I have done a comprehensive search online and my results have led me to postings on this site made by a user with the handle "SBR7_11."

I recently built a 9mm suppressed AR pistol.

The pistol has a 5.5 inch barrel, 15.5 oz bolt, 12 oz buffer, and traditional carbine spring. It uses a side charging upper and a hahn dedicated mag block for Colt metalform mags. It's a beauty.

The pistol functions perfectly with my suppressor. It is very quiet. The increased recoiling mass is working out precisely as I planned.

However, there is only one drawback. There is a loud noise- albeit only one. The noise of the bolt closing; hitting the barrel chamber face.

Without hearing protection, using a cheek weld, and closing the bolt on an empty chamber, my right ear is hurt by the slamming noise.

If I load one round in the magazine, chamber the round, and fire, the shot is very quiet, the bolt locks back due to the empty magazine, and all is well.

If the magazine is not empty, and the bolt chambers a new round, that chambering is loud. Steel-on-steel slap loud. Not good for my right ear.

SBR7_11 posted that he has cut the chamber face of his 9mm barrel(s) down so that when the bolt closes on a newly chambered round, the bolt is SLIGHTLY offset from the barrel chamber face, by approximately 0.010". I think this requires cutting down the chamber face of the barrel on a lathe and also the barrel flange so the barrel sits back into the upper by the same amount.

This would result in much quieter chambering of rounds, because the bolt never really hits the full chamber face; just the rear of the 9mm case.

Unsupported case length concerns may be justified here, but maybe not, with only 0.010" of gap.

According to SBR7_11, you need 0.135" protrusion of the case measured from the barrel face to the rear of the case.

I am posting this because this is the last piece of a truly quiet suppressed 9mm AR puzzle.

Let's freaking solve the puzzle, together!

I have limited privileges on this site. It says I am a "new member" so my posts are limited to 2000 characters. I apologize if I left out the detail above that a common 9mm bolt pocket depth is 0.125".

That is why, I think, SBR7_11 says your ideal case measurement out of the chamber is 0.135". That gives a 0.010" gap, for the noise reduction mentioned.

Photos as payment for your trouble:

I would talk to Rudy at Macon armory.

I have seen his posts on this site, I think. Is he the user "Mad Machinist?"

Thank you for your advice and opinion. Do you think you could provide such a service? I am tempted to buy an extra barrel just for the experiment, frankly.

And yes, I agree with you - have to trim the other side of the barrel (that shoulder part) so it sits back in the upper the same amount you trim from the face.

That's him.

SBR7_11 - Thank you for the wealth of information you have posted online over the years.

Rudy @ Macon Armory - ordered a 5" barrel, thanks for the help.

Rudy and SBR7_11 have been dishing out this knowledge for a while (here are several threads about a more severe issue, but still, the answer is always the same- make your 9mm barrels with the correct chamber depth):  

again
again
and again

Use a 0.744" long 9mm case.  Put it in the barrel.  If your case sticks out from the chamber face (the outer chamber face) by 0.135", that will provide a 0.010" gap between your bolt and barrel face when in battery with a round chambered if your bolt pocket is in spec at 0.125".  You can have up to 0.754" cases according to this:





The rest of you -

Take a look what your bolt looks like against your chamber face through the ejection port when in battery.  With a round chambered, do you have a small gap between the chamber face and the bolt face?

I do not.  My bolt, manufactured by Spinta Precision (it is a blem I got from them recently) is perfectly in spec, with a 0.125" deep bolt pocket.

My barrel, a 5.5" Modern Series from Ballistic Advantage recently purchased has a chamber that is too deep.

The 9mm luger round should index off of the case mouth.  

My gun functions perfectly.  However, it is not perfect enough, as I am slamming steel-on-steel each time I chamber a round.

There is no reason people should have to worry about things like this.  There needs to be a better 9mm AR standard.  This site should set it.  So go set it!

This is an interesting thread, I just had the gas key on a gen I PSA single screw bolt shear off while running in an upper with I believe a Faxon lightweight 16" barrel, PSA has already generously accepted the bolt back for warranty so I cant check it but I will when the new bolt comes back, Only reason i didnt go with rudy is his barrels were heavier at the time I built this.

I guess, technically, there is nothing wrong with my Ballistic Advantage barrel..... in that no function is influenced by the chamber being too deep, with the ammo I am using. This is why I am betting when I contact them about it, and I send it to them, they are going to say it is in spec.

I am having a hard time finding the ACTUAL Colt spec for a 9mm AR barrel chamber.

This type of weapon is non-standard. The round indexes off of the case mouth though, so there has to be a 9mm AR barrel chamber spec somewhere.

Right now it seems like a fucking free for all with these barrel makers. Rudy uses technically sound methodology, which comforts me.

I will be sure to give an update when I get the barrel I ordered from Rudy. I didn't want to modify the Ballistic Advantage barrel. Assuming the new barrel fixes my issue, I will then start to explore options with Ballistic Advantage. I can always sell it for cheap; I mean, it has 3 rounds through it, is in pristine condition, and works. Just not ideal for a suppressed weapon.

Correct. I use a 9QT 12.5 oz buffer with sliding tungsten weights, made by Clint at heavybuffers.com

Update for the group:

Case protrusion measurement with standard factory ammo (4 different types):

Ballistic Advantage "Modern Series" 5.5 = 0.1165 inches

Macon Armory "Guaranteed to Feed" 5 = 0.129 inches

Spinta Precision Blem Bolt Pocket depth = 0.125" (standard; this is correct).

The Macon Armory barrel leaves a 0.004 inch gap between the bolt face and chamber face when a round is chambered.

The Ballistic Advantage barrel actually allows the bolt to bottom out on the barrel chamber, like a hammer to an anvil, even with a round chambered.

Test fired 3 shots suppressed with the Macon Armory barrel. Right ear not hurting.

Success.

Will contact Ballistic Advantage tomorrow and let them know that their barrel might have an issue. Hopefully they will correct it going forward.

Note that the threads on the MA barrel are longer than the BA. Also, the chamber face sits a little more forward in the upper with the MA barrel.

I belive the Macon Armory threads are not intended for suppressor use; they are too long and there is no thread relief before the shoulder. I believe 1/2x28 threaded length should be shorter for suppressors so that they index on the shoulder. This is a common issue with some barrels because they follow some kind of AR15 thread length for flash hiders or something. Spacer needed. I use a 3 lug adapter which bottoms out, but will work in this case.