Just installed an 80 degree LP in my AP5-p and it sounding quieter than my full size AP5.  Same CGS Mod 9 can on both hosts.  Anyone experience the same?

Would it be worth throwing in a 90 degree LP in full size to make it sound as quiet as the k variant?

I have little experience with this, but just thought I'd ask...

Any chance your ammo was subsonic in the shorty and super in the full size? That makes a pretty big difference and 3" of barrel isn't trivial for 9mm.

This was my first thought as well. The velocity out of the full length barrel, depending on the ammo, is potentially noticably higher. Try 147g in a non +P+ load out of the full length and I think you'll find the the full length to be quieter.

Both are subsonic loads with k and full size.  I reload and have a chrono…

What are the chrono numbers for both guns? I am willing to bet that the ballistics answer already provided to you is still the answer. With the shorter barrel, odds are the subsonic load is not reaching the same speed as it is in the longer barrel.

Dont recall off the top of my head on the K but the full size was around 980 fps

Next time you go shooting chrono both and you will most likely see slower speeds on the K due to the shorter barrel, hence it is a bit quieter.

I dont doubt that but I wouldn’t have thought full size would be louder being both are subsonic loads and less 1000 fps.  My guess is that full size is unlocking a little earlier contributing to a bit more sound over k with 80 degree lp - more of a high pressure contribution than velocity increase   Hence me asking if anyone runs a 90 degree lp or even an 80 degree lp on their full size?

I considered the PDW locking piece as well as a reduced power recoil spring for my SP5. I did a test to see how far the carrier was moving by spearing grease on the inner rails, shooting suppressed, and noting the displaced grease. It stops well short of the back of the receiver, so instead of changing the locking piece (and possibly recoil spring), I chose to change the rollers to adjust the bolt gap to the narrower side of the range. This will increase the delay slightly from a larger bolt gap.

That said, it might be worth checking the bolt gap on both your hosts as your results aren't what one would expect.

I feel like the full size is quieter with same ammo/can, but it could be that it’s further away from my ear?.

Its probably one of two things.

1. As already noted it could be that the ammo is going super in the longer full size barrel.   However it sounds like you have already discounted this.

2. The 80 degree LP is delaying the amount of time it takes for the bolt head to unlock from the trunnion.  This means more hot/high pressure gas is being forced through the baffles of the suppressor vs. blowing out of the ejection port so it probably sounds quieter, especially to the shooter whose ear is right by the ejection port.

You can certainly use the 80 degree LP in the full size gun as long as it runs properly.  Its possible depending upon the recoil spring, hammer spring, and bolt gap that the full size gun won't have the ommph to reliably run/eject running the 80 degree PDW LP.   However my experience is that the full size guns generally run just fine with the 80 degree PDW LP and a suppressor attached.

Also just a note there is no 90 degree locking piece for the 9mm MP5 series that I am aware of (unless somebody started making custom LPs)

The 90 degree MP5 LPs you see are online are  for the MP5/40/10 series of subguns.  Technically the #24 90 degree LP  is the "low pressure" ammo LP for he MP5/40/10 and the high impulse ammo LP is 60 degrees.

The MP5/40/10 LPs are shaped differently and are more of a "T" shape vs. the 9mm LPs that are more of a "square" profile.  The MP5/40/10 locking piece is a T shape because the ejector runs through the side of the MP5/40/10 bolt head so it has correspondingly shorter (top to bottom) rollers and a locking piece shaped to match those smaller rollers.

While an MP5/40/10 LP will fit a 9mm bolt carrier group the 9mm bolt head will flop around quite bit more as its a T shaped LP in a Square shaped bolt head hole.

I have personally test fit them but have  never tried to run this combination of 40/10 LP and 9mm bolt head as it doesn't seem like a good idea given the play between the 9mm bolt head and  40/10 locking piece.  There might also be some issues with the taller 9mm rollers engaging the shorter angled caming surfaces of the 40/10 LP.

(The 9mm LPs won't fit a 40/10 bolt head as you can't put a square shaped LP into a T shaped hole)

Hope this helps.

From memory my full size is quieter using the same ammos

Its really splitting hairs though

Theres a bigger difference between them and my SD per say. But even then the SD is not leaps and bounds more quiet than the MP5/K with the appropriate ammo

Using the same ammo, my MKE K model is also quieter than my full size using a tirant 9. Stock locking pieces.

Even if the ammo isn't going supersonic in the longer barrel it is most certainly going faster and just the physics of it; more energy is going to be louder given the same volume of space in the can being used to reduce that noise.

I wouldn't think it would be enough to be noticeable to your ears but there's also tons of factors like the air temp that can play into this.

For example if you shot the K early in the morning when it was colder out while the suppressor was cold it would be quieter than if you shot it late afternoon when the air temp was higher and the suppressor had already been used to shoot a full mag.

This is opposite in other cases though.  Like 22 handguns vs rifles.  Less gas burning in the barrel/can means more sound.  More burn, less sound, under 1050fps or whatever.  More energy is being expended into the projectile vs escaping gas from the muzzle.


OP when I suppressed my K I was initially disappointed by how loud it was at the ear.  Switching cocking pieces made it quieter at the ear.  Either cocking piece sounds the same to an observer downrange.  So yes I think the timing of unlocking is what you're hearing.