geissele describes it as SSA like. Why not use the large pin SSA? Maybe I missed something.

Because the HK BCG has the firing pin safety that must be activated before it will fire. To do this, the hammer on HK rifles is a bit taller. That is why Geissele made a MR556 trigger in the first place.

As to “buying an HK lower”, I could do that but I already have 4 Colt SBR lowers. HK416 uppers can be had for half of $6000. I just picked up an excellent condition 14.5” upper and a complete lower parts kit for under $3k.

I would like to have a SSA-e like trigger in my colt lower for these uppers. The only Geissele option that would work (because of the firing pin safety) is the MR556.

I’ve sent Geissele an email and I guess I’ll give M60Joe a shot too.

[GH41]

Because the HK BCG has the firing pin safety that must be activated before it will fire. To do this, the hammer on HK rifles is a bit taller. That is why Geissele made a MR556 trigger in the first place.

As to “buying an HK lower”, I could do that but I already have 4 Colt SBR lowers. HK416 uppers can be had for half of $6000. I just picked up an excellent condition 14.5” upper and a complete lower parts kit for under $3k.

I would like to have a SSA-e like trigger in my colt lower for these uppers. The only Geissele option that would work (because of the firing pin safety) is the MR556.

I’ve sent Geissele an email and I guess I’ll give M60Joe a shot too.

I am blind as a bat. I completely missed the HK upper mention.