As I was taught, the various locking pieces are designed with a wide range of factors in mind. Some of the major ones are bullet and powder grain weights, average velocity, timing/bolt carrier weight, ad infinitum but with varying degrees of influence.

HK may recommend a specific style of locking piece for a given weapon, and/or a specific projectile and charge combo, but it may be an average.

With an add-on suppressor to a K model, you will be altering the gas pressure on the bolt and unlocking sequence, depending on bullet performance, and may have cycling problems.

A different locking piece may resolve those issues. The "universal" locking piece you refer to may be be a composite average shoulder angle to allow for performance throughout a broad range of ammunition types, suppressed or unsuppressed.

9mm ammunition grain weights and velocities have crept up the scale since the 70's and the design of the various locking pieces, combined with the variations within the weapons system allow for the continued service life of the weapons system.

 

He further said that the 100o is recommended by HK now instead of the #28 for suppressed or non-suppressed. A "universal" ( my words ).

Everyone else that I spoke with says the #28 for suppressor.

Does anyone know for sure?

So, you don't know if you should believe the factory advice.
Who is "Everyone else" you spoke with? Other Hk factory people?
Do you have a clear understanding of the history of why a LP other than #16 was made and used? I use a standard MP-5 LP with and without a silencer no problem on my 5K. Not exactly sure what kind of proof you are looking for.

 

wh: Not other HK factory people but owners of companies that supply us the parts and other SMG shootin buds. Their mutual concerns are the increased stress on the receiver and bolt head from the pressure and higher cyclic rate. In reality I may never shoot the # of rounds to even cause this but with an $18K+ weapon I do want to take good care of her. The #16 ran like a champ on mine too; just concerned about the unknown (this is my first SMG) stress it might be creating.

Bottom line is I'm going to get the 100o and replace the #16. Assuming she purr's the same I'll maybe try the #28. I'd rather put that xtra $125 toward ammo

 

As has already been mentioned, the performance of the system is affected by the interaction of its parts. It is impossible to know how all the parts will interact when the specifics of the individual parts are not known. To make a blanket recommendation, even from HK, is not wise or prudent. Before anyone starts changing out parts *before* a problem happens (prophylactic measures), discover if there *is* a problem. You may discover that your combination of specific suppressor, current wear on locking piece/rollers, bullet choice, etc., works just fine with things exactly as you have them now

Rule #1 in problem is solving is to determine if a problem actually exists.

 

My phone call to the H&K Virginia office was returned today and here is the advice I was given. Gerald said that HK still recommends the 80o/#28 for suppressed use and was rather surprised that Matt in the Al office told me that the 100o was for both.

As the discussion progressed he said that if I was not running +p+ rounds and especially if I was running 115gr vs 147 that the odds were that it would be fine and not abuse the gun with the 100o LP.

Keep in mind that the advice Matt gave probably came from a LE perspective. LE might need a universal LP, for function, if you needed to go suppressed then unsuppressed because the 80o is known to have potential cycle issues when not using the suppressor.

Additionally he said that if you knew you were only going to use the can then stay with the 80o/#28.

 

Not sure about Gerald guy. But I could have told you not to listen to Matt.

The long and short of this thread HK's recommends a #28 for a MP5K PDW
with a suppressor attached. So why do something different.

 

Because this is now old information compared to what they're installing now at the factory.

I love the 80 degree in my pdw. Shoots fine either way, and slows the bugger down.

 

It depends on your gun, but on my K gun the 28/80 is perfect. It works suppressed (that is what it is for) and slows the gun down without the suppressor without jamming. Some Ks won't run with the 28 without a suppressor.

So try out the 28. If it works both ways, you are set.