Quoted:
What's the ideal locking piece to buy to run in a full size omega, suppressed 87% of the time?
I tend to shoot cheap factory ammo too

Quoted:
There is are a couple of factors to consider when trying to choose the correct locking piece.



If you move up to 147gr or 158gr ammo plus a suppressor you may want to seriously consider the lower angle MP5K-N/MP5K-PDW locking piece (80 degree / #28) which is designed for suppressor usage along with higher grain weight bullets.   There are supposedly 70 degree and 90 degree MP5K LPs out there as well but I have never personally seen one.

Quoted:
There is are a couple of factors to consider when trying to choose the correct locking piece.

The "standard" MP5 locking piece is 100 degrees and HK makes locking pieces that go both up and down in angle.   The greater the angle in degrees the easier it is for the action to open up.   So in general the more pressure in the system the lower the angle locking piece you want in the gun.  

What can affect the pressure acting against the bolt/rollers/locking piece

- Barrel Length (longer barrels require lower angle locking pieces)  -This is why the LP in a standard MP5 is 100 degree but is 110 degrees in an MP5K.
- Grain Weight (higher grain weight bullets require lower angle)
- Power Charge (higher power charge requires lower angle)
- Suppressors* (Suppressor use can also require a lower locking piece angle)

* the one exception here is that you do not want to use an MP5SD locking piece (115 or 120 degrees) in a non-SD suppressed standard MP5.  The reason the MP5SD doesnt appear to follow the standard rules is because the A. the barrel on an SD is closer to K length than standard length and B. The barrel is ported just past the chamber further bleeding off pressure.

So while it may seem like a good idea to use a MP5SD locking  piece in your standard MP5 with a suppressor, its not advisable as you are actually going the wrong way in terms of LP angle as the SD LPs are 115 or 120 degrees.

Most MP5s running  bulk pack 115gr 9MM ammunition will be fine with a suppressor and the 100 degree LP installed.    

However, you will want to be aware if you notice excessive recoil, monitor your polymer buffer for signs of excessive wear/imprinting, and also make sure you are not getting signs of any roller dents in the rails as there are other factors that determine how easy the bolt group unlocks.  (bolt gap, recoil spring strength, and hammer spring strength)  So while many standard MP5 may be just fine with a 100 degree LP and a suppressor it is possible that a combination of other factors like bolt gap, recoil spring wear, etc. could end up damaging your receiver via roller dents over time.

If you move up to 147gr or 158gr ammo plus a suppressor you may want to seriously consider the lower angle MP5K-N/MP5K-PDW locking piece (80 degree / #28) which is designed for suppressor usage along with higher grain weight bullets.   There are supposedly 70 degree and 90 degree MP5K LPs out there as well but I have never personally seen one.